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My central heating has a thermostat dial ranging from 10 to 30 degrees Celcius. I'm wondering whether it's likely to be cheaper to crank it up to 30, and turn it on for, say, an hour, 3/4 times per day, or to turn it down to say, 15, and leave it on all day.


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I think there are too many variables involved to give a general answer. It'll likely depend on the heating system, size of the space, insulation (or lack thereof), floor/wall coverings, furniture, number of occupants, other heat sources, etc. – Tester101 Nov 28 '12 at 15:34
@Tester101 I'm not so sure I agree. Please take a look at my answer and comment to let me know what you think. – mac Nov 28 '12 at 20:50
up vote 5 down vote accepted

I tend to think of questions like this about heating and cooling efficiency from a thermodynamic perspective, where its primarily a question of energy loss. The energy loss to the surrounding environment goes up when the temperature difference between the inside temp and the ambient temp goes up; the energy loss goes down when this temp difference goes down.

Lets say for the sake of argument that your ideal temp is 15*C. So one option is to leave the thermostat at 15*C. Lets say that the other option you are considering is you'd run the thermostat at 30*C until the space reaches 30*C, then turn the heat off until your inside temp reaches 10*C, at which point you'd set the thermostat back to 30*C, and repeat.

Your proposal basically implies that you're willing to accept your inside temp being slightly higher than ideal sometimes (up to 30*C when you want 15*C), and slightly lower than ideal other times (down to 10*C when you want 15*C). What are the concequences of this for energy loss?

At those times when the inside temperature exceeds your ideal temp, you'd be losing more heat energy to the environment than you would if the space was at your ideal temp. At those times when the inside temperature is less than your ideal temp, you are losing less heat energy to the environment than you would if the space was at your ideal temp.

My conclusion is this: why subject the people occupying the space to somewhat uncomfortable temperature swings? Set the thermostat to your ideal temperature when you're in the space, and set it to something low when you're not.

A related question I've heard is: Why turn down my thermostat when I leave? The heater is just going to have to work harder when I get back to bring the space back up to temperature. While this is true, the whole time that you were gone, you were losing less heat to the environment because your inside temperature was getting closer to the temperature outside. That is your energy savings. That doesn't get taken away when you come back and turn the heat back on. Yes the heater may run for longer when you come back, but all you need to be concerned with is energy loss, which has gone down on the net.

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The last paragraph is the most important point (and more likely the point the OP is trying to ask about). – auujay Nov 28 '12 at 21:34
If the heat is turned back on at 10*C, the average indoor temperature is 20*C. It is not a wash, the thermostat set to 15*C will use less energy. The least practical energy consumption is achieved by setting the thermostat as low as you can stand it when occupied, and as low as it will go when unoccupied. – bcworkz Nov 28 '12 at 23:08
I would think that air leakage would have a much higher influence than heat leakage and it doesn't vary as much with the temperature. I would also think that using the thermostat as intended, the approach bcworkz recommends, would achieve the highest efficiencies since these issues have been legislated for some time. Out-thinking the engineers is a fools game. – Tim Quinn Nov 29 '12 at 23:34
@TimQuinn The same math applies even with air leakage. You wouldn't really care about air leakage if it didn't result in heat leakage--we open windows on a comfortable day. A given amount of air leakage results in more heat loss (or gain) if the temperature difference is greater. – mac Nov 30 '12 at 14:39

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