Quote:
Originally Posted by Piwoslaw
At night, or when you're not at home, the lower temperature differential between indoors and outdoors allows less heat to be lost.
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That's it exactly. Here's Fourier's Law of Conduction, which states that energy loss is proportional to the temperature gradient:
If the temperature differential from the inside of your wall to the outside is twice as large, you'll lose heat twice as fast through the wall once transient conditions stabilize.
No matter what you're heating with, less heat will be required if you let the house cool down overnight and while you're at work.
Furnaces are horribly inefficient when they first turn on, as you spend lots of energy heating up the furnace and its ducts. Running the furnace all at once, shortly before you get home / wake up would be ideal.
I've heard several reports (one of them carefully controlled and documented) of heat pumps using more energy when allowing the house to cool at night and when noone's home. I know that some heat pumps will engage the auxiliary electric heater (30-50% as efficient) if they see more than one degree difference between the temperature and the set point. What I'd like to know is, if you were to manually disable the aux. electric heater, could you then use a programmable thermostat + HP to save energy?