Does turning down the heat for a short period waste energy and waste my time?

Jim100Flower

I went to lunch today with my smartest friends and boasted that I turned down all six of my smart thermostats with a single voice command. Nice, but they all agreed that turning down the thermostat for a short period, a 2 hour lunch, will not save energy. It seems the situation is akin to highway vs city driving and there is a minimum period, I’ve read 8 hours, for a turn down to save energy.

Google is wishy washy on this topic, so I thought I’d ask the experts at HeatingHelp.

I claim that if the heat is down for long enough for my house to cool down even a little bit then I am saving energy. The rate my house loses energy is lower when the difference between the inside and outside is smaller. A short turn down of the heat can shrink that difference a bit so that I am losing energy at a slower rate and there will be less energy needed to bring the house back up to temperature.

Further, if there are energy costs to starting up the boiler, they may be incurred more often if the heat is left on (starting a few times) than if it is turned down (starting once).

I think turning down the heat for a two hour lunch could save me a tiny bit of energy. Am I right?

I have a forced hot water boiler fueled by natural gas and feeding baseboard radiators.

Jamie Hall

What you are leaving out of the equation there is how much energy does it take to bring the house back up to temperature? A surprising amount. Trouble is it varies with the house and the system, so there is no one hard and fast answer.

Jim100Flower

Whatever energy it takes to bring the house up to temperature it is going to take more energy if I've been losing energy at a faster rate (by leaving the heat on and the in-out difference higher)

Jamie Hall

Well, actually, no. You are neglecting the heat capacity of the structure and the furnishings which -- depending on what it and they are -- can be substantial. If you were dealing with an empty box, and heating only the air, then fairly large and quick setbacks make sense. The only sure way to find out is a controlled trial -- which is extremely difficult to do. However, to give you just one example, the main place I care for cools at roughly 2 degrees per hour. Even with the boiler at full song, it takes a bit over an hour to recover from a 3 degree setback -- which I use there. I do not think, based on boiler run times, that there is any significant fuel savings either way.

HVACNUT

I think you and your smartest friends have way too much time on your hands.
You never mentioned how much lower you set it from your "at home" temperature.
And like you said, the outdoor temp will have a large bering on the heat loss of the home, so there really is no one specific answer. A home with less than 3 air exchanges per hour will fair better than a similar home with 5 air changes per hour.

Jim100Flower

@HVACNUT it is great to have too much time on your hands!!
I generally use a 5F setback.
I agree there is no one specific answer as to the degree or extent of the effect but there is ONE answer as to whether the savings is zero or a benefit or a loss, whatever the degree.
I am using forced hot water so I am thinking in terms of boiler ON/OFF versus air exchanges.
Suppose at my normal indoor temperature, say 65F, the boiler runs for 5 mins per hour.
If the thermostat is constant at 65F during the 2 hour lunch, it runs 5, then 5, then 5 again in the third hour.
If the thermostat is set back to 60 during the 2 hour lunch, it runs 0 then 0 then something less than 15 in the third hour.
In the second case the room has lost less energy due to the smaller T[inside] - T[outside], so it runs for a slightly shorter time.

Larry Weingarten

Hi @Jim100Flower , Seems to me your thinking is correct though you will likely only see savings if the house is very leaky. The unspoken bit is that the house becomes somewhat less livable because of the setback. I'd look at this another way; which is to do a blower door test and survey the house with an IR camera so you really do know what's going on and where any substantial heat is leaking out. Fix those things and setback becomes a moot point!

Yours, Larry

Icarus

Fundamentally, a smaller Delta between indoor and outdoor temps uses less energy, regardless of how long it takes to bring it back up to temp, assuming all else being equal. In reality, in a well insulated envelope without it being too cold out (large delta) it probably isn’t going to make enough difference to even notice.

Icarus

nicholas bonham-carter

Outdoor setback would be the most effective fuel saver, with a constant lower temperature setting.
At 50 degrees outside, the system water could be a lower temp to keep warm, whereas at 0 degrees, the system would automatically raise the water temp.
What is the interior temperature when you return from lunch, and how long does it take to recover?—NBC

GW

Oversized systems, let the temp sink all you want. Right sized systems, set back little to none.

Same answer with loose vs tight (good or poor thermal envelope) home.

So to wrap it all up, a tight home with a big system, shut the thing off if you want. Loose home with system that barely beats the place, don’t move the stat

Also, warm air is quicker than hot water (all other things being equal) Radiant is slow, don’t yo yo a radiant system.

As Jamie said, no one quick answer, even though so many TV heads claim they have the perfect answer

Jim100Flower

Thank you. What I’m hearing is…
A short setback will not waste energy but it won’t save energy unless it is long enough for the house to cool down and that happens sooner for a leaky house and on a cold day. If I tighten up my house, the setback won’t be needed.
The time to recover to a comfortable temperature depends on the leakiness of the house, the size of the boiler, and the furnishings in some rooms make them slow to warm up.

@nicholas bonham-carter you asked the temperature on my return from lunch - it was essentially unchanged, so, for this lunch I did not save any energy. Your question prompted me to look at my smart thermostat data from the Kitchen zone. This graph is centered around the time I left for lunch and set the thermostat back. The thermostat did not call for heat any time during the period graphed.

@nicholas bonham-carter I have an outdoor reset. I don't know if it is working.