Re: Does it really save that much?
"Your heating bill is driven by the heat loss from your house. The rate of loss is set by the standards of insulation, but if you see those as set, then your heat losses are proportional to the delta between inside and outside temperature, multiplied by how long the heating is maintaining that difference." -- Ledswinger
Sorry Ledswinger, I do understand the physics, but I'm still not convinced. Say at 5℃ outside, your house, at 20℃, is 15 K hotter. You drop the thermostat to 15℃ when you are out. Eventually, your house will get to 15℃ and your heat loss rate will be two thirds of what it would have been. But, until it gets to that temperature the reduction of your heat loss is less than a third. And when the heating goes back on, it has to work harder to raise the temperature of the house.
So if your temperature drops to 15 quite quickly, your have more serious problems that your thermostatic control. And if it drops quite slowly, your house is so well insulated that you may as well keep your heating on. You probably don't want to drop your temp by much more than 5K, not just because your heating has to work harder, but because you don't really want to temperature cycle your house and its contents by much more than this on a regular basis.
So, a themostat thats 5K lower when you are out can save you money, but maybe not as much as it seems. But these things have to be even better than that - because their opportunity to save you money is only when you are out and you didn't expect to be. And the most common scenario I see quoted for this "late home from the office" is only going to be a few hours of cooling. Finally, there is only a very brief period in family life when you are all out, or all in at the same time, so unless your the kind of office worker who thinks the spouse and kids should shiver along with you when you're stuck in the data centre, I just don't think the maths adds up for most situations.