If You Think Thermostat Setbacks Don’t Save Energy, You’re Wrong!
Some people argue against thermostat setbacks, saying they don’t save energy anymore. The reasons behind that thinking, however, are flawed. Setting back your thermostat when you’re away or at night definitely can save energy (depending on a few factors, as noted below). Two simple and definitive reasons show that thermostats can save energy: Physics and studies of energy savings from thermostat setbacks. Let’s take a look at each of them.
Physics
As I wrote here recently, your HVAC system is like a faucet and your building envelope like a cup. The leakier your cup is, the more you have to open up the faucet. One thing I didn’t write in that article is that is that the leakiness of your cup depends on the temperature difference between inside and out. In the world of physics and building science, we call that temperature difference ΔT (pronounced delta T).
In another post, I wrote about heat flow and showed the basic equation for heat loss or gain through the building envelope in a home:
Let’s focus on winter because that’s when most homes have the biggest temperature differences. I ran some simple calculations using this equation for two scenarios. In both cases, the house has a total UA equal to 300. (UA is the product of U-value, which is the reciprocal of R-value, and the area.) In both cases, I’ll use 30° F as the outdoor temperature.
Scenario 1: No setback
In this scenario, we’ll leave the thermostat at the normal setting. Let’s take 70 °F as the indoor temperature (despite the strange geography of thermostat setoints showing that Texans are likely to use that setpoint whereas Vermonters live with a cool 63 °F setpoint). That gives us a ΔT of 40 °F. Thus,
Q = 12,000 Btu/hr
Using a time period of 8 hours, we get a total heat loss of:
Total heat loss = 96,000 Btu
That’s the amount of heat the home loses through the walls, floors, and ceilings if we leave the thermostat alone. Yes, this is a simplification that ignores other forms of heat loss and the heat gains inside the home, but it gets at the heart of why setbacks work.
Scenario 2: An 8 °F Setback
Now, let’s look at the same house (UA = 300) in the same outdoor conditions (30 °F outside) and set the thermostat back 8 °F for 8 hours. Since our heat loss equation is linear and 8 is 20% of 40, we would see a 20% reduction in the heat loss during that period if the temperature dropped those 8 °F immediately.
Instead, let’s make it a bit more realistic and say that it takes 2 hours for the temperature to drop those 8 °F. That means our ΔT doesn’t get to 32 °F until the third hour so in this case we’d have 2 hours with ΔT between 40 °F and 32 °F and then 6 hours with ΔT = 32 °F. I made it simple and assumed a steady temperature decrease rate of 4 °F per hour, so here are the numbers:
| t (hrs) | ΔT (° F) | Q (Btu/hr) | Qt (Btu) | |
| 1 | 38 | 11,400 | 11,400 | |
| 1 | 34 | 10,200 | 10,200 | |
| 6 | 32 | 9,600 | 57,600 |
Adding up the total amount of heat loss in the far right column, we find:
Total heat loss = 79,200 Btu
This number is 18% less than the 96,000 Btu of heat loss when you leave the thermostat alone. This bit of basic physics yields a simple result:
Lowering the ΔT reduces heat loss.
Well, actually we didn’t need to run any numbers to show that because the equation shows it all by itself, but it’s instructive to see how some sample numbers work out, I think. So, setting back the thermostat and letting the house temperature get closer to the outdoor temperature reduces heat loss or gain through the building envelope. The reduction is proportional to the ΔT. Your heating system has to supply an amount of heat equal to what the home is losing.
Thermostat Setback Studies
Yes, real homes behave in more complex ways than my calculations allow for, but the above scenarios capture the basic physics involved. To verify that this actually happens in homes, all I really need to say is:
Michael Blasnik has done multiple studies and found that thermostat setbacks save energy and money.
In case you don’t recognize the name, Michael Blasnik is your go-to guy when you need residential energy data. I wrote a couple articles about a big study he did with Advanced Energy in 2009 (one on REM/Rate and another on radiant barriers). He’s been in this field as long as I might’ve been had I figured out what I wanted to do right after college (early ’80s) and he knows his stuff. I mean, he really, really knows his stuff.
The table below shows the results of 7 studies that he conducted for a gas utility company in the northeast US (first 7 rows) and one study done by another group (RLW Analytics, last row). As you can see, the savings are significant, and these numbers are the incremental savings. That is, they’re the savings that can be attributed to that measure only after accounting for the effects of the other measures taken to improve the homes in the studies. The percent savings range from 5.0% to 8.2%.
In addition to Michael Blasnik’s work, there are plenty of other studies available, and Canadian engineer Robert Bean, has compiled a list of 22 of them. Check out his comments about programmable thermostats preceding the list on the Healthy Heating website.
Conclusion
I like to consider new information and question what I think I know about the world. When warranted, I’ll change my views to incorporate the new information and replace old views. The introduction of the Nest Learning Thermostat last fall gave me a chance to look at the argument against using thermostat setbacks.
Yes, it’s true you have to consider the equipment because if recovering from a thermostat setback means that your heat pump kicks the electric resistance heat on, your bill may be higher. Right-sized heating and cooling equipment and equipment that can adjust its capacity to meet the loads also impact the effectiveness of setbacks. So does a lot of thermal mass in homes (though most furniture doesn’t have a lot of thermal mass). Is it possible that setbacks won’t save money for you? Absolutely. The title of this article refers to the general case, though, and for a great many homes, thermostat setbacks can save energy.
My first reaction to people who say that thermostat setbacks don’t work has been to say that they need to go back and study the basic physics of heat transfer. After going through all this, that’s still my view. Yes, if you have a right-sized heating and cooling system, you need to be careful with setbacks because the house may not heat up or cool down quickly enough when coming out of setback. But to recommend that no one will ever save energy with setbacks is foolish.
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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IMHO setback stats are great
IMHO setback stats are great for the typical oversized HVAC installs. On the other hand if HVAC equipment is sized correctly it hurts the effectiveness of setback stats since recovery takes so long. If I was putting a new system in I’d go with the properly sized system instead of the setback stat.
Personally I have a programmable stat that is set for 68 in winter 24/7. In summer it’s set for 75 all the time except 12-2 weekdays where it goes down to 73 to precool the house. We have variable peak pricing between 2-7 weekdays, and the thermostat automatically raises the setpoint based on the cost of power that day. OG+E (our electric company) controls the thermostat during peak pricing through radio control. So in our case a programmable stat makes a LOT of sense.
http://www.ogepet.com/programs/smarthours.aspx
Question. Ir there not
Question. Ir there not another piece of these scenarios missing. In the 2nd scenario there is the amount of additional runtime required to restore the original temperature in the home. The amout of run time to acomplish this would depend on the efficency and size of the unit. While this may or may not exceed the original cost of not setting the thermostat back it should be considered to provide a properanalisys of the costs involved.
I may be off base and I am not in this industry. Just my 2c as an outsider looking in and thinking something is missing.
Thanks
Allison,
Allison,
Great to see two things in your post.
1. This is the first time you have stated the value of modulating equipment, they are also much more comfortable.
2. As we both agree, “proper sizing” does not save any money and oversized heat pumps will save money by running less backup.
Since Load calculations on existing existing homes are total SWAG, lets focus on Modulating systems, and stop the myths surrounding load calculations.
Recall, Us HVAC guys, who drive an hour in traffic, to do our tired load calc stick, entering wild guesses into our legacy software, that we can make say anything we want, so we can provide a printout to an home owner, inspector, or utility guy who has no clue what our numbers mean, chasing a rebate based on bogus performance metrics created to lie, paid for by our children or neighbors, administered by huge costly bureaucracies all in the name of saving energy, only none of this saves any energy at all.
Why would anyone think we should be treated like Adults
Glad to see your movement to modulation and away from the old load calc parlor act.
Best,
Pj
Manual Thermal Control Is
Manual Thermal Control Is Always Best
Great post! The finer points of thermostats are actually much more complicated than any published article covers. You must study each premium thermostat individually. If it does not have settings for swing, deadband, fan delays, early recovery, etc., it’s not really of good use. Dual fuel setups are even more complicated because you must decide whether to use the thermostat or fossil fuel kit for balance point control. You must also decide on an economic or thermal balance point.
If you actually live in the house, you may adjust a dual fuel setup to your preferred balance point. If you are an installer, you will just guess at a balance point, the same as guessing at the sizing of the HVAC units. A homeowner without degrees in physics, chemistry, and business is at a total loss to understand their home climate system today!
Manual Thermostat control always results in the lowest costs and optimum comfort. No thermostat out there can compete with a trained human! The Nest thermostat has serious flaws, as does its competitor, the Honeywell Prestige thermostat. The ideal thermostat for all setups has yet to exist.
You guys are forgetting the
You guys are forgetting the effect of utility rates and participation. If you live in an an area that offers TOU rates and/or VPP then a programmable stat makes a LOT of sense. More areas are getting smartgrid technology, this is what will drive which thermostat works best for your region. For us the energate works best because it’s the stat our local utility uses to communicate with. No upfront cost (they even pay for installation) is icing on the cake.
http://energateinc.com/products-sol-products-smart-thermostats.php
Bob:
Bob: Setbacks are great, if not essential, when you have time-of-use (TOU) rates!
Rob Z: Catching up means replacing the BTUs that the house lost. In scenario 2, that would be 79,200. If the heating system had kept the house at 70° F, you would’ve had to put 96,000 BTUs into the house.
pj: I’ve never been opposed to modulating equipment, and I’m certainly not opposing load calculations.
Tom DC: I purposely didn’t talk about the workings of thermostats and whether or not programmable thermostats save energy because, as you point out, there’s a whole box of Pandora’s goodies within. Even though we have a programmable thermostat here in our home, we control it manually most of the time.
Bob: I’m
Bob: I’m not forgetting TOU rates. I think they’re great. Also, I just noticed that you said you pre-cool the house before the utility takes control of your thermostat during peak cooling hours in summer. If you’re not home during that period, I suggest not doing that because it just accelerates your heat gain.
Allison: Truely setting back
Allison: Truely setting back the temperature can save energy, But — it all depends.
On a single stage furnace it is pretty much a slam dunk — if the customer uses the set back option and if they were not already setting it back manually.
On a multi stage unit, air conditioner or particularly a heat pump with electric resistance back up, it is a different animal. The efficiencies are different in different modes. And the heat loss or gain in the duct system changes with the mode of operation.
Anyway — done well it works for gas furnaces.
John P.:
John P.: Absolutely! As with most things in the world of building science, you have to look into the details of each particular application to know how it works. I did mention the equipment issues, but I was mainly debunking the argument of our guest poster, who claims that setbacks just don’t work at all.
Thanks for commenting!
Well written and informative.
Well written and informative. I followed the thread as well (ie Building science fight club)
I think Ted put out some good points which most of us energy geeks agree with, fix the shell, educate the customer, take the whole house approach test in test out.
Then he lost us in an barrage what can be best described as personal opinion and unsupportable arguments in an attempt to win his case.
Micheal put forth a well tuned response that was informative and well thought out. It was obvious that this was not his first time thinking on the subject nor writing about it. At the end of the day it was not even an argument.
Setbacks work. A better than 6 percent average savings on a part that can be bought for as little as 40 bucks and installed and programed in well under an hour. Man whats not to like!
I think that the systems that Ted proposes are beautiful state of the art equipment and technology. I think that for those that spend most or even all of their time in their home such a system would make sense. Safe Comfortable and Efficient, wow make sense to me.
But affordable also plays into the equation. These system are costly and run constantly. Not everyone can afford the unit and or have the financial ability to condition their homes 24/7.
Furthermore conditioning space unoccupied for 30 percent or more of the time and maintaining a constant temperature during sleeping hours is not for everyone.
Almost everyone can afford a setback and the benefits are well documented.
Pre-cooling doesn’t make
Pre-cooling doesn’t make sense? How does removing heat from the home for 2 hours when power costs 4.5cents not save money vs paying 23cents to keep the house @ 75? Say the stat is 75, and TOU is 2-7. Dropping the temp from 75 to 73 from 12-2 means the house will need to warm back up to 75 before the system cycles a single time at the higher rate. Our local utility recommends a 2 degree drop for 2hrs prior to the TOU rate hike. They arrived at these numbers from last summers “smart study” where they installed energy monitoring equiptment in volunteers homes to see what works and what doesn’t. Our schedule is somewhat unpredictable so the thermostat is difficult to program for occupied/unoccupied periods.
Our utility is much more involved with conservation/TOU/VPP plans than most because they are trying to delay the construction of another power plant. So for them it’s more about the investors bottom line than just getting government subsidy money.
Allison,
Allison,
I agree that in *most* cases this is true – but in my case, both at home and at work, it is not. I have demonstrated this many times over the past several years with direct empirical evidence, typically measured by live energy monitoring and site temperature/HDD measurements.
In the case of an air-source heat-pump heated home or office in the mild climate of the Pacific NW (specifically Western Washington), it typically doesn’t make sense to put into place nightly setbacks for heat – especially in the case of an office. (FYI, electric resistance backup heat is disabled)
Example: The office operates at 71* during the day, and at 6pm I let the temperatures drop to 66*. Daytime exterior temperatures are typically 55, with night temps nearing freezing. The building’s interior temperature will slowly decrease and finally settle at its nightly setback by about 2am. The energy use problem happens during the morning warmup cycle at approximately 5am. This is when it’s coldest outside, the heat-pumps operate at their lowest COP and will run a full power for several hours. The outdoor units will go through as much as a dozen freeze-thaw cycles and it takes a good 4 hours for the building to come back up to temperature.
If I leave the variable speed heat-pumps at the same temperatures overnight, they only need to ‘bump’ the system 5-6 times a night at a low speed to maintain temps, so they don’t face nearly the same freeze/thaw cycle for the outdoor evaporators.
Facing a weeknight with the same humidity levels and HDD profile, I’ll save approximately 20% in kWh by leaving thermostat temperatures at their daytime setpoints.
That said, it does make sense for us to put in place setbacks for the weekend, as the energy loss from the building envelope over a period of 72 hours is more than the energy use of our Monday morning warm up cycle.
Lets not forget COST rules
Lets not forget COST rules over USAGE. If you are on a TOU or VPP rate structure, the 2 are not necessarily the same. Each utility has it’s own incentives/reasons to vary rates, and it’s important to be involved to lower the total COST of energy. IMHO all areas will eventually have smartgrids and smart thermostats making the whole program/manual thermostat discussion irrelevant. Giving up partial control of your thermostat to your local utility is what will reduce your cost the most after all is said and done. Smart utilities will give strong incentives to allow them to install thermostats they can partially control depending on their needs/costs. Acceptance will be slow due to some consumers concerns about “big brother” and skepticism about lower overall costs. Seriously, smartgrids are coming to all, it’s just a matter of WHEN.
The elephant in the room we
The elephant in the room we are not looking at:
The building shell and how good it is. How do we normalize the data based on that?
Whatever, nothing against set backs – if you dont want to tackle the shell.
If you dont want to go through the BS – then just default to equipment and machines – and dont worry – because the machines will take up the slack.
PS. I dont like Man J either. I just use 1000 sq ft per ton as a minimum, on a retrofitted shell. 🙂 Starting to try some 2000 sq ft a ton this summer. Fingers crossed.
I don’t think anybody could
I don’t think anybody could deny the different heat gain/loss with a change in temperatures. What may be in question could depend on different dollar efficiencies depending on source (e.g. that heat pump with resistance backup). And besides saving money keeping the customer comfortable is paramount, which will vary between an AC sized to maintain and one larger sized to handle extraordinary conditions — not to exclude duct leakage and contractor quality.
But I really appreciate seeing the numbers for heat loss with different temperatures. A homeowner very seldom gets to explore this quantitatively.
Glen G.: I
Glen G.: I forgot to mention it in the article (maybe I’ll add it), but Blasnik found that the programmable thermostats the gas utility installed had a payback of less than one heating season. Also, for anyone who’s interested, here’s the link to that
Home Energy Pros discussion on the Nest and setbacks.
Bob: Pre-cooling may not make sense because you’re increasing the ΔT, and thus accelerating the rate of heat gain. If no one’s home until the evening, all that pre-cooling may have ‘evaporated’ by the time you get home. Do a little experiment and don’t pre-cool and see if the temperature when you get home is any different. If it’s the same, your pre-cooling energy is all wasted.
And yes, I’m a big fan of TOU rates and would like to see more of them. Georgia Power has them, but they don’t really help people who have natural gas heat or water heating. Thanks for your constant reminders! I need to write about this topic.
Steve A.: Yes, yours is exactly the kind of scenario I was alluding to when I said you have make sure setbacks work with your equipment. I’ve revised my wording a little bit to make it clear that my title doesn’t refer to every specific case but only to homes in general. Still, even as you’ve indicated is true in your case, setbacks do still help sometimes. Many homeowners don’t understand COP or even how heat pumps work and what kind of supplemental heat they’re using.
Christopher C.: Actually, the building envelope is never the elephant in my room, as I discuss it constantly. Of course we want to minimize the amount of heat loss/gain, but we can still save energy with setbacks, too. The numbers from Blasnik’s studies are based on homes that had several energy improvements made to them – insulation and air sealing mainly. I think you’ve been reading this blog long enough to know that I’d never advocate for just letting the “machines will take up the slack.”
M. Johnson
M. Johnson: Absolutely! All of those factors play into the net result. And speaking of comfort, that’s relative, too, as you can see in this article on Green Building Advisor:
The Strange Geography of Thermostat Settings
Turns out that Texans like their thermostats set at 70° F in winter and Vermonters like it at 63° F.
Bob wrote:
Bob wrote:
> If I was putting a new system in I’d go with the properly sized system instead of the setback stat.
Why not do both? Setback would only be an issue during coldest and warmest days. Even on a design day, it doesn’t take much excess capacity to achieve fast recovery, and systems sized per careful MJ will have at least 10% margin in cooling mode, and even more in heating mode. If it turns out that recovery really is too slow on design days, then avoid setback as appropriate.
@PJ: How do you decide what size system to install, especially in new construction? I’m sure you’re aware that multi-stage and modulating heat pumps & air conditioners will not achieve their rated efficiency if grossly oversized.
Around 25 years ago
Around 25 years ago Brookhaven National Labrotory on Long Island, NY did an exhaustive study on how much setback thermostats could save. They delved into much more criteria then you have mentioned in your mathematical analysis, including heat source, and it’s efficiency under variant modes of operation, and the true time it takes a home to lose temperature over a period of time. Dropping eight degrees could thak the entire setback period. I personally have dropped my thermostat back ten degrees, and three hours later the house had only lost one degree. General calculations on cost of operation drop the calculated load by 25% due to internal heat sources. There is a flywheel effect that effects heatloss, and temperature drop. Brookhaven’s conclusion on it’s analysis, was that if the setback was less then twelve hours, and less then ten degrees, there was no savings at all.
You might be able to get more information from The Brookhaven National Labrotory in Upton, NY 11972.
I read the report when it first came out, sometime inthe 80’s.
Allison, here are more
Allison, here are more studies that demonstrate setback savings (thanks to our Canadian friend, Robert Bean):
The Effects of Thermostat Set-back and Set-up on Seasonal Energy Consumption, Surface Temperatures and Recovery Times at the CCHT Twin House Facility
Programmable Thermostats – Part 1
Stephen F.
Stephen F.: Thanks for the tip!
David B.: Oh, how could I have left Robert Bean out of my article?! (I’m sorry Robert. You have one of the most complete lists of thermostat articles I’ve seen anywhere, so I’ll post that in the article above, too.) Thanks for putting those links down here, David.
I figured Chris might come in
I figured Chris might come in on my side: “The elephant in the room we are not looking at:
The building shell and how good it is. How do we normalize the data based on that?
Whatever, nothing against set backs – if you dont want to tackle the shell. ”
Allison, not sure what house you have that will drop 8f in 2 hours, or how you are able to leave it 8f for the remaining 6 and magically have it back to comfort levels, but the size of the equipment, and thus the ductwork, must be enormous and, your house pretty crappy. A decent structure might see that backslide 2 days a year.
My clients homes don’t drop that fast, and when they let that occur they take a LONG time to recover at anywhere close to design.
So change your delta loss savings assumption significantly on BOTH sides of the 8 hours please.
The bigger “elephant in the room” is the incredibly broken assumption that heat replacement always comes at the save efficiency. That this equipment is like some plug and play toaster, that rated efficiency is what is magically achieved under all and any conditions.
You seem to only be looking at the loss side, not the replacement side. Methinks you are missing the forest for the trees.
I don’t think you truly understand how important it is to maximize heat replacement.
This is accomplished by banging hard on the condensing side of combustion equipment as well as refrigeration equipment. Low, slow, cold, maximum surface area for btu transfer. (Sorry if that looses anyone.) Think load matching. Think maintaining momentum, not accelerating and decelerating.
Long recovery used to increase efficiency (single stage non-communicating, leaky shell), now hard recovery costs efficiency.
The saving of efficient delivery way overtrumps delta savings by colder spaces. And you don’t deliver efficiently at full throttle. Look at the efficiency curve on an ECM motor, and apply that to how you look at this whole equation.
Sorry if you don’t like car analogies, but If you put the petal to the metal on your car it simply is not going to deliver power as efficiently as if you drive it gently.
Sure setback may show a tiny reduction in heat loss, but if you open the carburetor getting back up to speed you’ve just given up more than you gained. And now you have to buy a v8 instead of a v4.
You guys can argue theory all you want. Are any of you tracking results – besides Chris?
Thought not.
There are simply too many cases where this prescriptive recommendation fails, and I am seeing it. It fails often enough that we need to abandon the assumption and tell people they need proper diagnostics before a recommendation can be made.
You guys want to make blanket recommendations, go ahead – but I’m calling foul and accusing you of malpractice.
Ted K.: I
Ted K.: I’m not really sure how to respond to you because it appears that you haven’t really understood what I wrote above nor have you even acknowledged all of it. I will issue this one response, though, to some of your points.
not sure what house you have that will drop 8f in 2 hours
How fast the ΔT drops is irrelevant in my example. Drop it more slowly and it makes the final number closer to no setback but the conslusion is the same: The house loses less heat with a smaller ΔT. This is a red herring.
The bigger “elephant in the room” is the incredibly broken assumption that heat replacement always comes at the save efficiency.
Ah, now you’ve got me. Oh, wait, you were the one who argued that increased efficiency is a good reason to use setbacks. Besides, nowhere did I assume that and in fact even discussed that issue here in the comments with Steve Allwine.
I don’t think you truly understand how important it is to maximize heat replacement.
First of all, that statement doesn’t make sense with the comments that followed it. Second, few homes have modulating equipment, so why do you assume that that’s what they have?
You guys can argue theory all you want. Are any of you tracking results – besides Chris? Thought not.
Did you read the second part of the article? The part where I gave Michael Blasnik’s results of studies of hundreds of homes? You know, where they went in and tracked the results?
You guys want to make blanket recommendations…
It’s easy to knock down a straw man.
I’ll be the first to say that setbacks don’t save energy in every home all the time. You have to consider everything. But to issue the blanket statement that you’ve made that setbacks don’t save energy is to ignore physics and studies that show otherwise.
Now please, it’s time to stop making people think you know what you’re talking about regarding setbacks because you don’t.
Summary of Research Findings
From the Programmable Thermostat Market:
“Analyses from recent field studies have suggested that programmable
thermostats may be achieving considerably lower savings than their estimated potential.
In particular, a study from the Energy Center of Wisconsin showed no statistical
difference in heating intensity among their sample of single-family houses when
comparing households with programmable thermostats and those without. Allison, you go ahead and tell people how much they will save with setback.”
OK Allison, you go ahead and tell people that setback will save them energy.
I’ll tell them they may save, they may not, and they may actually pay more.
I don’t think 1-5% savings are anything more than anecdotal. Add, the homeowner was more energy aware after the install, so can you really attribute that tiny savings to Mexican Jumping Bean thermostat?
When people set back they want recovery, and recovery expectations are what HVAC guys size to.
Are we insulators, hvac installers, and roofers or are we building scientists? I think simplistic assumptions and rote recommendations are a huge disservice to homeowners and our industry.
I know they are in the way of my efforts to help people save real energy when I install a propane hybrid and the people program a 7 degree setback. I have lot’s of other real world examples.
Again, take a step further back and look at the whole picture, not just at the “science of reducing losses by heating less”. Look at the efficiency of btu delivery. I think the opportunity for savings there is significantly greater, but it requires thinking differently.
@Ted, virtually your entire
@Ted, virtually your entire argument is based on the premise that the equipment everyone should aspire to doesn’t work very well with setback. I’ll grant you that setback can cause problems by causing unnecessary up-staging. But as Allison just pointed out, modulating and multi-stage represents but a small fraction of the market.
Given an unlimited budget, I might consider such equipment for my own home. But the reality is, money is always a constraint.
I love working with clients who set out to build ‘green’ homes, which nearly always start from a baseline that includes modulating or multi-stage equipment. I can ALWAYS save them more energy by shifting money from the equipment to the duct system, envelope, and third-party QA, as appropriate.
Single-stage 15+ SEER systems done right with appropriate use of setback usually cost less to operate than 16-17 SEER equipment, and always have lower lifecycle costs, especially when comparing with the higher efficiency systems. Moreover, I get no comfort complaints with single-stage done properly.
You want to push high-end equipment? Fine. Just don’t waste our time with your dribble about how setback is an outdated strategy. Not everyone lives on your planet.
David,
David,
You don’t like the idea that I’m saying setback doesn’t save energy. You feel that for some, it may. Does that mean you are going to say everyone should implement the strategy?
Are you going to recommend setback to all of your clients? I don’t think you will.
In the 1970’s the prescription applied to enough of the population that it may have had a tiny bit of merit as a blanket recommendation. It no longer does. In fact the presumption that it “always saves” may cause more harm than it helps.
Let me be blunt. Blanket recommendation are stupid. The only way to hang a lantern on the stupidity is to say the opposite. Is the claim that “setback never saves” stupid? No less than the claim that it always saves.
So, if anybody says setback saves I’ll say it costs, and bring real world clients as proof.
(Do I have clients that benefit from setback? You don’t think I’m going to share that I actually recommend setback in certain circumstances, do you? Not until people agree setback should be abandoned as an rule of thumb recommendation.)
Ted wrote:
Ted wrote:
> Are you going to recommend setback to all of your clients?
For cooling, generally yes. But for heating, it depends on system and client. The majority of my projects involve heat pumps, in which case I typically oversize (based on cooling capacity) by 15% to 25%, so fast recovery from setback isn’t an issue except on extreme days exceeding design. My clients have no problem understanding that.
It makes no sense to keep the AC set at 74 all day (or whatever) while house is empty. Many of my clients tell me they turn down their AC to 72 or 73 at bedtime, then kick it up to 78 or so at daybreak, and down to 75 or so when they return in the evening.
For heat pumps with electric supplemental heat, I always recommend an conveniently located override switch. I explain to clients as long as they don’t need the strips for setback recovery, feel free to use setback. Just don’t expect the heat pump to recover quickly when it’s below freezing outside. If the balance point is in the low to mid 20’s for example, the system will be able to recover 4 or 5 degrees fairly quickly when it’s above freezing outside. Also, you’ve deliberately ignored a point I raised elsewhere — that a lot of folks simply aren’t comfortable sleeping without turning down the heat several degrees.
One final point: When you first raised this issue in various venues last year, *you* were the one making blanket statements. Since then, your position has evolved somewhat. Good for you. But to accuse me (of all people) of making blanket recommendations is laughable.
Sounds like our disagreement is now only a matter of degree: I say setback can save a significant amount of energy for most folks, especially in cooling mode. Your argument seems to be that only the worst homes with oversized equipment will realize appreciable savings from setback. If that’s a fair representation of your position, then we’ll just have to agree to disagree. I’m done with this.
David,
David,
Load Calculations on old houses are total Bullsht, have zero value other than performance art, and “proper” sizing has no correlation to energy savings.
The truth is on old houses, you have no idea what the load is at any outside condition, sometimes there is no load,most of the time it is less than you figured, sometimes it is more than you figured, and for about .01% of the time it might be what you figured. How silly can we get.
SEER is a sales term, a lie, not a valid performance metric. Use EER and you will find AC units are commodities and there is not much difference.So when you state,
“Single-stage 15+ SEER systems done right with appropriate use of setback usually cost less to operate than 16-17 SEER equipment, and always have lower lifecycle costs, especially when comparing with the higher efficiency systems.”
this would of course be true. Look at the EER’s.
You continue,
” Moreover, I get no comfort complaints with single-stage done properly”.
You can’t know comfort until you are comfortable, so this statement only confirms your point of having not given your customers the best comfort option, modulating equipment, but how would they know? Oh well,
Pj
.
I think my biggest problem
I think my biggest problem with setback methodology is twofold. Admittedly, my problems are with only a specific subsect of heating systems out there:
1. Outdoor reset vs Setback. We deal with hydronic systems, and often the savings from setback are used as an argument against outdoor reset water temperatures on modern boilers and the like. The problem is that reset can usually save at least as much, if not more than setback, especially in a modern, well constructed home. And when running it generally provides superior comfort. in the Air world I assume there is an analog to outdoor reset with modulating equipment but being a hydronic guy, I don’t know for sure.
2. Comfort. You may get a thermostat back to room temperature at 6am when the owner wakes up… but you are not going to get his walls, floor and ceiling to the temperature they were when you went into the setback period for at least several more hours. In fact, I’ve lived in slab homes in which the nighttime setback schedule basically meant that the building surfaces stabilized at an uncomfortably cool level constantly. Thus the phenomena of the “cold 70” thermostat setting and all of the fairly simplistic (and often misleading) arguments about radiant being more comfortable at 65 than air is at 68.
You can save the most energy by turning the thermostat down to the bare minimum for human habitation as well… but that kind of defeats the purpose of a heating system. So comfort must be addressed in this conversation.
If you have to come out of setback to a thermostat setting that is a few degrees higher to counterbalance that ( and I know people don’t, because they don’t understand why they are uncomfortable in the morning… but if you want to get apples to apples that’s what you’d have to do) and then bleed off that extra heat over time what’s that do to your savings? it may not eliminate them, but it certainly reduces them.
I guess I’d feel a lot better if we’d all stop making blanket statements like “setback saves money” and specify the circumstances that matter. Setback saves money when using fixed temperature, fast response and/or oversized equipment. It saves a lot more in leaky homes than in tight homes. and it saves money by sacrificing some comfort… perhaps not enough to ignore the setback, but maybe you’d be better off with a slightly lower, full time setting than a bouncing setback schedule.
otherwise, I suggest a new blog for ultra green living… SHUTTING OFF YOUR HEAT SAVES MONEY AND THE PLANET. Sweaters are the new vogue!
Allison, Can you or anyone
Allison, Can you or anyone else reading this suggest a relatively easy to use programmable thermostat? I work with Affordable Housing builders who require the most user friendly products available.
Thank you,
Kim
thanks Allison for a great
thanks Allison for a great article.
We have ten programmable t
We have ten programmable t-stats (elec baseboard heat) in our modest 1260sf house (+ basement) only to minimize the temp swing – we don’t set back – we hold steady at 71F in winter. If we did set back, the humidity would rise as the temp falls – causing the HRV run to react and over-compensate – quickly chasing the temp down to the new setpoint. Then when it re-heats, the RH would drop far below the min setpoint. That’s nuts. Ok, so now maybe you can guess that this is a fairly efficient place… the heat loss is 10kW in Zone 7 (!) as a result of our recent DER project. We would not only risk comfort, but potentially durability due to condensation. All to save 5% (at most) of $1000 annually. If that’s the price of comfort and I get to maintain my sanity too – so be it. So in our case, I have to agree with Kidd… and I would not recommend set back to ‘everyone’.
Really good discussion BTW – topic is dear to my heart.
This has been one of the most
This has been one of the most interesting discussions yet on this blog! However, in reading the posts much of the time I can’t help but think of the tale of the blind men describing the elephant. One problem for me is that I have no idea where all the writers live, and climate zones make a huge difference in this matter. It would be great if each writer in some way gave an indication as to the climate they primarily deal with.
when is this coming out in
when is this coming out in paperback???
Paul M.:
Paul M.: If you thing this is long, check out my article with 58 comments over at GBA:
Who Knew the Stack Effect Could Be So Controversial?
Scenario 3: An 8°F
Scenario 3: An 8°F Setback with a real world recovery.
I’m OK with the drop rate in your example, but you failed to allow for the time it takes to recover. Say, for example, that the furnace would take 1 hour to recover from a 8°F setback. (Some more, some less, depending on sizing, climate, etc.) During that recovery hour, the average ΔT would be 36°F. Heat losses including that recovery time are shown below. (I hope my columns come through unscathed.)
t (hrs) ΔT (°F) Q (Btu/hr) Qt (Btu)
1 38 11,400 11,400
1 34 10,200 10,200
5 32 9,600 48,000
1 36 10,800 10,800
Total heat loss = 80,400 Btu
Still a “win” for set-backs, but not quite as much as Scenario 2.
Even this scenario is slightly flawed in that the drop rate and recovery rate would depend on the ΔT. The savings are found in the area of the temperature curve below the steady state temp line.
If someone’s home loses heat
If someone’s home loses heat at an amazing low rate, they can’t save 5% using setbacks. Why? Because they’ve spent so much money on tightening and insulating that they’ve prevented “normal” setback savings.
For extra credit (or as homework for tonight) rerun the calculations for a house that drops only 1°F per hour. Just so everyone comes up with the same answer, assume that this house can recover 3°F per hour because it’s furnace is matched to the home’s needs.
After reading skimming your
After reading skimming your article. You said you heat to 68 but dididn’t specify what you suggest setting it at when away from the house? You did use 8 degrees in your equation.so I am guessing you set it to 60 when away.