Do I really save energy by turning the thermostat down at night?

  • This is the only information I have about energy use in my home. Not terribly useful. David Brooks / Monitor staff

Granite Geek
Published: 1/17/2022 4:09:57 PM

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.”


David Brooks bio photo

David Brooks is a reporter and the writer of the sci/tech column Granite Geek and blog granitegeek.org, as well as moderator of the monthly Science Cafe Concord events. After obtaining a bachelor’s degree in mathematics he became a newspaperman, working in Virginia and Tennessee before spending 28 years at the Nashua Telegraph . He joined the Monitor in 2015.



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