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I often hear a claim saying that reducing the thermostat's setpoint by one degree C will reduce the overall yearly energy consumption by 7%. I've heard heating control manufacturers, energy consultants, and heating installers repeat this claim.

However I have not been able to find out where this number comes from, and indeed I suspect that this number may vary a lot from building to building.

Does anyone know where this number comes from? And how reliable it really is?

lindelof
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    It will vary by building, by local climate, by hot water consumption, and most importantly by current setpoint temperature. I'll try to find time to write up a proper answer. – 410 gone Jun 14 '14 at 15:40
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    imagine a building where it is always 20C outside. If you keep your thermostat at 21 and I talk you into lowering to 20, you will save a lot more than 7%! OTOH if it's -40, I doubt the savings are so much. – Kate Gregory Jun 14 '14 at 18:07

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Kate is correct, and it is a good example.

Turn it around the other way: 7% for a 1 degree setback amounts to an average heating regime of 14 degrees C. Multiply by 365 days a year, this corresponds to a 5100 degree day heating climate. That's about a 9000 F degree days. That corresponds to a very cold climate.

It's worse: A house has to be badly designed to no solar heat to some degree. My house, not anywhere close to R-2000 standards doesn't require us to turn on the furnace until the outside temperature is close to freezing. In addition internal waste energy gives the house a nudge. Lights, cooking, domestic hot water, all help warm the house.

EnergyNumbers has another good example. As the climate gets closer to inside temps, the benefit of lowering the set point increases. Since a 5100C degree day heating season is at the cold end, the claim is actually quite conservative.

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