The Easy Way to Switch to a Heat Pump

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1. Great article. A small point to clarify the article. Add the word coil to your image and description of the Ac or Heat Pump “coil”. Describing the indoor coil portion of the system as a “heat pump” may be misleading to some readers not familiar with the terminology.

   1. Matt: The reversing valve is usually outdoors, and you have to change the out both units when you do this. But yeah, I’ll add something to clarify that. Good point.

2. Good article, Allison. This is a good option for those that want to “ease into” a heat pump. I many milder climates, if the heat pump is properly sized the back up heat should never come on. Hopefully, after a while they will realize that they never use the gas furnace and will get rid of it completely. Personally, they are missing one of the biggest benefits, though. My main goal for getting rid of all gas appliances is the health and safety factor. Piping an explosive gas (that gives off poisonous fumes after being used) into a house just makes zero sense to me and always has. I turned down many design jobs where they wanted the gas furnace or water heater inside the conditioned shell.

   1. Russ: Thanks for amplifying and extending my closing point about not using the furnace and also raising the issue of combustion safety.

      1. Heat pump with training wheels.

3. Running a downstream heat pump in heating simultaneously with the furnace operating is not a second law issue. The heat pump indoor coil will be operating at a much higher condensing temperature due to the higher temperature air entering it from the furnace. The system will then either trip on a high pressure or temperature overload switch or the compressor will eventually fail. I know this for a fact because I had a dual-fuel system on my first house in the 1980’s and the installer screwed up the controls. After he replaced the compressor, he put in the right controls.

   1. Roy: Thanks for the correction. I’ve revised the article.

4. There are two other types of dual-fuel controls available depending on your preference. One has an outdoor temperature switchover point that is based on the relative cost of gas furnace and heat pump heating. It switches to gas heat when it is more economical than the heat pump. Someone has to look at your local utility rates and equipment efficiencies to figure out this economic balance point. Another strategy is to simply use a two-stage thermostat to switch to the furnace when the heat pump can’t meet the load. This maximizes heat pump operation, but you do have to be sure that it disables the heat pump when it switches to gas heating.

   1. Roy: Thanks for the info about additional ways to set up a dual fuel heat pump. Using an economic balance point makes sense in some cases, but it can change more frequently than the outdoor temperature balance point.

5. Are some ksystems like this set up where the evaporator coil is at the inlet to the furnace and thus under negative pressure? That is how I recall it was at my previous house. Any idea which is more typical?

   At a few houses where the coil was under negative pressure I remember having to help the owners correct improperly installed trapping to correct condensate not draining.

   1. Easy as ready the installation instructions. Also, The 2 stage Tstat by B*******
      has a few models hav the adjustments for 2nd stage switching and differential between 1st and second. W/or w/o Wi/fi. When you HP falls below the demand..2nd stage sweetly cycles on and looks out HP on stage one. You get maximum heat pump output capacity w/o any balance point nonsense. If you want to play w EBP…they hav aps for that too.

6. I live in the central NJ, balance point is 38°F for Ama na HP 16 Seer, 9.7 HSPF 1.5 Ton.

7. I guess this is true. Good luck finding competent HVAC people willing to do this for something besides an arm and a leg. Great idea in theory though

   1. What would be your approach for vetting a contractor to perform this work?

8. I just replaced a 22 yr old electric heat/ac system. My new unit also has a heat pump. The installer told me if temps will be under 30F, I just change my thermostat to “EmHeat.”

9. I will admit I have not done a deep dive into past blog posts, but I am wondering, do you deal with people wanting to replace boilers with heat pumps (or as primary source with boiler backup) for use with old fashioned radiators? For some added context, I live in Minnesota.

   1. You can do a heat pump system called a ductless split that is the best way when u have a boiler we have been adding cooling only unit to houses with boilers because of no duct work is installed but u do end up with multiple microwave size indoor unit on the upper walls

10. How much work/money it’s it to plumb a heat pump over an old/existing furnace vs getting a furnace designed to work with the heat pump from the beginning. And when you set a balance point what kind of controls do you need? Particularly integrated vs. Mom integrated system. In essence, about what are you paying to salvage the old furnace, and how easy is it to replace it it fails. I’m just assuming that the existing system is often end of life, and that is a significant reason for installing a new heat pump.

    Also, if you haven’t addressed this already, can you talk about refrigerants and what is on the horizon and if any of the current ones look like they will be sunseted. It would be a pity to put in a heat pump only to have it need a conversion in ten years.

    Thank you!

    1. I don’t think any furnaces are designed to work specifically with heat pumps. You do need a thermostat that will control a dual-fuel system appropriately.

       As for what is on the horizon for refrigerants, that is a whole different subject that deserves its own column. I can’t wait to see it. A big transition is currently underway and it is going to be interesting.

       1. Roy, do you know of specific thermostats that are designed to operate dual fuel? Is this common functionality, or only niche products?

          1. Sorry, but I can’t make any specific recommendations on that. Most heat pump manufacturers should be able to provide you with the appropriate dual-fuel thermostat. If you use a third-party thermostat, be sure that it won’t run the furnace and heat pump simultaneously.

          2. Nest thermostat supports the duel heat sources up to 2 stages.

       2. Thank you RoyC. All kinds of “balancing points“ with the refrigerants I’m sure. I too am looking forward to seeing an everyman’s analysis done by someone who possesses the technical skill and approachability/understanding to do a good job translating the dry tomes done by industry and government that represent so much work. Thanks for your answer. Definitely a role for graphs.

    2. Pricing is always contingent with having a well regarded company come do a full assessment of the existing hvac system along with taking into consideration the size of the home, location, and several other factors. I am a sales rep with a residential hvac company in Maryland, and converting from gas over to either dual fuel, or full heat pump is at an all time high. Our company has installed countless conversions in 2023.
       Yes there is a refrigerant change on the horizon. Most manufacturers will be using R32 and R454B is another that will start being used at some point. R32 has been in use since 2016. R410A has a GWP(global warming potential) of over 2000 and that is why it will be going away. Last, R32 has a GWP of 675 and 454B GWP is 466. Both of these will be the refrigerants used in all new equipment starting in 2024.

       1. What about propane? I know several European HP are starting to use it.

11. The challenge is going to be what happens to the gas utilities when they lose a significant portion of their market, both from dual source and straight heat pump conversions. Existing assumptions on how to apportion capital costs as fixed or variable costs will need to change, which could show up as higher fixed costs or higher minimum monthly gas bills. It’s a similar kind of problem to what California is having with solar. This isn’t a problem for early adopters, but if there is mass adoption. But, utilities always want to plan for the worst.

    As a side note, one of my friends with a dual source heat pump uses a different way to switch to gas, which is the price of electricity. In the Chicago region, they have introduced variable pricing based on electrical usage and supply. When the price goes high, he changes to using gas. As more people move to dual source heat pumps, this could drive earlier in the winter season use of gas.

12. Last April we replaced an 8 SEER air conditioner with an 18 SEER heat pump, linked to my NG furnace. I’m in Wisconsin. My HVAC installer set it up to switch to the furnace when the outdoor sensors said the temp was 20 or lower. This year we needed at least nighttime heating into early June, but from the day it was installed my furnace hasn’t run.

    1. Teresa, would you be willing to share more specifics on your project?

       I’ve been pursuing this, but had trouble gaining confidence in a successful outcome (which it sounds like you had – yay!).

       Did you require a new thermostat to control the dual fuel setup? Was the new HP the same brand as your furnace? Any consideration needed to matching the heat pump with the existing blower in the furnace? How much did it cost?

       Any insights would be appreciated!

       1. Chris,
          Sure. We did get a new thermostat, but I don’t believe it has anything to do with controlling the dual-fuel status (maybe there’s something buried in the menus, but it looks fairly basic). It is an upgrade to a smart thermostat, which we didn’t have before and which is subsidized by the state’s conservation program. I think the setting to switch to the furnace at 20 degrees was somehow set by the installer. If I have any control over it, I’m not yet aware.

          The heat pump is not the same brand as the furnace. What both the salesman and the technicians were concerned about was electrical. My furnace is old enough (1999, God help me) that they were worried there might not be an available connection that could be used to link the air handler and the heat pump for the sensor-controlled switchover from heat pump to NG furnace. But what they needed was there — they had to pull a wire over my finished basement ceiling is all.

          The heat pump plus thermostat were just under $11000, installed. There’s a bit of a rebate from our local program and of course, tax credits in the IRA. We think it will slot in at about $8000, in the end. Despite my excitement to go electric, I felt obliged to inquire what the cost would have been for a traditional air-conditioner at say, 16 SEER. My HVAC company quoted the exact same price, but without the rebate and tax credits, which happily made the decision a no-brainer. I confess I was a bit shocked at the price tag, but that’s been happening to me a lot.

          I can go a little bit more in the weeds, but I am not an HVAC pro, and I trusted my contractor for most of this.

       2. Heat pumps typically have two extra controls at the thermostat, one for auxiliary heat when the heat pump is not keeping up with the demand, and emergency heat in case the more complicated compressor system breaks down. In an all-electric house, these turn on electric strip heaters in the air handler. Both of those can trigger the gas furnace on a dual-source heat pump. A more sophisticated thermostat that understands a dual source heat pump could make different decisions than a generic heat pump thermostat. But, one has to use a thermostat with that extra control, which also may require more wires from the thermostat to the air handler. If it all comes from one brand, a communicating thermostat may be able to use fewer wires, as all the boxes talk over a HVAC network instead of using on-off controls.

13. Great article as usual! I’m enjoying your book as well. In 1992, we bought a brand new home in Chesapeake, VA that had a heat pump and a natural gas furnace! I wondered at the time what incentives the builder had to put such a system in a new home! It worked great, very economical and comfortable home!

14. Allison, hello from Northern Nevada! Been a long time.

    One issue I may have missed in your article and the comments is air flow and duct sizing in existing homes with gas heat. I believe that on the heating cycle, heat pump systems operate with higher air flow and lower supply temperatures.

    How do those factors play into the situation when conversion is considered?

15. Allison, this is a great article! I have been trying to pursue this retrofit at my house recently.

    As an aside, I used one of your previous blog posts to troubleshoot a humidity issue in a utility battery system I was designing.

    Here are the hangups I have run into trying to replace my AC+furnace with a dual fuel heat pump…

    1. IRA rebates not yet available. These are to be administered by each state, and here is the headline from my state’s energy office REBATES FROM THE IRA ARE NOT CURRENTLY AVAILABLE (https://energyoffice.colorado.gov/federal-funding-incentives/inflation-reduction-act-tax-credits-incentives)

    2. To qualify for the rebates (in general, noting that they are not available yet), one contractor told me that the Heat Pump’s inside/outside units need to be “matched” (combined unit efficiency certified per AHRI). Not every combination is tested, so it’s unlikely to find a new HP outdoor unit that has been certified with my existing furnace.

    3. Locating an appropriate contractor for this type of retrofit. I have discussed this idea with 3 different contractors and not been able to gain confidence that I will end up with a reliable and efficient system. The equipment matching, controls/thermostat coordination, and integration of existing equipment (furnace blower) with new equipment provided by the contractor (heat pump) give me concern.

    I have a 36 year old AirCon (estimated 9 SEER by energy auditor based on its age) and a 9 year old high efficiency gas furnace (Carrier).

    My current thought is to buy a new simple AC unit to replace the old one, and then plan to replace both furnace+AC with a heat pump in about 10 years when they are both approaching end of life.

    Am I off base in this approach? Am I misunderstanding any of my 1-3 hangups above?

    Would appreciate any feedback or insights!

16. There are heat pumps that work to -40 C, so keeping your old expensive to run furnace is not necessary, as well as it’s expensive natural gas connection that charges you connection fees. My Mitsubishi Zumba pump has been doing 99.9% of the heating, only needing a tad of electric resistive heat backup when it’s -28C or below. If it was half a ton more it would never need resistive heat, but there was no space for it.
    I suppose if your house only has 100 amp service or you don’t want to spend enough to buy a heat pump capable of it all, you could do what this article talks about. But I have a feeling you’ll have wished you got rid of the old outdated furnace technology.

    1. Stephane,
       Just because I’ve can buy a heat pump which works down to -40C doesn’t mean you should, particularly if using the heat pump for air conditioning. I believe there is an older article from Alister describing the issues with using only a heat pump. The issue is that sizing a heat pump for the worst possible temperature gives an air conditioner which is way oversized. It will short cycle and will not do a great job of dehumidifying in the summer. This is especially true for older houses which are not well insulated. Even new houses which only reach 5 ACH are probably not great candidates for only a heat pump in moderate cold winter climates. A far northern or southern latitude which gets very cold in the winter and hot in the summer without being a passive house is probably a poor match for a heat pump with electric auxiliary heat.

17. Fabulous article, thanks. I live in Toronto, where it gets to zero degrees now and then, but generally around 15 in the winter.

18. I recently moved into a late 1960s house that has a central AC/furnace. I want to update to a heat pump because we have solar PV, and also because our system is definitely oversized (5ton for a ~2600 space), and very loud (system in a closet off of the living room, so when the blower runs you need to shout to hear each other). I’m in California, temperature might dip to at worst upper 40’s at night in the depths of winter so I don’t need aux heat. Should I just get rid of the furnace? Bonus if this would mean I could “reclaim” some of the closet space since the furnace is taking up most of it.

    Thank you

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