Dear Umbra,
Many of my friends are environmentally minded and do lots of things to try to have a smaller carbon footprint. Yet when I tell people I turn my heat down when I leave the house even for an hour or two, and that I turn it down to 50 at night, they say, "I thought it takes more energy to reheat the house than to keep it at a constant temperature." Please clarify. If it is better to turn the heat down, then there is a LOT of room for education on this topic, as even many people in the environmental community are confused.
Sarah W.
Brunswick, Maine
Dearest Sarah,
Either by evoking the confused masses or using all caps, you have convinced me to revisit this topic. I will try to explain, in basic terms, just why we should turn down the thermostat. But let me be crystal clear up here in the thesis paragraph: Turn down the thermostat at night and before you leave the house.
Get with the program.
Let's say a typical, nice heat setting is 68 Fahrenheit while at home, 58 at night or while away. (Your 50 is probably lower than most people will try, but bravo to you.) The heater will save fuel as it falls to 58, and expend about the same amount of fuel as it rises back to 68. Therefore, these two transitional phases cancel each other out. And while the heater is set at 58, for as long as it is set at 58, it is merrily saving fuel (aka energy, money, and the planet). Because as we all know, it takes more heat to keep a house at 68 than at 58. Overall, then, fuel is saved.
To bulk up this answer a little bit and remind us all that insulation and sealing the house are important aspects of keeping ourselves, but not the planet, warm, let's discuss air-movement dynamics briefly. Remember, air lives a life of heat equality. Hot air wants to rush out and share the heat with nearby cold air until all air is the same temperature. This happens, as fervid readers may recall, through the processes of convection, radiation, and conduction. The stack effect is an example of convection: Hot air in a building not only rises, but is of higher pressure. As it rises, it pushes against any cracks in the ceiling or roof, escapes, and leaves a low-pressure area at the bottom of the house. The cold air rushes in to the low-pressure area, and must in turn be heated.
Our heaters are fighting an incessant battle on our behalf, warming all the new air. If we are not there to be warmed, or are sleeping under a cozy duvet, we can turn down the thermostat. Programmable thermostats are very helpful and quite cheap.
I repeat: Reheating uses less energy than keeping it hot while you're gone. No organization -- reputable or disreputable -- disagrees with this advice. To quote the EERE, "This misconception has been dispelled by years of research and numerous studies."
People, turn your thermostats down.
Incessantly,
Umbra
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Comments View as Flat
rufwork Posted 12:30 am
03 Dec 2008
My two semesters of physics wish to be heard...
"Because as we all know, it takes more heat to keep a house at 68 than at 58."
It might be more precise to say, "When it's colder outside than in, that cold will leech away the heat in your house more slowly if your house is at 58 than 68 because there's less heat from your house to leech."
Of course being accurate doesn't mean it'll read nearly so well. ;^) Still, I think the amount of heat/energy needed to heat a gas of volume X (surrounded by a perfect vaccum, let's say) by one degree is going to be give or take the same no matter what temp it is, right?
Ah, vive la physics.
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sindark Posted 1:40 am
03 Dec 2008
Metric
Some metric temperature conversions in parentheses might be appreciated by readers in countries that use a more rational scale than Fahrenheit (which I cannot even spell without help).
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sindark Posted 1:45 am
03 Dec 2008
Fahrenheit and Celsius
68 Fahrenheit = 20 degrees Celsius
58 Fahrenheit = 14.4 degrees Celsius
50 Fahrenheit = 10 degrees Celsius
Personally, it has always seemed more sensible to have a temperature scale where zero is the freezing point of water, not the temperature at which a frigorific mixture of ice, water, and ammonium chloride stabilizes.
Wikipedia has this on the origins of the Fahrenheit scale:
"According to a letter Fahrenheit wrote to his friend Herman Boerhaave, his scale built on the work of Ole Rømer, whom he had met earlier. In Rømer's scale, the two fixed reference points are that brine also freezes at 0 degrees and water boils at 60 degrees. He observed that, on this scale, water freezes at 7.5 degrees. Fahrenheit multiplied each value by four in order to eliminate the fractions and increase the granularity of the scale (resulting in 30 and 240 degrees). He then re-calibrated his scale between the freezing point of water and normal human body temperature (which he observed to be 96 degrees); he adjusted the scale so that the melting point of ice would be 32 degrees, so that 64 intervals would separate the two, allowing him to mark degree lines on his instruments by simply bisecting the interval six times (since 64 is 2 to the sixth power)."
I also recall it being gently mocked in an episode of James Burke's "Connections."
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karimetzger Posted 1:54 am
03 Dec 2008
Radiant Heat?
I've wondered this myself quite often - we keep our radiant heated, well insulated house (that can almost heat on lightbulbs alone) at a temperature around 20 C. We were informed because it is in-the-floor, radiant heat (which I'm not sure should matter anyway), it is way too much energy for the boiler to heat it back up if we take it down at night. It does take more time to heat the house back up if we let it fall, than if we had a conventional forced-air system.
However, I still am in the habit, and have been since childhood, of turning it down at night to more like 16-18C.
So, same rules for radiant heat? Or not?
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sustainablemer Posted 2:53 am
03 Dec 2008
radiant heat
i also had a home with radiant heat. i don't know how much energy it takes to reheat a space with a radiant heat source, but i found i simply too slow in responding. 12-24 hours might pass before my thermostat showed the increased temp requested, say 60F to 68F. was my system defective, or is yours really responsive?
emmer
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WaterConsNYC Posted 4:06 am
03 Dec 2008
Thermostat Setback
Umbra, as usual you are the bee's knees.
A couple of small enhancements. For those with physics-lust, the underlying reason this works is that a house loses heat at a lower rate when the indoor-outdoor temperature difference is less. Keeping the temp at 70 deg F or higher overnight or at similar times means you're losing heat faster.
It's true that you give back a little bit of the savings because you have to keep the heating boiler and water hot, but you can minimize this issue by getting a controller that changes the temperature of your heating system water with outdoor temperature (warmer outdoors, cooler water; colder outdoor, warmer water).
And, as Umbra notes, caulk and weatherstrip any leaking windows. The low-budget way to detect leaks uses a damp hand held up to the edges of the window, Or you can spend $750 on a thermal camera and get results that are 20% more accurate. The conclusion is the same.
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swolpow Posted 8:23 am
03 Dec 2008
Still confused
Dear Umbra, Thanks for the clarification on my question. The only problem is perhaps my question wasn't clear enough! Folks seem to know that setting the temp down somewhat overnight is good. But it seems to me that you CAN set it down so much that you negate any savings. If I understand you correctly, the only savings occur when the temp has stabilized at a lower than usual value. There are no overall savings during the dropping period because these are wiped out by the reheating period.
For example, let's say my house is 68 and I leave for two hours. Let's say the house loses one degree every half hour. So, if I set it to 60, it will fall to 64 and then rise to 68 again when I come home and turn it up. It will never reach (and stabilize at) 60. This, if I'm understanding correctly, results in NO savings and NO loss: 4 degree drop, 4 degree rise. And, it would be more comfortable to just leave it at 68 rather than having to wait for the house to warm up again from 64. On the other hand if I set it to 67 when I leave it will take 1/2 hour to drop to 67, then it will save a bit (the difference between heating to 67 and heating to 68) for an hour and a half . So, in the first case I save nothing and in the second case I save a little bit and I'm warmer. Is that right?
If so, then it matters how low you set the temp. You need to make sure that it will stabilize at some lower temperature for some length of time, otherwise there is no net savings.
Same question when I turn the heat down to 50 at night. In the shoulder seasons it's often only 54 or 56 in the morning - in other words it's still in the dropping phase and has not stabilized. This suggests that I would have saved more setting it to 60, which it would have actually reached at some point during the night.
Someone please clarify!!! This has been bugging me for years.
Thanks,
Sarah
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WaterConsNYC Posted 10:14 am
03 Dec 2008
Temperature Setback
First, setting back the temperature if you go out for an hour or so will provide some savings but the best way to do this is to get a seven-day programmable thermostat and program it with the general rhythm of your life.
The bottom line to your confusion is that the amount of fuel required to bring the temperature back to 70 degrees after spending the night at 66 degrees is far less than the amount of fuel needed to keep it constantly at 70 degrees full time. As the temperature goes down the rate of heat loss decreases significantly and since the house stores quite a bit of heat, bringing the temperature back up in the morning requires less energy. It may seem counter intuitive but there are millions of homes saving fuel in this way that provide it works.
Now, there can be a partial penalty in that if the heating water temperature is set at 200 degrees at all times, then even though the thermostat is set at 66 degrees the boiler will start up for a few minutes every hour to heat up the boiler and boiler water. But the mass of the boiler and it water is a whole lot less than the house. But that's why its best to have a control that has the heating system operate at lower water temperatures most of the heating season. For more www.energystar.gov
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WaterConsNYC Posted 10:17 am
03 Dec 2008
One More Thing on Night Setback
If you want to really nerd out over this here is most recent, over-instrumented research:
http://www.homeenergy.org/article_full.php?id=566&art ...
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spaceshaper Posted 1:32 pm
03 Dec 2008
Observations..
Sarah, it's not necessary for the temperature to stabilize at the lower level to save energy. If the 'stat is set at 50 and the morning temperature is 54, it means you have used NO energy for overnight heating. This will ALWAYS more than offset the rebound usage - WaterconsNYC gave a good explanation why this is so. Set the overnight as low as you are comfortable with for maximum energy savings.
Karimetzger, radiant floor heating may just be the exception to the rule. High thermal mass, slow response heating systems like yours are designed to work most effectively when operated at a fairly steady rate. You might save a little without sacrificing performance by using a programmable thermostat set to wake up an hour or two before you do. You could also set it to cut back an hour or two before you retire as the system will stay warm for a while. Radiant systems are capable of providing comfort levels at lower ambient temperature settings so you are already ahead of the game.
Programmable thermostats are the bomb. One of the easiest and least expensive energy upgrades you can do - worth it even in a rental. Once set you never have to think about it and it'll never forget to dial back if you decide crank up the heat temporarily for some reason.
And don't forget to insulate/weatherize as well as you can afford!
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lilylily Posted 4:21 pm
03 Dec 2008
Radiant slab or high thermal mass exceptions
Umbra is right that setting the thermostat back will result in energy savings for most homes. The less the inside/outside temperature differential, the less the heating costs. But for high mass homes, especially those with radiant floor concrete slabs, it could cost some people more money to allow their temperatures to fluctuate. They might be heating them when they're not home by overshooting their needs.
Take the example of someone who awakes and goes to work for nine hours, leaving the house empty. They only need full warmth for a short time in the morning (and when they return in the evening) of workdays. With a low mass house, they'd use less power to get air temperature up to 68 degrees briefly in the AM then let it relax while they're gone. Someone with the thermal lag of a high mass home would take longer to heat it for a short morning use.
I've designed energy-efficient homes for 30 years, and work with a leading energy researcher/engineer who conducts workshops for architects and builders after doing scientific thermal studies on literally thousands of homes for an energy company. We've designed net-zero-energy homes that actually work well. He uses radiant slabs and super-insulation as his main features to save energy. With that solution, a setback thermostat is unnecessary and counterproductive. These are rare now, but with pending climate change regulations, will soon become popular if not nearly mandatory in some areas of the country.
For the rest of you, don't heat when you don't need heat. And stop leaks, add insulation, build smaller, rely more on radiant heating, wear a sweater, and exercise more to keep your metabolism higher so you're naturally warmer inside.
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DaveMart Posted 7:25 pm
03 Dec 2008
Winter Deaths caused by cold
Your advice is highly dangerous to older people.
I do not have the figures for the US but in the UK over 25,000 a year die in excess of the normal mortality in the winter.
This is due to both the blood thickening which can cause heart attacks and to bronchial problems.
Please check with an MD and publish prominently the dangers of reducing temperature for the old.
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jwebb Posted 11:01 pm
03 Dec 2008
Correcting
I believe that this answer lacks some scientific help. Usually Umbra gives specific numbers- such as in turning the heat down this study found that X kW were required to raise the heat back up as opposed to Y kW to maintain the temperature. Even the link she uses for EERE says: "But when a heat pump is in its heating mode, setting back its thermostat can cause the unit to operate inefficiently, thereby canceling out any savings achieved by lowering the temperature setting". If my heat pump has to run continuously for 2 hours to heat my house back up when i get home instead of running off and on for 2 hours while I am away, the amount of energy used is more for the continuous cycle. Show me some studies, facts about the heating system, not about insulation which wasn't the question.
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swolpow Posted 11:21 pm
03 Dec 2008
Would also like some hard data
Thanks everyone for trying to answer this question. I think I agree with jwebb and would also like some more data. I've checked out some of the sites folks have mentioned and they all address the issue of turning back the heat a moderate amount for long periods of time - which everyone seems to agree is better, and for which there seems to be corroborating data. The question still remains whether it's proven to be better to turn back the heat drastically and/or for short periods of time. The fact that the house loses heat faster when the inside-outside temp difference is greater makes sense to me. So, that would be a point in favor of setting the temp back any time. But I'd still love to see some more actual numbers. Maybe there aren't any! I don't know much about heaters - I some type of oil-fired forced hot water system. It cycles on and off when reheating the house (on for maybe 3 min, off for maybe 10 min). On very very cold windy days it can take 6-7 hours to reheat the house if it's at 50 in the morning. Although I usually don't find the cold very troublesome, it would be nice to know that it's resulting in significant savings.
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rufwork Posted 11:54 pm
03 Dec 2008
WaterConsNYC's link is money.
Thanks for the link to Home Energy Magazine Online, WaterConsNYC. That articles rocks (and is perfectly detailed).
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wagonfullapancakes Posted 4:34 am
04 Dec 2008
to still confused
Newton's law of cooling - the rate of heat loss is proportional to the difference in temperature (i.e. inside vs. outside) - means that the house will lose energy faster at 68 than at 50. In order to keep the house at a constant temperature, you need to add heat energy at the rate it's being lost.
The energy lost in cooling from 68 to 50 is equal to the amount of energy you need to add to get it back to 68. But the entire time it's less than 68, you're saving energy relative to how much it would take to keep it at 68.
Of course, from a practical standpoint, most people won't want to let the heat go down to 50 if it takes forever for their system to bring it back up to a comfortable temp. If your schedule is predictable, though, you can have a programmable thermostat start warming things up a while before you get home.
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myrtoashe Posted 4:51 am
05 Dec 2008
Excess winter deaths?
It appears true (see the British Medical Journal 2001 article by Annabel Ferriman). In England (and New Zealand) 10's of thousands of mostly elderly people die in the winter. This appears to be a problem that does not depend on income, but does depend on indoor temperature. It is also linked to outdoor temperature, but improves on rainy days, suggesting that the issue is exposure to cold (folks stay inside on rainy days).
As an American physician, this surprised me so I did some reading on it. Interestingly, it turns out there is little central heating in New Zealand, and the same is true of older British housing. As most of the deaths are related to cardiac problems, the reasons for the excess deaths apparently are related to increased blood pressure in cold environments (see webMD) and increased strain on the circulatory system (increased oxygen demand for example).
So, as much as I hate to say it (as I keep my own house around 63 F (or 17.2 C) day and night - the bedrooms are at 59 F (14 C)) anyone suffering from high blood pressure and elderly people with possible heart problems should be cautious with the thermostat advice above. They can still save on heating costs with insulation, and insulate their bodies wearing wool and/or synthetics, possibly a wool cap in the house, and using a sleeping bag at night.
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Pangolin Posted 4:06 pm
05 Dec 2008
Mold caution
As a rental maintenance manager in Northern California I ran across a repeated problem. People who had turned their heat down or off to save money were getting mold blooms in various places around the house. This was a particular problem on exterior walls behind furniture or drapes and in bathrooms.
After a while I figured out that these were all residences where the occupants weren't heating very much but were still taking showers and cooking. The water vapor, having nowhere else to go, was condensing on cold surfaces. Where this combines with poor air circulation mold thrives.
If you aren't heating a modern building you have to open windows, drapes and blinds daily if you are going to occupy it. Simply breathing in an enclosed space puts water vapor in the air and showers will add vaporize several gallons of water. The poor air circulation in these buildings will provide dead air spaces for mold to thrive.
Bottom line; heat the house or open the windows.
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WaterConsNYC Posted 3:22 am
07 Dec 2008
A Few More Notes on Setback
Pangolin, your point is well taken but the temperatures at which mold production condensation will occur are usually 50 F/10C or lower.
Myrtoashe, I've seen the same kind of data but in the US and Canada where we almost always have central heating elderly folks with circulation problems and who sense "cold" more than folks without such problems are not likely to set their temperatures back so far. It does raise a point of whether housing regulations in northern cities that may allow landlords to set overnight temperatures back to 55F/12C are going too far. New York apartment buildings, usually with steam heating systems, are notoriously overheated.
Swolpow and Jwebb, the issue has been around for 30 years. Oak Ridge's Energy Analysis unit did major studies in the 1980's and ACEEE and ASHRAE have published a lot of technical papers on night setback. There's lots of data and i provided a link to one of the more recent, heavily instrumented studies performed by the Canadian housing folks, who generally do very good work. The more that temperature is setback, the greater the savings but in reality unless you're in a very cold climate or have insufficient thermal integrity, most homes will not get down to 50F/10C. So you may set the temperature at 50F/10C but your saving won't be any different than setting it at 55F if the temperature never goes below that. Further, temperatures of 50F or below are probably lower than most people will find comfortable. If it's taking you several hours (as opposed to two) to bring your home back up to 68F/20C then you have significant thermal envelope problems.
Yes, heat pumps behave differently but only a tiny fraction of North Americans have heat pumps and most of them are in the southern portion of the US. Air-source heat pumps are for homes with moderate heating loads since they turn into electric resistance heaters under 35F/2C.
Personally, I strongly recommend a programmable thermostat set for your general lifestyle and not to manually set and reset the thermostat. There are two reasons for this: 1) you have to get up at 4 am to reset the thermostat at the coldest point of the night, which is even worse than my cat getting me up at that hour to feed her, and 2) if I set back a thermostat while I go out for only an hour or two there's a good chance I'll forget to reset it when I return.
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spaceshaper Posted 9:42 pm
08 Dec 2008
Heat pumps etc.
One of the difficulties of discussing building-related energy strategies on a national/international forum like this is the vast range of climate conditions which individual contributors take for granted. Just looking at the US alone, residents of Miami, Albuquerque, Minneapolis, San Diego and Seattle will encounter very different challenges in achieving desirable comfort conditions in their homes at reasonable energy cost and will tend to have a locally-biassed vision as a result. As an example, WaterConsNYC suggests "only a tiny fraction of North Americans have heat pumps". Without arguing over the specifics of what constitutes a "tiny fraction" I can tell you that air-source heat pumps are the most common systems here in the Carolina Piedmont and I believe they may be the heating/cooling system of choice in as many as 25% of US homes. As such they deserve a little attention. In my experience as a design professional in the home construction and renovation business I have found from observation that programmable thermostats do indeed offer significant year-round energy savings on heat-pump installations, especially as they function in the cooling season too of course. If there are cross-climate generalizations to be made I think it's fair to say that programmable thermostats, appropriately set, can offer significant reductions in home energy use in most ducted air and fast-response radiant heating systems (i.e. wall radiators rather than heated high-mass floors) and probably in all air-conditioning systems.
And swolpow, if it takes as much six or seven hours to reheat your house on cold windy days after overnight setback it sounds very likely your home could benefit from a relatively inexpensive weatherization program. You don't say what kind of heating system you have or where you live, but in a typical low-thermal-mass US home with decent insulation and draft-stripping a recovery time of as little as twenty minutes should be achievable - maybe an hour at the most.
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human power Posted 2:53 pm
09 Dec 2008
How depressing
It is so sad to read so many comments that take for granted that most people are too frail to enjoy indoor temperatures below 10 C. If we require such extravagant use of interior climate control, no wonder our global climate is out of control.
How about this folks: set your (central heating) thermostat as low as you can take it and use a portable heater to heat one room (and keep the door closed).
Of course, if more people would stop using fossil-fool powered wheelchairs for transportation then more folks would find comfort at temperatures ranging from 8-48 C.
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