Heat Pump or Furnace: Which Is Better for Comfort?
With all the incentives for heat pumps and the talk of electrification these days, you may be thinking of going with a heat pump. Heat pumps can work in both new and existing homes. But can you be comfortable with a heat pump? If you’ve been reading this blog for a while, you know my answer. Let me explain.
The temperature of heated air
First, let me say that air temperature is not the main factor that determines thermal comfort. Mean radiant temperature is more important. But for this article, I’m going to focus only on what forced-air furnaces and heat pumps do: blow warm air into the conditioned space.
A furnace burns a fuel (gas or propane) and transfers most of the heat of combustion to the air passing over the heat exchanger. That air then gets blown into the different rooms in your home to keep you warm in winter. The furnace-heated air coming out of the vents in your home is about 125 °F. (There’s some variation, but this is typical.)
A heat pump doesn’t convert another form of energy to heat. Instead, it just moves heat. In winter, it moves heat from outdoor air to indoor air. And yeah, there really is enough heat to heat your whole house even on a cold day…assuming the system has been designed and installed properly. The air temperature coming out of a heat pump, though, is only about 95 °F.
Why a lower temperature is better
We don’t need 125 °F air to heat our homes to 70 °F. Even if you set the thermostat higher, say 75 °F, you can still heat your home with 95 °F air. Think about it. You add 95 °F air to 70 °F air. The heat from the warmer air transfers to the cooler air and keeps you warm.
So why is lower temperature air better for comfort? Well, to get the same amount of heating from lower-temperature heat pump air, you need more air flow. That additional air flow is the key to comfort.
With high-temperature air, the furnace blasts you with hot air for a short time and then goes off. Then it comes back on a while later and blasts you again. So you’ve got what can be a noticeable variation in temperature over time. The big difference in temperature between the room air and the heated air also creates variation in temperature from one part of the room to another.
A heat pump, by contrast, adds a greater volume of lower temperature air. That extra air flow and lower temperature means a more uniform temperature. And that means you’re not as likely to notice temperature differences from place to place or from time to time.
The importance of good design
Now, the big caveat here is that you need a heat pump that’s sized properly for the heating load in the house. You also need a system that can provide enough heat even when the outdoor temperature is low. That means having the right kind of heat pump or having auxiliary heat available for the cold days.
Then there’s the distribution system (ducts plus supply registers and return grilles). The ducts and fittings need to be selected and sized to distribute the right amount of conditioned air to the rooms. And the duct system needs to distribute that air to the right places.
One of the most important rules for designing a duct system is not to blow air on people. You want to put the supply registers in places where they’ll blow the air into the room in a way that it gets well mixed with room air before entering the occupied zone.
That last point is the answer to what HVAC professionals call “cold blow.” Yes, heat pump air is cooler than furnace air. If you blow it right at a person in the room, they may feel a draft. The problem isn’t the heat pump, though. It’s the distribution system. (ACCA Manual T provides guidance for selecting, locating, and sizing registers and grilles. It’s rarely used.)
The reality is that properly designed heat pumps can provide more comfortable heating than furnaces.
Allison A. Bailes III, PhD is a speaker, writer, building science consultant, and the founder of Energy Vanguard in Decatur, Georgia. He has a doctorate in physics and writes the Energy Vanguard Blog. He also has written a book on building science. You can follow him on Twitter at @EnergyVanguard.
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The problem is they’re being installed into duct systems that are vastly undersized in cold climates.
Yes. That is a real problem. They need to update the ducts to match the system. Again, not the heat pumps fault. In heating dominated climates, if they do a load calc they will discover that the existing AC is probably way oversized. If they install a smaller tonnage system, the ducts will be more appropriate (rarely perfect, but much less in need of replacing).
Had a low as ambient mini split heat pump installed at a place I worked at a few years ago and when the outside temp was below zero the thing froze up and destroy itself after struggling to maintain temp. Probably not the best choice for heat in this area.
Or it was improperly sized or installed. Did the installer do a Manual-J load calculation?Right sized heat pumps work. It’s pretty simple, really. Math is math and a BTU is a BTU, regardless of how it is generated.
Today’s heat pumps are not efficient in cold weather climates. They utilize the outside ambient air to hear the home. They use a reversing valve that runs the refrigerant backwards to heat your home in winter. If your feel the warm air coming off your A/C. condensing unit outside in Summer, that’s the heat being removed from your warm house. The heat pump just runs it the opposite way in Winter. If they outside air is too cold then you need an electric strip heater installed inside the main supply duct to supplement the not so warm air to heat your home. This adds a lot of costs to your electric bill. However the good news is the Carrier Company along with Trane Company are coming out with a more efficient heat pump that works in colder weather soon.
Jim: Robert OBrien is a topnotch HVAC contractor in New York. He knows how heating and cooling systems work. Your statement about heat pumps not being efficient in cold climates is not correct, however. Yes, the efficiency does drop as the temperature drops, but even in cold climates, the temperature is in the efficient range most of the time.
Can a Heat Pump Work in Minnesota?
Now, how about we address the topic of this article: thermal comfort.
Cold climate air source heat pumps work in very cold climates, and without resistance back up heaters. They do drop off in efficiency, but not to a COP of 1, like when using strip heat.
Ground source heat pumps work even better, though, since they are not impacted by ambient outdoor air temperatures. Ground source heat pumps use the earth as the heat sink to exchange energy with. When rejecting heat from the building when in cooling mode, we move the heat to the cool earth. In the winter, when heating, we take the heat from the ground and move it into the building.
really? A heat pump doesn’t convert another for of energy to heat? so electricity isnt needed to run the pump in a heat pump? The heat pump just magically condenses or expands the refrigerant to extract the heat? You guys just keep pushing your narrative of how great electricity is until we are all forced to use only electricity and watch the sh*t hit the fan. Oh, and what is producing most of the electricity at this time??? COAL! So lets stick to natural gas or propane, for a cleaner burning future.
Todd, heat pumps use electricity to move BTUs, not create them. That’s why the are so efficient. Electricity production is getting cleaner. Natural gas will always be dirty and dangerous.
Natural gas will always be dirty and dangerous? are you kidding me? Natural gas is a clean burning fuel. It has to be refined but you sumize electricity is superior, I haven’t drunk the coolaid on that, all energies have there problems, and the toxic chemicals that are generated by all! The problem is a path has been declared we must go down and that reduces our choices and our future. Open your eyes to more than one path.
Todd,
I studied energy resources in college and have over 30 years experience designing and testing HVAC systems. If that’s “drinking the coolaid”, so be it. I get asked by a lot of people if switching from gas to electricity is really a good idea. My answer is this: We’ll have to do it eventually. We need to stop kicking the can down the road. There will be growing pains, just like switching from anything old to anything new. On the safety side, would you rather be locked in a sealed room with a running gas-burning appliance or an electrical appliance? For those of you who then say, it all depends on where your electricity comes from, yes, you are 100% correct. But, electricity can and is changing to be cleaner and renewable everyday. Fossil fuels will always be dirty and of a finite quantity. The sooner electricity gets cleaner, the healthier our future generations will be. The best time to do the right thing is yesterday. The average persons’ interests do not align with the big companies who have a vested interest in not changing. Stop helping them resist change. I was BPI certified for combustion safety (now expired) and tested many houses, including homes of friends and family. I was shocked at how dangerous some of the houses are, purely because of gas burning appliances. Piping explosive, poisonous gas into a living space is just plain stupid, when you have any alternative at all. I’d rather drink coolaid than suck fumes.
In the true spirit of Allison, the troublemaker, yet another article that provokes a commenter to stir up clouds of dust. We’ll let that dust settle.
In the meantime, we really need to get out of the middle ages. My vision of the future does not involve scenes from Mad Max. We’ve been burning fuels for millennia, exploding them for well over 100 years (combustion engines), enough is enough. It’s been a wild ride, but it has to stop.
We’ve built our last house in a neighborhood where, quite properly, there is a natural gas line under the sidewalk. I did not even bother calling the gas company with a request for a hook up. Why would I, when, as Russ put it, “piping explosive, poisonous gas into a living space is just plain stupid.”
Change is hard. It’s the most stressful thing to do. Resistance to change is the most natural phenomenon. No hard feelings, Todd. And about your doomsday scenario where the grid collapses, people are working on it. I am working on it, too: installed a modest PV array on my roof that is producing 54% of my yearly energy needs. There, that’s my nano-grid, today.
Russ your over simplifying things. We have plenty of natural gas in this Country. It is totally safe if installed by a properly trained journeyman Sheet Metal Worker, aka Union educated. I was a SMW for 40 years installing Commercial heating and cooling systems. Going green on electric energy is a false narrative pushed by special interests so they can get richer. Commercial solar panel farms and wind driven turbines destroy bird populations, effect marine life in ocean installations, and are visual pollution on our earth. The idealists think that we are saving the planet in America, while the gross polluters like China, India,etc are contaminating our World at a immeasurable pace. Just takes some critical thinking skills to see what is really going on.
I would have to disagree with your position on natural gas.
“On the safety side, would you rather be locked in a sealed room with a running gas-burning appliance or an electrical appliance?”
When are any of us going to be in a locked sealed room with a gas or electrical heating appliance? If any measure of the building code is followed, this is not going to happen. It’s a pointless question. Our current natural gas heaters use sealed combustion which means there aren’t any exhaust gases getting into the heated air. Your argument is not really a valid one.
#2 “We have to do it eventually so let’s do it now”
that’s not how technological evolution works efficiently. Newer technology is based on current technology. Improvements in new technology can also apply to current technology, making upgrades to current technology has plenty of advantages.
If we want to look at overall efficiency, we can’t just throw away what’s existing. It would be foolish to brush off what we have today because “electricity is the future”.
We should transition by keeping what is effective, efficient, and what suits our needs based on our availability of electrical and gas resources and the environment we are in.
Heat pumps are going to take-off in warm climate areas, and are gradually moving to mixed climate areas. They are not yet good in cold climate areas.
Should we burn more coal and natural gas at power plants at 50% efficiency to create electricity which will operate at 100% efficiency in cold environments?
When we can use natural gas directly and get 92-96% efficiency, we are using half of the BTUs to heat our home than when we convert it to electricity first at 50% efficiency.
Switching to electric without the infrastructure to support it in a more efficient manner is quite simply a stupid and completely counter productive idea.
What about end user costs?
Electricity generates 3412 BTUs of energy per kw.
1 Gj of natural gas generates 984,000 BTus of energy
There are 288.4Kw in a Gj. At 92% efficiencty ( a common furnace efficiency there are 265 usable KW of energy when generating heat.
265kw of energy at
$0.10/kwh = $26.50
$0.15/kwh = $39.75
$0.17/kwh = $45.05
$0.20/kwh = $53.00
Also, don’ty forget the flat fees on your bill that need to also work in to the /KWh cost.
265Kwh of energy from natural gas is $6 to $15/mBtu after energy, transmission, distibution and monthly flat fees. depending on where you live.
Right now, after all fees on a regular usage month, I am paying about 18.9 cents for electricity after cost of energy, distribution and transmission fees which is $52.52/Gj.
For natural gas after all fees on a regular usage month, I am paying $17.81/Gj.
The cost of electricity for heat is 3x the cost of Natural Gas for heat. A heat pump cam reliably operate at a COP of 3.0 at what, 0c or 32f?
Heat pumps are cost effective at 32f or higher. My average temperature is below 32f and down to -15f average and as low as -40.
I am cost effective above 32f, and more energy efficient at probably 20f. It makes no sense to use a heat pump (using mini splits as an example because they are the most efficient system) for 4-6 months of the year, the months where I use 80% of my HVAC energy. My average heaviest heating month use 25 Million BTUs, which costs $445 and if it wqas electricity, that would be 6,390 Kw for a total cost of $1208.21 using electric heating. There would be minimal chance of any sizable efficiency gain with a mini split type heater.
A blanket statement of Just do it now is worse for the environment.
Here in central Alberta, we would need to burn twice the volume of gas to turn the gas into electricity just to turn it into heat energy because generators are only about 50% efficient after you bring in the transmission losses.
Doing all that instead of just burning the gas directly where I can use 96% of it’s energy is a bad idea to the tune of 2x the emissions.
I would need 6,390kwh of energy on top of my regular monthly winter load of 1200.
My home would draw 7,590kw/month instead of 1,200.
That puts infrastruture demand at 550% of todays demand. That costs time, money, materials, labour, energy, and all those things cause emissions. And when we get to a more decentralised power system through solar and wind, it’s useless because we won’t need that kind of grid capacity. It won’t pay back.
Let’s not forget to mention with the demand increase for power, rates would skyrocket. Making it probably 10x more expensiv than gas and still “saving” by generating double the emissions from electricity generation at a power plant?
No man, you grow in a sustainable fashion, you don’t abandon what you have for what someone says is the next big thing.
Especially when we consider things like Natural Gas CoGeneration, Energy storage, and finding uses for CO2 emiosions that are helpful.
All electric is not for today, and it will never be the answer. We will always have a need for fossil fuels in some capacity. Right now, fossil fuels make WAY more sense. As technology changes, we should adjust, but jumping in neck deep is completely unrealistic.
Patrick M,
You missed some of my points and you ignored others.
1. “On the safety side, would you rather be locked in a sealed room with a running gas-burning appliance or an electrical appliance?” This was a rhetorical question. The point being that gas burning appliances give off deadly poisonous fumes, period. Assuming that installers will follow current code is dangerous. We can’t even get contractors to pull permits. I was a BPI proctor and trainer for several years. I tested homes that had atmospherically vented gas furnaces and water heaters in hall closets behind unsealed doors. Pipes rust. Heat exchangers crack. This makes my argument extremely valid.
2. “We have to do it eventually so let’s do it now”. I did not say “let’s do it now”. The people who are asking my advice are people who already have a gas furnace that is either dead or dying. They need a new one and are asking me if they should get another gas furnace or switch to heat pump. I’m not endorsing that everyone rip out their perfectly good gas furnace and install heat pumps. Not at all. How long will the new one last? 10 years, 15 years, 20 years? I lived in a house that had a 25 year old system that showed no signs of giving up. Do you not believe that within that period of time the economics will change to favor electricity? I believe it will. Sooner than later. You can disagree, but I would not want to be stuck with a dangerous, polluting system if I had had the option to switch a while back and chose not to because it didn’t quite pencil out *at that time*.
You also seemed to have missed or ignored a very clear statement I made: “For those of you who then say, it all depends on where your electricity comes from, yes, you are 100% correct.” Please stop using coal powered electrical production as an excuse. It is an abomination and should have been outlawed decades ago. If your electricity comes from coal, I’m sorry. You have more serious concerns to worry about. Not only should you not install a heat pump, you should use gas powered appliances and lighting. I understand the economics. I understand source energy. I understand infrastructure. I understand local climate considerations. I’m endorsing a measured, smart switch to a safer, cleaner, superior technology. Nothing in your comment disproves anything I said in mine.
Electricity is just as dangerous as natural gas or any other form of gas.
Electricity will always cost more than natural gas unless there’s a huge interference by political party people.
Not too sure where you live but let’s just say you live somewhere in Ontario Sudbury I can tell you right now A natural gas furnace Cheaper to run and still more efficient than a heat pump.Natural gas is not dangerous people that don’t know what they’re doing with natural gas are dangerous.
Natural gas furnaces are inexpensive both in equipment and fuel. On a pure cost comparison, natural gas at the current Ontario rate is only 1.375 cents/kWh² equivalent compared to 11.3 cents kWh for electricity³.
Eugene, don’t know where your numbers come from, but you’re not suggesting you can use gas lamps at the dinner table, are you? It looks like your natural gas equivalent rate is for the embedded energy in gas. You agree that in order to compare gas to electric, you need to use the unique COP’s of a gas furnace and a heatpump, right?
Here is an example: our GA rates are roughly $0.80/therm for gas (although it can vary wildly), and $0.10/kWh for electric energy. Let’s say we have a very decent 3-ton heatpump with a COP=4, and a very decent 3-ton gas furnace with a COP=0.95. We’ll run them for 1 hour, i.e. 36000 BTU’s, or 10.55 kWh of heat energy.
Heatpump will use about 2.64kWh, gas furnace about 11.1kWh (or 0.38 therms). That 1 hour will cost you $0.26 in electric energy, and about $0.30 in natural gas. So at least for us here in GA, it’s roughly a wash.
Note: Does AFUE for gas furnaces include electric energy cost to run the blower? I am too comfortable right now to look up “For the exact definition of AFUE, refer to the federal test method 10 CFR Part 430, Appendix N to Subpart B.”
You did not do a very good job reading anything I said in my statement.
Clearly says where Sudbury where Ontario do you want more Canada.
And what works for you down in the Southern parts of the States doesn’t work for us up here.
Information came from a website which it gives you the numbers of cost between electricity and natural gas here in Ontario Canada.
And the information came from a organized source here in Canada in Ontario
You can find the same thing on the Web by typing in or asking Google cost comparison between electricity and gas in Ontario.
I work in the ICI sector And when it comes down to reality you can have a perfect environment and test anything or you can place a product in real life and see how it works. You can be green all you want but in the end if the product doesn’t work like it’s supposed to then why should people waste their money and spend it if they’re gonna constantly maintain it repair it and Replace it or upgrade it.
Your environment again is different from Georgia than where I am in Ontario.
In the Atlantic province we will find out with due time especially after this Winter with the amount of increase of heat pumps we’ll see how reliable they are and cost effective.
Your government is also probably a lot more organized than our government.
Can’t compare the 2 markets when you price comparison electricity to natural gas I really don’t have a clue about Georgia that’s why I stated if you read my statement where I was from again.
Green want to be green that’s fine but green only works when it benefits everyone
And everyone here in this form knows they rather a product that’s gonna be dependable reliable and cost effective.
Because at the end of the day you gotta have a pay in order to buy it.
And a lot of my bigger projects that I’ve been privileged to work on were dealing with geothermal. Out of the project’s dealing with the government and the hospital are choosing geothermal for part of their heating and cooling source. Which is a true form of green. A lot more reliable.
In the end it all breaks down to cost once it’s gonna cost you after it’s running what’s it gonna cost you to put it in and what’s it gon to cost you to maintain it.
From a residential point of view All families want something that’s reliable dependable and cause effective.
Can’t think of the last time my gas ever shut off but I could tell you how many times I’ve gone without electricity.
Again we forgot about infrastructure.
The gentleman’s article in here has nothing to do with what we’re talking about but unfortunately someone started up with thinking gas is unreliable and dirty and dangerous.
Do anything that’s dangerous is the people who don’t understand things.
Gas products are reliable dependable cheaper and every year become more efficient.
OK, I couldn’t resist, to follow up on the AFUE definition, 10 CFR Appendix N to Subpart B of Part 430 – Uniform Test Method for Measuring the Energy Consumption of Furnaces and Boilers, does include blower energy:
“8.2 Electrical input. For furnaces and boilers, during the steady-state test, perform a single measurement of all of the electrical power involved in burner operation (PE), including energizing the ignition system, controls, gas valve or oil control valve, and draft inducer, if applicable. For boilers, the measurement of PE must include the boiler pump if so equipped. If the boiler pump does not operate during the measurement of PE, add the boiler pump nameplate power to the measurement of PE. If the boiler pump nameplate power is not available, use 0.13 kW.
For furnaces, during the steady-state test, perform a single measurement of the electrical power to the circulating air blower (BE). For hot water boilers, use the circulating water pump nameplate power for BE, or if the pump nameplate power is not available, use 0.13 kW. “
Todd
I agree that the “energy” (electricity) consumed by a heat pump is converted to heat at 100% efficiency (COP=1). The rest of the heat output from a heat pump is pumped at an efficiency of 1-COP.
I disagree that most of our electricity is produced from coal. EIA data (https://www.eia.gov/tools/faqs/faq.php?id=427&t=3) shows that in the U.S., only about 20% of electricity is generated from coal. Natural gas has the lead at about 40%. Nuclear and renewables (wind, hydro, solar) have about 20% each. Coal might have had the lead in the past, but natural gas slowly took over due to economic reasons.
So, If we go with the EIA data, the energy of choice for all our needs only provides 20% of our energy and one of those, Hydro, is being dismantled due to the impact on fish, so we really only have 10% of the energy needed for our day to day lives? I think we need to get off the one horse mindset (electricity) quit demonizing gas and oil and show the advantages of those fuel sources.
Where was the demonizing of nat gas in the article?
This blog has written that natgas furnaces (96 percent eff) can be as efficient as a heat pump. Obviously NatGas is cheaper because its a primary source of energy whereas electricity is a secondary source of energy.
Well Nat Gas doesn’t work without a blower motor so what’s your point?
Besides on the national level coal isn’t the primary source of fossil fuel for power generation.
https://www.eia.gov/tools/faqs/faq.php?id=427&t=3
Most people don’t know that heat pumps are efficient not always suitable to the location applied due to various reasons. But to the note of effective and efficient heat pumps using the correct type with the right coefficiency of 1 to 3 or 1 to 4 meaning one unit of energy (Electrical energy applied yeilds three to four units of heat energy gained from heat pumps – generally speaking in properly applied climates and locations). Next is the type of heat pumps for the right location would be air to air heat pumps vs air to water heat pumps or even water to water heat pumps such as geothermal heat pumps. Geothermal heat pumps are amazing although the overall investment cost is usually way higher than anyone can afford unless there is a source such as government assistance programs in place to offset the price (Subsidiaries). Most people think heat pumps are good and they only speak to air to air heat pumps have a system installed and sold to them by region that is dictated by the us government by ASHRE. These regions say specifically that each region has a temperature swing from hot to cold seasons and are not designed for the hottest times or coldest times of the year. That is if they were designed for those highest and lowest times the house structure would get effects such as a home is clamy or to moist because the unit is running at a higher rate than designed and slams the temperature to set point to fast without the benefit of tempering the space adequately. But overall if everyone had enough money and vendors/ contractors actually did what they need to do such as heat load calc’s and using the right materials competent employees the issue of heat pumps and one type fits all mentality is pushed out. Then only, and only then heat pump talk would be better understood and liked. Hope this is looked at in a sense I don’t know more than one person or another but rather the facts of one who has taught it and knows it wanting to share some thoughts, by no means am I trying to best anyone just thought topics were missed for a wholesome conversation.
Apologies had the coefficient written wrong it was 3 to 1 and 4 to 1 heat energy gained for electrical energy applied.
Agreed !
Ding, Ding. Bullseye! Great article.
A similar thing is true for oversized AC. Colder supply air temps exacerbate stratification.
The one BIG issue we are running into is the defrost cycle and actually blowing really cold air. Some manufacturers have figured out ways to deal with it, but many haven’t. What are your suggestions?
This is not an us against them issue. Thanks for your diplomatic approach to presenting the facts Russ.
You left out a discussion on gas variable speed condensing furnaces that blow warm air slowly for a longer period of time. With these systems, which I have used since the late 90’s, you don’t hear the furnace nor feel any air movement. You are simply quietly comfortable. No emergency heat $$$ required.
I just replaced my gas furnace with mini splits. There is no question for me that the heat pumps are way more comfortable.
Allison, I believe that the warmer air produced by the furnace tends to dry interior air faster than the lower temperature air produced by the heat pump. In cold climates the inside humidity drops rapidly as the outside temperature drops. People often install humidifiers on their furnaces to help keep the humidity at a comfortable level. (Between 40 and 50 percent relative humidity.)
I’m not sure how much of a factor this would be when deciding between a furnace and a heat pump but hotter temperatures do dry air out faster so this may be a consideration for some people.
Commenting for those who, like me, may hesitate on a heat pump because they want that experience of nice hot air coming out of a vent somewhere. The air coming out of my ducted mini split heat pump registers is discernibly warmer than the temperature around it. One of my design decisions was to have floor registers, but I was warned that I would not get that warm air coming out of the register experience with a heat pump. The warning is wrong. The air from my ducted heat pump is plenty warm enough to give that experience of standing over a stream of hot air to warm up when you come in from the cold.
Hi,
Another great article (and thought provoking).
I’ve learned a lot from your articles over the past 2 years and enjoy that they are design-heavy.
For this novice, could it be possible to have a duct/vent design that is adequate when the heat pump is blowing cold air (summer) but inadequate when it’s blowing hot air (winter)? And for reference, my 60’s house in Texas is very similar (age, design, materials, ATTIC!, etc.) to yours. And how probable is it…
Thanks, Dr. Bailes!
Hugo
I have noticed that articles advocating for changes in our homes and lifestyles to work against climate change usually call for people to be “green citizens” and make sacrifices. My personal experience is that these improvements lead to a house that is much more comfortable, quieter, the air is cleaner, and the bills have gone down to boot. Unfortunately, these changes are too expensive; but if folks understand the benefits, regardless of their politics, self-interest and subsidies are motivation enough! Thanks for emphasizing comfort here.
For a home with a boiler and hot water baseboard (or radiant) heat, are their systems to replace the boiler with a heat pump to heat the hot water?
Bob, You have put your finger on a big problem we face in the northeast US. The short answer is NO, not yet. But they seem to be coming. There are hydronic (air-to-water) het pumps available now. But the refrigerants they use are not capable of reaching the roughly 170 degrees F. you need for hot water baseboard or radiators. They do work with low-temp radiation, European style panel radiators, fan-coils similar to what are used with mini-splits, and radiant floors and ceilings. But removing your current radiation distribution system and installing a system designed for 110 degrees F. warm water which requires that every room have home run supply and return lines is very disruptive to install and expensive. The current refrigerants used, like R-410a are potential climate disasters when they leak, with Global Warming Potential 2080 times worse than CO2 we emit when burning gas. Mitsibishi is doing a webinar this Thursday about a new heat pump product using CO2 as refrigerant (GWP-1). These new high temp air to water heat pumps are coming and will be able to replace boilers without needing new indoor distribution systems.
Hello Gene,
What you say is true but somewhat overgeneralized. Here in Montana we have had some good success placing otherwise “high temperature” hot water baseboard and such systems on outdoor reset, allowing them to work with lower water temperatures during milder weather conditions. As the article notes, lowering the supply temperature necessarily results in longer thermostat cycles which achieves higher comfort. While conventional wisdom has it that baseboard systems must run too hot, the devil is in the details as many baseboard systems were originally oversized and many homes have received conservation retrofits that dramatically lower design heating loads. We also have spent time retro-fitting distribution systems using low temp baseboard, panel radiators and retrofit radiant floors. Retrofitting air or water distribution systems to use lower temperatures is a super good idea and will make the best use of any fuel used including electricity, (not a fuel). In Europe, companies like Viessmann are producing air to water heat pumps that use propane as a refrigerant to reduce GWP as well as to achieve a larger working temperature range. We hope to see this equipment soon.
Yes Bob,
Geothermal using water to water heat pumps as well as air to water heat pumps are available and especially appropriate for low temperature hydronic systems especially radiant – where these comfort considerations regarding forced air heating just sound silly. From the perspective of hydronics, eating a building by filling it with hot air, (again and again and again), is a problematic endeavor at best.
Allison, Isn’t part of the problem with residential air distribution the low quality supply grills commonly used. In commercial buildings air is supplied at the ceiling with square or round diffusers that use the Coanda effect to “stick” the air to the ceiling for some distance (the Throw) before it mixes with room air and drops to the occupied zone. Even then sometimes a person will feel a draft. But residential supply grills are cheap and are often not installed in a place where they have space to mix with room air before hitting an occupant. Another issue is in heating dominant regions the grills are put in the floor to supply the air low under windows, which is not ideal for cooling. That’s a tricky matter of what do you optimize for: heating or cooling?
I’m currently taking care of 190 heat pumps in Oklahoma, and definitely do not recommend this system if you zone gets below 20°f.
No way I’m getting one of this system for my house, furnace is much better in extreme cold conditions
Hey guys good discussion- I’m not fully in either the heat pump or the gas heat camp. I have installed a lot of heat pumps and a lot of gas furnaces. Mostly just replacing what the customer has. Converting over is always problematic. If you don’t have gas the best choice is a heat pump. I had a 80% gas furnace in my house in the early 1990’s. Since I do HVAC I decided to try a heat pump – Heil brand. I had to run a 60 amp circuit to the crawlspace for the aux. 10kw heat kit. It worked ok but…. it got a terrible ordor every winter like dirty socks- I had to clean the evaporator coil every winter. The heat wasn’t hot enough to kill the bacteria on the coil – I guess. Anyway evenually I got Goodman to sell me a 95% 2 speed gas furnace and a 2 speed 16 seer A/C at cost (since it was for personal use) about 8 years ago. The evaporator coil leaked right away and I had to replace it. But after that the gas heat has been working great my biggest gas bill has been only $120. Like someone mentioned the low speed is quiet and comfortable. I did add an extra 8″ return grille in the hallway to lower the static pressure to about .8 (still too high. I also added a 4″ thick filter box- that uses generic filters and even the merv 13 filters last a year. Mostly no more smell.
Gas is explosive and electricity can kill you. For most people the choice depends on the utility services available. Changing from gas to electric or electric to gas means the cost of piping or wiring negates any savings. I do hate old electric furnaces it is so inefficient and cost 3 times what a heat pump costs to operate I have seen 100 amp breakers just for the heater and monthly $500.00 heating bills.
I lost a bid on a hotel property that had separte spilts in every room cause they saved 100 bucks a system from some joker that used electric furnaces with regular 14 seer condensing units. I was pissed- and they called back to show me the terrible installation the other company did. – Whatever.
I think to next generation is an outdoor mini-split type unit- with a variable speed fan and compressor so they don’t need aux. strip heater – with a regular air handler that fit into the existing ductwork. These mini splits use a biflow electonic TXV in the outdoor unit and no TXV in the air handler – which is why both pipes have to be insulated. I have installed a few mini splits with wall cassettes or floor model or ceiling tile drop in’s – not too impressive. They also have terrible remote controls or optional wall thermostats OEM only. They need to work with regular heat pump t-stats that would help but mostly using the existing ductwork be a big improvement. It’s coming. The existing mini split “air handlers” are a joke- supply plenum is something like 4″ X 18″ and really provide really low static pressure.
Gas heat is very good heat. Natural gas is cheaper than LP gas. Your comments on needing the correct duct size Is correct and your home should have adequate insulation for a heat pump. With that being said todays inverter technology heat pumps have over a 20 SEER for air conditioning and the heating factor is going off the charts compared to heat pumps of yester year. I’ve been working on heat pumps since the 70s and most of the negative reviews are accurate for the antiquated standard heat pump. Todays inverter heat pumps are excellent. I installed a BRYANT EXTREME HEAT PUMP which is an inverter EVOLUTION SYSTEM ( Modulation compressor) in my own home 20 miles north of Philadelphia. We do get very cold and damp weather. I have locked out my electric backup heater so it’s only the heat pump heat. It can produce 120 degrees at the air vent down to 5 degrees out side. Do your home work and look up inverter heat pumps. The ductless units are also inverter technology systems.
Simply. Heat pumps will not work in all zone climates. Like Canada. Our winters get well below -30°C. You need a back up heat source. The idea doesn’t work here. The problem is the government is giving grants because eventually they want to get rid of using natural gas.
Steve: Heat pumps work in Minnesota. They also work in Canada. You should do a little more research.
Can a Heat Pump Work in Minnesota?
I live in BC, Canada. The house is 3800 square feet and 40 years old (meaning that the insulation may not be good enough). I would like to replace the very old furnace with a new heat pump for both heating and cooling, but I am worried about Vancouver In winter, if the heat pump power enough without any electric auxiliary heating , can the indoor temperature reach 21 degrees Celsius?
This blog-response chain has evolved into an argument about whether heat pumps cost less than gas furnaces to operate. That was never addressed in Allison’s article. He was talking about comfort related to frequent fan cycling with a higher supply air temperature.
Everything I read about heat pumps and gas furnaces makes the statement that heat pumps are more efficient, but that can (and is probably meant to be) deceptive, because it says nothing about which system will cost less to operate.
1. Are heat pumps more efficient than gas furnaces? YES, if you understand what that means. Because they are able to deliver about 2.7 times (=COP) as much energy as they use to do that (by pumping it from outdoors, even down to 5 F), they are said to be 270% efficient (vs a condensing furnace that converts about 92% of the energy in the gas to indoor heat. I think we can all agree that 270 is bigger than 92.
2. So do heat pumps cost less to operate than gas furnaces. I haven’t checked gas and electric rates everywhere, but the answer in New England and I suspect most places is NO they do not cost less that gas furnaces to supply the same heat energy. It all depends the relative cost of electricity and gas, and since we make so much electricity by burning gas, the ratio of the cost is similar all over the US.
3 So if Heat Pumps don’t cost less than gas furnaces why install them? Many replies above have given reasonable answers. Gas systems have definitely killed people in violent explosions whereas heat pumps just quietly leak there high global warming refrigerant into the atmosphere, warming the planet. Lot’s of folks living along US coastal waters are a bit concerned about the rise in sea levels caused by warming oceans. No one has mentioned the fuels used by folks who don’t have access to gas: oil or propane. Do heat pumps heat for less money than those fuels? YES they usually do, but not by a huge amount. So why all the talk about making our homes ALL-ELECTRIC? Two reasons: 1. Burning oil, gas, propane or coal releases carbon dioxide which changing the climate everywhere and causing all sorts of expensive problems. NIMBY you say? Look again. If its not in your neighborhood yet you can be sure its coming. 2. WE CAN MAKE ELECTRICITY WITHOUT EMITTING CARBON DIOXIDE and we can do it now. And we are constantly expanding our clean electric output. We all have make our small contribution to the trend away from fossil fuels to maintain a decent climate on our planet.
So no one should be promising reduced heating costs by switching from gas to heat pumps, or implying it by saying the heat pumps are more efficient. People want to know the cost difference, not the efficiency. The best we should say is that if you need a new heating system heat pumps can provide comfortable heat in all climates and also provide cooling. But don’t expect the heating cost savings (if any) to pay for the installation. They won’t over the average 15 year life of the heat pump
You can have your cake and eat it too. I installed ground source heat pumps to heat my house and shop in Iowa, Net result = no propane bill but electric cost doubled. Then installed a solar electric system on shop roof, net result = electric bill drops 90%. Payback on the solar is under 9 years.
Allison, for a clothed person sitting on a chair in the middle of a room, where air temperature is at a “comfortable” set point, isn’t the radiant heat transfer the major component driving comfort or lack of it?
Comfort can be very subjective, but it seems to me that for the above person a change in the window’s U-factor and/or SHGC, for example, will have far more dramatic effect than modest changes in cfm’s or air temperature coming out of the supply register. I may be wrong.
All of the factors of window u factor, air flow and temperature are all relevant to comfort. There are other factors such as humidity, duct design (such as having the air passing across your heat gain and heat loss) and if you room or house is a cement slab which needs to be addressed or wood construction with insulation underneath. Slab homes tend to feel colder due to heat loss through the floor. More modern heat pumps have air flow settings for comfort or efficiency. Comfort settings slow down the air as to deliver warmer air and efficiency move air faster so the air feels colder. The duct design shouldn’t blow air directly on you. Some contractors install air conditioning diffusers for heat pump applications and that’s when you get stratification of the air from ceiling to the floor making the floor cold and higher up warmer then thermostat gets satisfied yet about from 3.5 feet down your freezing. Not all contractors or designers understand the actual application of proper duct design.
You can have a heat pump and have gas as your backup heat instead of electric backup. Is is referred to as dual fuel heat pump or as hybrid. You can lock out the heat pump or lock out the gas backup at what ever temperature you desire. Your hvac contractor should know all of this. You can run heat pump only or switch to emergency backup heat if your fossil fuel system isn’t as advanced as it should be. Today’s modern thermostats can take the place of a fossil fuel kit and be set up with outdoor temperature sensors to create the desired comfort you desire.
You can have a heat pump and have gas as your backup heat instead of electric backup. Is is referred to as dual fuel heat pump or as hybrid. You can lock out the heat pump or lock out the gas backup at what ever temperature you desire. Your hvac contractor should know all of this. You can run heat pump only or switch to emergency backup heat if your fossil fuel system isn’t as advanced as it should be. Today’s modern thermostats can take the place of a fossil fuel kit and be set up with outdoor temperature sensors to create the desired comfort.
Dual fuel systems are a terrible idea. I have worked on and even installed a few. If you have gas just use a 95% gas furnace. The extra complications and cost of dual fuel is overkill. They don’t save money since gas companies charge $30 to $35 a month just for service all year round. I have one customer that shuts off the gas 6 months a year but then the gas company charges higher rates. If you have the gas shut off the heating system won’t work right. It will try to run the 2nd stage gas and shuts off the heat pump because of the switch over relay- so you get another service call. Even if you have a gas water heater don’t go dual fuel on HVAC -it is a bad and needlessly expensive idea. I work in Atlanta GA but even in a really cold climate if customer has gas just get gas furnace they aren’t dangerous and are efficient. They are more comfortable then heat pumps. I like heat pumps but they aren’t necessary if you have gas.
so correct
I’m going on year 36 in the HVAC business. In the 1980’s we installed many HP’s that we later replaced in the 90’s with gas furnaces because people hated that generation of HP’s. Partly because we poorly applied and serviced them but mainly because building envelopes were terrible. I have oil/HP and propane/HP systems in “hybrid” arrangements at both my properties and they work great, both homes could be exclusively HP but I like choices and redundancy. Besides HVAC is easy for me. Our 55 year old HVAC company is agnostic on equipment types. We survey the existing conditions, customer goals, budget, etc., and offer multiple solutions that solve the needs. We are now competing with “new” HVAC companies that sell only HP’s, every solution is a HP which we find interesting. “HVAC experts” selling cell phones 1 week, HP’s the next…what could go wrong.
What’s also interesting is how low/moderate income homeowners/renters/landlords, prompted by huge tax incentives and rebates in the IRA legislation to replace fossil fuel, will deal with high maintenance costs that HP’s require, not to mention unexpected repairs. Doubtful systems will be maintained, resulting in poor efficiency, reliability and short life. In the Boston metro area, I am seeing +$3,000 service tickets for multi-zone leak check/repair/vacuum/recharge routinely. Who knows what R410 or A2L refrigerants will cost in a few years. Say what you will about gas furnaces, at least they are easy to repair and they either work or they don’t unlike HP’s (with low charge).
I suspect I will see another round of HP hating before I retire.
I work in Atlanta but I spent a month or two working on HVAC’s at a couple hotels in Boston. It is very cold in Boston and It wasn’t even the coldest part of winter. That cold wind I still remember-yikes!. I think you are right gas furnaces are the way to go. As for heat pumps- they are great for southern climates.
Do you think blowing 95F air from heat pump which will becomes colder as it distributes , moves and mixes with room air and blowing to skin with 98F is comfortable in winter? Of course not! Check the comfort parameters; temperature, humidity and air velocity.
I think when we use heatpump the air should not blown to people.
The static ( like as radiators) or dynamic ( like as fancoils) heating is important.
On the other hand in places which natural gas is more cheaper and has a vast network such as Russia, CIS countries or in my country Iran we prefer use room gas heaters, combi boilers (which produce both domestic hot water and heating hot water) with room heating radiators room fancoils or the new equipment ; residential AHU (combined a hydronic heating coil with an evaporative cooler) and use ducts for distributing air.
Regards
makes sense … common sense
I simply gave an answer to comfort and options for gas heat and heat pump for residential applications. A basic heat pump with gas back up can leave you without heat due to the change over relay and that’s why I recommend an EVOLUTION SYSTEM by BRYANT. You have the option of selecting gas heat only or heat pump heat only. Problem solved. Your beef about where the fossil fuel comes from is a political problem so take that up with the government. You still generally need an air conditioner so getting a heat pump is not an unreasonable option. If installing a simple gas furnace with an air conditioner is your bag than stick to it. I get a fair amount of work for customers who choose to have the dual fuel systems. Thank you for not offering it to them it generates new sales leads. It’s not an argument it a preference. I like an educated customer and I explain the differences and offer both. If done right ether system be satisfying to the contractor and the customer. Every application is different and I won’t offer the duel fuel system if conditions in the home are inadequate. If you don’t like dual fuel then don’t get involved with it. Some of the arguments are antiquated because I’ve explained the differences. There is still comments on 90 deg heat coming out of the registers. If you read what I stated with an up to date system or even with a modern thermostat with an outdoor sensor can eliminate that problem. I feel that a lot of argument about this is just wanting to argue about personal opinion because I don’t have these issues with the heat pumps I install. I’m aware of complaints that are presented but these issues from past history of heat pumps and have been addressed.
You can debate the merits of gas furnaces vs. heat pumps, but it is becoming a moot point due to government regulation. The State of Washington will require heat pumps in all new homes starting next July. Natural gas bans are happening in many places and will likely continue to happen, especially since the Red Wave turned into a Ripple this week. Most if not all of the equipment manufacturers are shifting their development efforts to heat pumps.
As a past owner of both types of systems in various climates, I do prefer heat pumps when it comes to comfort.
Gas has been a money-saving fuel compared to oil for the past 20 years or so. And condensing furnaces and boilers were a great at wringing almost every Btu out of every CCF we paid for. But when we burn it we emit carbon dioxide… 30 billion tons of it each year, worldwide. And it lasts, for 100’s of years, so it keeps on building up, making the planet hotter. When we began this century we were not aware of the damage that burning fossil fuels causes and the new gas appliances seemed ideal, but now we know. People all over the world are suffering floods and droughts and hurricanes and losing their lives and property. It’s sad that we invented these really efficient gas appliances, but now we know we all have stop burning gas or suffer the results… and those results are REALLY uncomfortable. We don’t yet have all the heat pump options we need to make the transition in every case, but we have to do it and we have to plan to make the changes as soon as we possibly can.
Warm air, whether it is scorched by an oil or gas flame or warmed by a heat pump is a compromise in comfort at best. Warm air heat cannot compare to low temperature hydronic heat supplied by in floor radiation or radiant panels. Warm air heat is popular because it has a lower installation cost and is usually combined with cooling. Hydronic heat can be powered by natural gas, propane, oil, wood, water to water heat pump, electric etc. When combined with outdoor reset controls and water temperature modulation hydronic heat is as comfortable as it gets. Forced air is for cooling. The temperature in my own home doesn’t vary from its setting throughout the heating season. 40 years experience designing and installing comfort.
Thomas, I have never used radiant heating before, so I have some questions for you. (1) How does it work for people who like night setback? I don’t use night setback, but I know many that do because they like to sleep under blankets and need lower temperatures for comfort. (2) What about air filtration? Do you just not filter the air in the winter, or do you still run the central air handler?
Roy C I know you addressed Thomas and he does have a good input on the topic. I just wanted to interject on room or whole house radiant. Yes you can do night time set back. As far as air quality there are several factors as far as running the air through your air conditioning ductwork for filtration. If your ductwork is in a non-condition space and even though it’s insulated you will have a heat loss in the ductwork and you’ll be blowing cold air into your bedroom. Untreated air blowing through conditioned space will also feel cold. If your radiant is zoned you lose control over all zones because it will try to average the temperature. For air quality you will most likely need to run a small portable air cleaner or purifier with UV light in your bedroom. Radiant heat is excellent heat. But not for a healthy air change. So address what you want on what your needs are. But this off topic of the comfort between heat pump and gas furnace heat.
The air temperature coming out of a heat pump, though, is only about 95 °F.
If the delta t of a cooling system is at best about 20 degrees, and we’ve reversed the refrigerant flow into heat pump mode; how did you get to 95°?
If the return Air is 70 then the supply temp is less than skin temp and will feel drafty. Skin temp being what maybe 92? Often std heat pump output without auxiliary heat strips will be drafty because it is less than skin temp.
And standard non-modulating air handlers in cooling mode and in heating mode blow the fan the same speed because it’s the same volume of refrigerant. So we’re not slowing down the fan once we’re nearing temperature with standard heat pumps. This will be even further drafty as the volume of air is high and it’s low temp supply sure. Very few homeowners are investing in modulating equipment except for in the most affluent homes.
Home owners think their heat pump is broken because it blows cold into them.
P.s. looking forward to your book 😃
Chris: I and many other people have measure the air temperature coming from heat pumps. Sometimes it’s higher than 100 °F. Skin temperatures are 90 to 92 °F, but good design eliminates drafts. The vents should never blow directly at people. I think you must have missed that part of the article.
Your book will be in the mail to you by Friday!
I was just standing under my 2-head mini-split the other day (vanes pointing down for heating) and enjoying the warm breeze. It was quite pleasant. And in cooling mode the vanes change to a horizontal blow so the air mixes with room air before they reach us. Also pleasant. A bigger problem in our 1968 era home is that 2 heads (one each floor) are not enough to provide good heat DISTRIBUTION to all rooms. Bathrooms on outside walls get cold if the door is not left open. It’s not as bad with cooling because we are well-insulated and don’t need more than about a 5 F drop for comfort, and moisture from all over the house will migrate to the cooling coils because of the lower vapor pressure there. It dropped to 24 F outside 2 nights ago and our undersized system could not keep up with the 2nd floor heat loss, at least not at 70F indoors. So we turned off the heat pump and turned on the gas boiler yesterday, which surrounds each floor with a ribbon of warm baseboards. Heating distribution would not have this problem with a fully ducted system done well, but for mini-splits it definitely is an issue, unless your house is close to Passive House standards with an open floorplan. There are other problems with our current heat pumps, but heat distribution is the biggest comfort issue.
My other complaints are about the high GWP refrigerants (R-410a, and even the newer R-32) so that one leak will cause as much global warming as a year’s worth of burning gas (R-410a GWP = 2085 x CO2; full charge is 5 lbs). Also the replacement cost of R-410a is quite high. When they leak, most of the maintenance has to be done at the outdoor unit. Not so bad in summer, but miserable when its below freezing with 18″ of snow… and no backup heat in a mini-split.
Another issue is that they lack any user feedback or self-diagnostics, so you have to wait until all the refrigerant has leaked to realize there is a problem. Smart diagnostics with Wi-Fi would allow service people to diagnose from their shop. My boiler has gauges that tell me what is happening. I am a fan of air-to-water heat pumps for their better heat distribution, built-in backup heat, and self-contained outdoor unit that can be repaired off-site in bad weather. If we could just get one that makes 170 F hot water, we could use it in all the existing homes with hot water heat in our northeastern states.
One more thing. I am tired of hearing that heat pumps are more efficient than fossil-fueled boilers, implying a lower operating cost. If you have access to gas, a heat pump will not cost less, even with an heat pump HSPF of 13. It is usually less costly than oil or propane heat, but still with a very long payback. A high-temp air-to-water heat pump that could use existing hot water heat distribution might lower the cost enough to make the economics better, but operating cost alone at present will not motivate the large scale electrification that we need to do to control global warming. You have to have a failing heating system and strong desire for AC that is nicer than window units.
Poor Allison, he tried, he even plead on other social media for people to focus on the topic of this edition of News From the Front, in vein. For some reason, it turns into a dollars and cents issue for most people. But it’s backwards, weird.
If this was a car blog, and the topic was “Are SUV’s more comfortable than sedans?”, would we have so much digressing into sticker price territory?
This has been a very puzzling part of human behavior for me, well, the American human at least. We buy houses at prices that swing several dozen of thousand$ upwards just because, and yet we treat it with a stoic, “well, it’s the market here”. But when it comes to making sure that the castle we are buying is comfortable, “we love it, it’s a great location, and we’ll pay a few hundred dollars to an HVAC guy to tune it up.”
There is no MPG sticker on a house for sale, and most people do not expect it or demand it. There is no universal comfort level certification we can fall back on. Yes, isolated markets have a place for the HERS rating in the MLS listings, which can be a partial indicator of comfort, but what does the public know about a HERS rating?
All that for the 6- or 7-digit “investment” in which we spend half of our life. When we buy that $50k A-to-B vehicle, we scrutinize every little detail of comfort, acceleration, and seat lumbar support for the possibility of spending more than an hour or two per day in that vehicle. Well, we need to make sure this $50k is well spent! It’s just weird.
The problem with residential housing in the US is that the vast majority for the last 30-40 years was built “to code” by production builders. It’s akin to buying a car. Like a car you can pick your options but you can’t dictate the components the builder uses nor make changes once the options have been ordered.
Custom homes are expensive. Smaller “Green” homes are even more expensive o a $/sqft basis.
Renovations are expensive. People are loath to spend money on a home because they won’t realize a monetary benefit which is commensurate with the cost of the renovation. In my home we’ve needed new doors and windows for the last 8 yrs or so. They’re out of square, leaky, and some of the sills have developed rot. My spouse would rather fight with the HOA over the sill rot and have me wear a sweater and run a humidifier in the winter because when we sell, a buyer will spend $25k on inserts rather than $40k on full frame replacements that I would like to spend. That’s $15k in equity which we would gain on the sale to put towards another production built home.
And that is why the question of comfort as well as our hard earned equity comes down to building codes, but the powers of influence will hold those codes hostage delaying our comfort. Small scale improvements: custom homes, retrofits. Large scale improvements: building codes.
The question of comfort comparison between heat pumps and furnaces was bound to steer away from comfort.
First, comfort can be a rather private thing which gets in the way of discussing differences of perception in public.
Second, if both the furnace and the heat pump are using an air handler to distribute the goods, our comparisons eventually end up in the details of delivery of warm/hot air, neither of which is particularly comfortable for an average human.
Second, what exactly are we comparing to? How many of us have access to two identical test homes built next to each other, one with a heat pump, one with a furnace? A corollary to that is the fact that our test cases vary so wildly in age, technology generation, and build quality.
Feeling thankful for a home with four walls and a roof, I am sitting at my desk this morning and I haven’t even adjusted the thermostat. I feel hot still digesting the turkey from last night.
This post makes me even more inclined to try some experimentation with my twinned gas furnaces (120BTU each). Even on the coldest and windiest day during our recent winter storm, this pair only ran for 14.5 hours (according to Nest). On more typical winter days, this is in the 7-9 hour range.
It does this exceptionally well: “With high-temperature air, the furnace blasts you with hot air for a short time and then goes off”.
Heat and fan are independently run to relays which trigger both furnaces simultaneously, and both furnaces the common ducts for supply and return.
My thought, as an experiment, is to flip the fan wire to the heat terminal at one of the furnaces.
When heat is called in this modified configuration, one furnace should run its heat cycle, while the other would just blow its fan, ensuring the same amount of air flow, but with reduced heat at the registers.
I’d be able to see how much heat is called for each day as well as a visual representation of each cycle runtime (via Nest).
I’m curious about the comfort observations, but also what this does to efficiency (would a 7 hour runtime double to 14 hours on days with similar weather?).
Will post again if/when I run this experiment, but I’m all ears if anyone has comments about the scenario I outlined.
Sounds to me like your twinned 120’s are oversized if they’re only running for 14 hours/day on even the coldest days, and 7-9 hours/day on a typical cold day. Was a Manual-J done when they were installed in the first place?
They’re almost 20 years old and I’ve only owned the building for 3, so I have no insight into whether a Manual-J was done, but it certainly doesn’t feel like they were adequately sized.
The only places I’ve seen big twinned furnaces are in old homes with big ducts. These old homes were often very leaky and drafty, but with heating, bigger ant necessarily better. You’ll end up with hot spots and cold spots in the home, and the thermostat will satisfy, but the building is never going to be comfortable. If they’re 25 years old, they’ll probably need replacement soon enough. My suggestion would be to seal and insulate the building as best as possible and get the new equipment properly sized for the new load.
You’re right, it’s an old church that was built in 1892, so certainly not a modern enclosure or modern duct design.
Separating them into two independent systems will allow you zone it much better (assuming you can’t get by with just one unit). I never understood the logic behind “twinning”. You lose a thermostat and the ability to better control temperature in different parts of the building.
Steve, it sounds like you will effectively be running the twin furnaces in a “heat pump” mode, i.e. lower supply T, longer cycles.
Thanks for corroborating my theory.
Hello Steve,
Are the two furnaces ducted in parallel – share common supply ducting? If so, the ductwork should be sized to handle the blower load of both – there may not be a penalty in using a two stage thermostat to stage the two furnace burners even if you operate both blowers on stage 1.
If you operate the two furnaces independently, you will need to outfit them with backdraft dampers if they share common supply ductwork. You don’t have to run both burners all the time they are on.
Huge limitations to forced air systems…to my mind.
Dale