Far too often, a detailed analysis of any question produces the highly unsatisfying answer of “it depends.” So I’m delighted to say that today’s column comes down 100% on one side of this question:
Do you really save energy by turning the thermostat down at night in winter, or are you losing all the benefit and more when the house heats back up in the morning?
This is a contentious matter. I’ve heard people knowledgeable about HVAC (heating, ventilation, air conditioning) argue both ways. To settle it, I went to Dr. Alexis Abramson, dean of Thayer School of Engineering at Dartmouth.
She’s got academic chops (former professor of energy innovation at Case Western Reserve University, author on dozens of energy-related research papers) and government chops (chief scientist and manager of the Emerging Technologies Division at the U.S. Department of Energy) and business chops (technical adviser for Bill Gate’s Breakthrough Energy Ventures and co-founder of a start-up focused on building efficiency).
Just as importantly, she’s got practical home-heating chops: When she moved to Hanover she looked at the proposed HVAC design for new her house and made some big revisions.
So what’s the answer?
“There’s no doubt,” Abramson said. “Whenever you set the thermostat lower, you are saving energy – no matter what.”
So there you have it. To be honest, I was hoping for a different answer so I could get out of bed to a warm house without feeling guilty.
People who argue the opposite base their position on the idea that furnaces are less efficient when racing to quickly raise temperature back up than when moseying along and maintaining a constant temperature. That may be the case, Abramson said, but it will never completely overcome the savings you get during the period when the furnace is off while the house is initially cooling down. Not to mention the savings during the nighttime when the thermostat is set at the lower temperature. The physics does not lie.
This is absolutely, positively true for furnaces that burn heating oil or natural gas, she said, which basically just turn a flame on and off. The advantage is less obvious with homes that have electric heat pumps – which use a compressor/condenser unit to move heat from outside to inside, or vice versa in the summer – because those systems may operate less efficiently at much higher or lower thermostat set points.
Heat pump manufacturers usually recommend that you don’t change the thermostat very much but Abramson said she has not seen good data to back up their assertion that efficiency suffers, particularly if you’re only turning it down a few degrees.
“I suspect it also has more something to do with maintenance of systems – whether parts break when the system is turning on and off more often,” she said. “It’s kind of an unknown.”
One obvious point is that better insulation makes a difference regardless of thermostat or heating system. The less heat your house loses in winter, the less you’ll have to pay to reheat it.
A less obvious point is that we could all use more information.
I have oil heat and there’s no way for me to know how much I’m using at any given time, no equivalent of an MPG reading for the car.
How much does cutting the thermostat to 56 degrees at night instead of 60 degrees save me? Is it worth it?
I have no idea; the gauge on my oil tank doesn’t give anything near the detail needed, so I do what “feels right” even though it might be wrong.
Part of the change we need to make to fight climate change is to move most systems that depend on burning fossil fuel to systems that run on electricity, which can be produced more cleanly. (Yes, I know electricity is sometimes produced by burning coal and often by burning gas, but that’s changing.) Also, electric motors are generally more efficient than combustion motors.
If I had an all-electric house I could get the data I need about moment-to-moment usage and make better decisions.
Someday I’ll switch to a heat pump, partly because it will also give me air conditioning, which I’ve never had. Abramson said I need to think about it before my furnace dies, to find a dealer and be prepared.
“Think early about how you’re going to switch. … Don’t wait. When it’s an emergency situation, you kind of pick whatever they’ve got on the truck,” she said.
“The future is electric. It’s a much better way to manage impact on the climate.”
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Disappointing that the good doctor did not cite data or studies to confirm her assertion s. Surely, someone has tested how much fuel is/is not actually consumed by lowering the thermostat at night.
There should be graphs showing a variety of scenarios: number of degrees lowered, duration, outside temperatures etc. The plots would illustrate the optimal settings for fuel consumption which logically, should be programmed into a smart thermostat. Please keep digging.
Re: ”
Heat pump manufacturers usually recommend that you don’t change the thermostat very much but Abramson said she has not seen good data to back up their assertion that efficiency suffers, particularly if you’re only turning it down a few degrees.”
Heat pump efficiency drops with temperature. The outside temperature is lowest just before sunrise, so that’s the least efficient time to pump heat energy back into the house.
David, good article. Question: we have been turning our heat pumps completely off at night — reasoning that this means a little less wear and tear on them. Our house is well insulated, so we don’t lose much heat overnight. But we recently learned that we should keep them running, 24-hrs for best efficiency. What has your research turned up on this?
You might want to think more broadly about what you are trying to minimize. Is it oil and electricity? If electricity, you may want to consider minimizing carbon emissions and those are lowest at night at the present time per ISO New England ( https://www.iso-ne.com/isoexpress/web/charts ). Around 5am today the oil and gas sources started to climb, whereas earlier we had more carbon free electricity from hydro an nuclear. From this I might want to preload the house with heat. For those interested in looking at their oil furnace use, a smart thermostat like Ecobee will give you a minute by minute graph of when your system turns on and also show outside and inside temperatures. Likewise, a Sense energy monitor will show you when the burner on your furnace turns on and give an indirect measure of oil consumption.
Not true when I have tankless oil water heater my heat is always on 62 but I also use my gas mushroom to brake the chill and if I shut off for the night then in am I brake the chill with gas to the desired temp this is still calling for hot water every 8 min ridiculous. Having running on all the time on 62 temp small 3 bedroom house one floor no basement it eats 100gal in this cold weather in 22 days shutting off and only turning boiler when I need hot water I like cold in my house I can’t sleep when warm I save Oil double so if you have tankless oil heater furnace just shut it off it’s never going get below 51 no pipes will freeze especially when u not home all day working then brake the chill with gas heater I use big mushroom heats up whole house in 5 min and then I turn it on for little bit but summer I keep it off totally turning on only when need hot water and it takes 3 min so that’s a big savings. Guy from oil company told me that every time I turn it on the boiler being shut off for hours I use 1gallon of oil I rather do that then go 100 in 22 days crazy I don’t like tankless but have to deal with it am renting this house pay a lot just for rent the windows are old house not insulated good so that’s my way of saving money.
I will have a heat pump installed in the spring. I chose the regular compressor which can produce heat when the ambient temperature is 20F or above, not the super compressor which can heat when outdoor temperatures are a minus 20F. In our climate, is this a good choice?
Jim, I spent 12 years in HVAC in this area, and I doubt that “…keep them running, 24 hrs for best efficiency” is a valid point. It will be interesting too see what the experts have to say about that.
PS: Yes, I have a “water-source” (currently called “geothermal”) heat pump in a well-insulated house. It costs me somewhere between one-half and two- thirds of what conventional fossil-fuel systems do, for heat alone. Gotta love it.
The circulating pump runs at a constant rate for each zone. A simple hour meter connected across the pump will tell you how many hours the pump runs during the day. A time meter that reads minutes will give you finer resolution. That, combined with outside temperature and inside desired temperature may give you enough data to create a graph. I did that with my four heating zones was interesting but the hour meters, being old ran out of battery power after a while. I lost interest, being distracted by other interests. Mine is also more complicated because I heat mostly with wood.
I am married to a Filipina who feels cold all the time in NH winter, even at night, so I don’t dare consider turning down the thermostat to save money. A related question – should you put a timer on your water heater to shut it off at night? I think the repeated heating and cooling may shorten the life of water heaters. Materials, especially those in contact with differing coefficients of expansion, may weaken and fracture – has any study been made?. Besides, the small amount of energy radiated by the water heater simply adds to the overall heat available to your residence.
I do not know of any such study. I find lots of discussion online about the topic that reach the same conclusion about energy – cycling usage saves energy – but none discuss possible effect on equipment lifespan that I can find.
Thanks David I’ll give it a try. I’ve always lowered the setting at bedtime and gone back to daytime before we get up. We use propane forced hot air.
For what its worth, I heat my home with a wood pellet stove and this winter I have been running it overnight at its lowest thermostat setting, meaning it does ramp up and down to maintain a set temperature, about 68-70 (note however that the stove never turns completely off). In previous years I ran the stove on its “low” setting which maintained steady lowest-possible burn rate, with no input from the thermostat. The idea was to save pellets and by morning the house temperature would have dropped to around 60. Point of this story is that I have observed little change in the amount of pellets I am consuming (I keep close track year to year). If nothing else this might be useful information for other pellet stove owners.
I’m going to echo Bob Hargraves above,
I have a heat pump and I recently attached an electricity monitoring system attached to the breakers for the heat pump and in my limited testing so far this month the heat pump used slightly more energy when setback at night since it seems to consume quite a lot of energy in the first hours at dawn as it raises the temperature back up. It’s not a huge difference (less than 10%) and it may change as we head deeper into fall and winter but I’m not seeing any savings with a nighttime setback – likely due. to the reason mentioned by Bob, less heat energy available in the air.
Best discussion of this topic I’ve yet to find – thanks for that. I live in Mass., had a heat pump installed about 3 years ago but still have my old gas furnace feeding hot water baseboards. I’ve been getting conflicting info on what temps it’s best to run the heat pump at vs. the furnace. I was initially told heat pump at 30 or above, furnace at lower temps. Others say heat pump at 20 or so. So, what I’ve been doing is shutting off the heat pump at night and having the boiler set at an “Eco” setting on my Nest thermostat at 60 degrees – meaning it won’t come on until temps are lower than that inside. In the a.m. I have the boiler come on to heat up the house so it gets to around 68 by the time we wake up. At that point, the heat pump comes on and i turn the boiler thermostat back down. But I’m wondering if I’d be better off leaving the heat pump on overnight, maybe turning it down 5 degrees or so, and only using the boiler if temps fall below around 20. (I also wonder if I’m over-thinking the whole thing, but that’s another issue.) Any thoughts?