The belated arrival of cold weather to New Hampshire has brought back the eternal home-heating question: Do you save energy by turning down the thermostat at night?

You may recall that question being answered in this column three years ago. But that was before heat pumps started becoming a big part of New England heating, so I thought it was worth a revisit.

For the impatient among you, here’s the spoiler: Turning the thermostat down saves energy if you have a furnace but not if you have a heat pump. For those with a bit more time, here’s some explanation.

My 2022 column was based on an interview with Dr. Alexis Abramson, at the time dean of Thayer School of Engineering at Dartmouth who is now at Columbia University. “There’s no doubt,” she told me. “Whenever you set the thermostat lower, you are saving energy – no matter what.”

It’s basic physics of heat transfer, she said.

This is obvious with a wood stove. Nobody keeps the fire going full blast all night long to save wood; you dampen it at bedtime and kick-start it in the morning. I have a pellet stove which feeds the pellets by thermostat and when I forget to turn it down at night I use more pellets than when ramping up in the morning.

It’s not so obvious when burning oil or gas because it’s hard to keep track of fuel levels, however. Lots of people have a feeling that somehow the amount of work done by their furnace re-heating rooms in the morning is greater than the amount or work it would have done staying at that temperature all night.

The word “efficiency” gets tossed around a lot in a vague way, with extra ideas sprinkled here and there. I saw one guy online who was sure that heat absorbed by furnishings somehow changes the equation. It doesn’t.

However, that was for furnaces, and Abramson qualified her response for heat pumps because she had little experience with them at the time.

But other folks do have experience and they are clear: “Set it and forget it” is the correct rule for thermostats on heat pumps. Nocturnal temperature fiddling is counter-productive.

Why? Hydraulics.

This was explained to me by the folks at Efficiency Maine Trust. They have a lot of experience with heat pumps because Maine has been doubling down on this energy-saving technology. The state provides incentives for them because the state which burns the most home-heating oil makes it vulnerable to price and supply shocks, not to mention local pollution.

Heat pumps use compression and expansion of refrigerants to absorb heat outdoors and release it indoors or vice versa, which is why they can be used for both air conditioning and heating. They’re more efficient than furnaces because they don’t have to create as much heat.

But they do have to pump the liquid refrigerant back and forth, which is where the thermostat difference crops up.

“Here’s what our program manager says,” wrote Kate Rankin, director of communications at Efficiency Maine Trust, in response to my repeated, confused queries. (They were very patient with me.)

“The fundamental issue is related to the Affinity Law (aka Pump Law). Law 1a: Flow is proportional to shaft speed. Law 1c: Power is proportional to the cube of shaft speed. So if you have 10 times the flow rate during recovery from a setback, you need to use 1,000 times (10 cubed) the power during that period.”

In other words, raising an indoor temperature for a short period requires a heat pump to use more energy than maintaining it for a long period, unlike furnaces.

Which is, come to think of it, another reason to get a heat pump: The house can be warm when you get out of bed and you don’t have to feel guilty.