My apologies in advance, because I know we've already discussed this before, but for some reason I can't find any explanation through my various searches.

As I've discussed before, I've installed a ZWave-controlled thermostat in my home. I love it, and I can adjust it from anywhere I have an internet connection.

When our house is empty, we adjust the thermostat so that it doesn't use as much energy. The scene I've set up for "leaving the house" turns off all the lights, and sets the heat to 65 and AC to 80.

I'd like to hear an explanation for why this is more energy efficient than leaving it at a consistent temperature.

Obviously there are a lot of factors here, like outside temperature and how well the house is insulated, but is it always better to turn the temp down and ask the system to run a little longer than it normally would when you get back in?

If there's a thread where this has been sufficiently explained, I'd be perfectly happy with a link, but if someone wanted to explain again that would be great too.

There was a long discussion in the Nest Thermostat thread, starting late on the first page.

-jk

It basically boils down to a simple law of physics: the greater the difference in temperature between inside and out, the faster heat will move between the two.

Therefore setting the thermostat for a lessor difference in temperature (between inside and out) will slow down the transfer (loss or gain) of heat, saving energy by reducing the need for the heater (or AC cooler) to be run to make up for the transfer.

Thanks, but that's not the part of the equation that's puzzling us. We figured it takes less energy to heat a house to a lower temperature.

The part I'm wondering about is the decision to lower the temperature while out, meaning that when you get back the furnace is going to have to run for a longer time to heat a house back up to a comfortable level that it does to maintain a temperature*. The question is: does that energy it needs to bring the temperature back up off-set the energy saved by not having to heat the house to as great a degree, or would it be about the same to simply leave the heat on at the same temperature.

I'm assuming the answer is "no," so I continue to turn the heat down when we leave, but I'd like to wrap my head around a concrete explanation of why.

The greater the temperature difference between inside and outside the more energy needed to maintain that difference.

By lowering the temperature difference when you are not home you are lowering the average temperature difference. A lower average temperature difference requires less energy.

It's the same answer, really!

If in cold weather, you keep the house at a higher temperature, it will lose heat more quickly, requiring lots more energy to compensate for the heat loss.

If you allow the temperature to drop overnight, or while out for a few hours, then this will slow down the loss of heat, saving energy. When you later turn the thermostat back up, sure the furnace will run longer than "usual" to raise the overall temperature. But there's simply no way that this can cost more energy than it would by keeping the house at a high temperature all of the time.

It's not about _temperature_. It's about heat (energy) loss. The less energy you lose, the less it takes to replace that energy. Turning the thermostat down for a few hours reduces heat loss.

Cheers

If I may contribute, the vast majority of the energy consumed by a heating system, or cooling system, is simply to replace the energy transferred to the outside. The heating system does not 'heat' the house, it simply replaces heat that has been lost through the walls, windows, etc to the outside. Essentially all the money you pay to heat your house is being spent to replace energy loss, not make the house 'warmer'.

Imagine the temperature of the core structure of your house was maintained at a near constant temperature (which if you have a good thermostat and heating system, it is.) All winter long those interior walls and floors are at the same temperature. You never need to 'warm up' those building elements, as they never get cold. The interior surfaces of the exterior walls are of course not quite as warm as the interior structure, so the room air loses heat to those cooler wall surfaces.

The heating system adds heat to the interior air to compensate for the heat loss to the outside walls, and that added 'air heat' slowly but continually transfers to the outer walls, and hence to the outside world.

As the room air becomes cooler (such as when you 'turn down' the thermostat) the heat transfer to the outside walls slows, since the heat loss rate is driven by the temperature difference between the interior air and the outside air. During the time the interior air is cooler than your normal inside temperature, the heat loss happens more slowly. The longer the interior spends at this lower temperature the greater the difference in total heat loss compared to maintaining the thermostat at your normal 'comfort' level.

The slower the rate of heat loss, the less total energy needed to maintain the 'set back' temperature and then return the interior to the desired comfort level when you return.

If you turn the thermostat off, the rate that the interior loses temperature is an indicator of how well the building is able to retain heat, or how quickly it is losing heat. A well insulated building should change temperature very slowly with the thermostat turned off.

I think turning the thermostat down shouldn't cost - and will generally save - unless you have a heat pump that uses auxiliary heat coils or some other two stage system where the boost is inefficient.

Also, if your house is so well insulated that it never actually reaches the lower setpoint before you turn it back up, I suspect it would be a wash and wouldn't bother.

(Thermo was a long time ago, so I'm probably be misremembering.)

-jk

Is this one of those questions served by measuring the area under the curve?

I had a discussion with Doug (tanstaafl) about this very same topic a while ago, probably in that Nest thread.

The reason was I'm using a geothermal heat pump to heat my house. This is a very energy efficient system. On top of that, I've insulated my house well beyond average, to the point that it's almost a passive house.

For this reason, I don't even *have* a thermostat. I just set the room temperature to 21°C on the heat pump, and it makes sure the house temperature stays at that level winter *and* summer. (yes, in the summer it actually cools the house down). This puts us in the luxury position of never having to touch the temperature setting. When we leave the house, it's 21°C, and when we return, it still is. There's something to be said for the luxury of entering a heated house when you return from the cold.

A thermostat would also be completely overkill in our setup, because when I turn the heat pump completely off, it still takes more than a day before the house actually starts dropping in temperature, thanks to the large amount and efficiency of the insulation. And even then, the temperature drops very slowly. I once had to turn the system off for maintenance reasons -in winter, yay!- , and only after three days this became noticeable.

For this reason, I initially thought that it would mean that I would consume more energy if I turned my system completely off and then turn it back on again, having it do "overtime" to return to the original setting of 21°C, rather than keeping it at 21°C all the time. Doug convinced me this is not the case, it does consume more energy that way. But, thanks to the insulation, the difference in cost should be calculated more in the region of eurocents/year than euros. For this reason I don't care and just leave it on all the time. In any case, it's a heck of a lot more economical than the heating system of all others I know, meaning it costs me about half to heat my house 24/7 to 21°C than what others pay to heat their house to 21°C only in the hours they need it. The whole setup was a very hefty investment, easily 10-15% of the entire build budget, but needless to say I'm very happy it has paid off the way it did.

I think with heating, because of the low humidity, it's always cheaper to reduce the delta T.

However for cooling, if you live in a humid area, there's a lot of power wasted in just removing water from the air. The air con in my apartment in Singapore would work an entire day just removing humidity before doing any appreciable cooling.

You are right, but the cooling does not work by cooling and removing water from the air like a normal airco does... It uses the same floor heating circuit as is used for heating, but instead of with heating, when water of about 28-30 °C flows through the circuit, with cooling the water temp drops to about 18°C (no less because then condensation starts appearing on the floor). This works wonderfully well and uses almost no energy since the source temperature of the water is about 10°C to begin with. This system works even better than airco does IMO because it does not make the air dry and it uses alot less energy. The only downside is that is does not work instantly, it takes a couple of hours before it kicks in (but this is no problem because of the insulation).

Right, the answer is different for people with heat pumps, because they're less efficient at higher outputs. This is not the case for any other type of heating: electric, gas, coal, oil etc. -- in those cases, consider the energy flow across the boundary of your house to get the correct answer that turning down the thermostat is a win.

Peter

We're finding 21°C too hot to do anything much physically. I guess we're still moving stuff around, sanding and finishing timber, grinding back concrete worktops etc which we won't be much longer but...

I need to look at the setup for the boiler and the MVHR in more detail.