Losing efficiency where? As long as the efficiency of your heater itself doesn't vary, in terms of what fraction of the calorific value of the fuel is emitted into the living space (in air of whatever temperature), the conservation argument remains the same.

Peter

Sorry, but I don't buy this. If this were true, it would be no use at all to insulate your house. How do you explain then that, back in the days when people did not insulate their houses, they easily needed a 35 to 40 Kw furnace to warm their houses to a comfortable temperature? If they now insulate that same house properly, they could easily do with a furnace of about 10 to 12 Kw (or even less) to also warm up that same house to the same temperature. The more powerful heater obviously also consumes a lot more fuel (not sure if its also about four times as much, but in any case a lot more)

And what about passive houses, which don't even have a central heating system anymore, but simply a LOT of insulation in the walls of their houses?

I know for a fact that, due to the thick amount of insulation in the walls of my house, my heating system doesn't have to work very hard to keep the temperature on the same level. It only switches on maybe once every two hours, and even then only for a few minutes. If I turn it off, letting the whole house cool down (which, granted, could take a few days), the system would be running for at least a day, none stop, to get the temperature back to the comfort level it was before. It would certainly consume more power during that 24 hr heating up period than it would have if I would just would have let it run constantly, only switching on a few minutes every two hours, with a total of maybe two hours per day.

Doug is correct. Insulation, due to its low thermal conductivity, only reduces the rate of heat transfer. It does not prevent heat transfer. You will eventually reach equilibrium with the ambient outside temperature, if you're not actively heating/cooling, and the ambient outside temperature remains constant.

The house will absorb some energy from sunlight, especially through windows, so it may be possible that the inside will never be quite the same temperature as the outside.

It can very easily be a lot hotter inside than outside, especially in the summer. Something pretty obvious. But nonetheless an mildly expensive lesson learned back in the summer of 2007 when I "boiled" the contents of a planted and stocked aquarium when I left the house for a few days and forgot to turn the AC on. Ugh.

I think insulation works both ways, it will also keep heat out.

I guess that is why my house is cooler than outside during the summer ???

Cheers

Cris

Right. I wasn't considering that, as I didn't really consider that as the outside environs remaining constant. I was thinking more in the same terms as physics' legendary "perfectly frictionless surface."

Yep. This is why my family's cabin is colder inside in the winter time then outside. It's a tradition to head up there, nominate some poor soul to run in, start the fire place, then run back outside while the rest are hauling supplies.

Right, what determines heat loss is temperature differential and quality of insulation ("thermal resistance") and total exterior wall area. But when evaluating different thermostat/timer regimens for the same house, the latter two of course remain constant, so only the first needs to be considered.

Peter

OK, one more time.

The only reason you run your furnace is to replace the heat that is lost to the outside. The greater the temperature difference inside to outside, the more heat you lose. It doesn't matter if it's the inside air temperature, the wall temperature, or what you're cooking in the oven. Temperature differential determines how much heat you lose.

You can try and make it more complicated by bringing in factors like insulation and glass area, but they only affect the amount of heat saved or lost at a given temperature differential. Their efficiency factor is fixed. It's temperature differential that rules.

tanstaafl.

No, Doug is spot on there -- the discussion is about whether to turn the thermostat down at night or not, and the answer (in cold climes like ours at least) is an emphatic "YES!".

But you are also correct -- better insulation always helps, whether one turns down the thermostat or leaves it constant.

The best conservation solution is of course to do both: insulate better, stop air leaks, AND turn down the thermostat at night.

Our heating bill would easily increase by 50% if we didn't turn the home temperature down by 6 degrees (C) at night.

Cheers

That's actually not where I was coming from. I doubt that 8 hours of a few degrees cooler is going to have a significant effect on the heat of the walls, certainly not enough to cause the kind of cycling you referred to earlier. I was thinking more of turning the temperature down 20 degrees or more while away for a weekend.

You don't want to see what happens (to a house) when you forget to turn on the heat here. It happened just up the street and looked somewhat like Mr. Freeze had just started making an ice castle. Pipes burst, walls froze, place needed to be gutted.

Oh, but I'm not arguing that. That I agree on, that, eventually the inside and outside temperature will reach equilibrium. My only point is that, because of the use of insulation and it's low thermal conductivity, you can slow down this process in such a way that it's not economical anymore to turn the heater down at night. (because in the morning it would have to do a lot more effort to reach the same temperature level as it was the day before and thus consuming more energy).

If you're leaving the house for more than a couple of days, then I agree you'd better turn down the thermostat to save energy and thus cost. But not in the course of 24 hrs, thanks to the insulation. I actually tried this last night now, just because I was curious. I switched off the heater when I went to bed, and when I got up 8 hrs later I looked how much the room temperature had dropped: half a degree. Ok, so this is not yet the coolest period of the year, this night it was 5°C outside. But I don't think the dropping of the inside temperature will increase immensely if the outside temperature would start dropping below zero.

A lot of the newer central heating installation don't even have thermostats anymore. They have what's called a WAR module on the outside and one temperature sensor on the inside. The WAR determines the heat of the water in the furnace. eg. At 10° outside, the water temp is 30°C, at 0°C outside, the water temperature is 35°C. The temperature sensor on the inside then determines when the circulation pump starts running and the heating begins or stops.

There's one thing I haven't brought into the equation yet: water temperature. With the older systems, which are used to quickly heat up rooms with the use of radiators, the water temperature is usually 60°C or more. Then when the thermostat is switched on, the water quickly needs to be heated to that temperature and kept constant until the correct temperature is reached. This consumes a lot of energy.

In my house, what is called a "low-energy-house" (=term that is used by the government), the water in the tubes is only heated up to 30 - 35 degrees, but then kept constant, in other words, the system is never turned off. At the end of the year, my system will have consumed less energy than the first mentioned, and that is why our government is now pushing new home builders to invest that extra euro to reach the "low energy house" standard. Because it IS more expensive: you need to insulate a whole lot more and make sure the house is air tight - because of this you now also need a ventilation system, which was not needed up to 5 years ago - but all those costs can be recuperated in an X amount of years (in my case: 7 years), and since the heat pump is claimed to last 25 years, the rest is pure profit.

Why am I saying all this? Because I want to make my point that it is not necessarily more expensive to let the heating system run at the same temperature all the time instead of turning it off/off every day, providing you did the homework and insulated the house, made sure it's as air tight as possible and use a heater of less Kw which uses low water temperature and an over dimensioned radiator as a heat release system. (like eg. floor heating tubes are because essentially the whole floor is a radiator then). Then, and only then will the bill at the end be less, and significantly less than a normal house with an old school heating system (trust me: I know because I live in such a house and it's me who pays the bills )

I'm not saying you can stop heat transfer/thermal conductivity, that was never my intention to claim and if it came across like that then I apologise. I know you can't do that, it's a law of physics and as Scotty always said: "you cannae change the laws of physics!" But you can bend them a bit to your advantage, by using modern techniques.

And mine is that, even though good insulation and good system design can reduce the cost of running the heater overnight to arbitrarily small values, the physics of the situation means that the cost if you turn it down is always fractionally lower still.

Peter

Let's agree to disagree then. I can only tell you that my heating cost is now considerably lower than it was before. That the government even subsidises people who are willing to invest into converting their houses into "low energy houses". Why would they do that if it was useless and would even consume more?

And most of all, I know of several people who have actually done the effort of keeping record of the expenses when then did the on/off method, then did the home upgrade, changed to low-energy and constant temperature and again kept record or their monthly costs. Even though the cost of energy had gone up since the year before (as it always does every year) they noticed a considerable lower monthly cost than before. Are all those people lying then? I don't believe this, because I've noticed the same thing myself.

The government did not pass the Law of Conservation of Energy and is not in a position to repeal it

All I'm saying is, if upgrading your insulation, eliminating draughts, installing a heat-pump, and leaving your heating on all the time produces a "low energy house" -- then upgrading your insulation, eliminating draughts, installing a heat-pump, and not leaving your heating on all the time will produce an even lower-energy house.

The only way it might be otherwise, is if the heating system itself is more efficient at low outputs than high outputs. This is, to a certain extent, the case with heat-pump systems (it certainly isn't with conventional fossil-fuel or electric heating), but even so, if the reheating after a cool-down is gradual enough, it still seems likely to be a win.

Peter

Of course it is. More efficient system, better insulation, etc. But your present system would consume even less energy than it does now if you took steps to minimize the temperature differential inside to outside. Maybe not a LOT less, with a hyper-efficient house like yours, but less.

Yes, your system "works harder" for a little while to bring a cold house back up to normal temperature than it does while maintaining a constant temperature. But all it is doing is replacing the heat lost during the period when it was "working less hard" because of the reduced temperature differential. Lower differential = less heat lost = less heat to replace.

As you [Scotty] said, "You cannae change the laws of physics."

tanstaafl.

I'm about to do some major renovations to my house - adding a second floor of about 900 sf - so this discussion is very timely!

Thermodynamics was a very long time ago, but I have to agree with the folks who say turning down the thermostat at night saves power (on most systems). Especially on cold winter nights when the delta is greater. (One system that probably wouldn't see savings is a heat pump with an auxiliary heating element when the element kicks in - it's way less efficient than the heat pump alone.)

I currently have a typical hvac setup for my neighborhood: forced air with a natural gas furnace and elec AC; gas also heats our water. My current (web enabled) programmable thermostat is set in winter to heat the house to 68 at wakeup, drop back during the day, up again in late afternoon and evening, and down to 60 at night. Our house is older, and loses heat all too quickly in the winter, even in the not-too-terrible climate of greater DC. It's hard to compare winters or factor the water out, but our gas usage did go down after adding the programmable thermostat about four years ago.

We're investigating geothermal as part of our renovation. The startup costs are high, but there are some nice tax credits and we intend to be here at least 15 more years. And I hate the noise of my AC's compressor in the summer!

-jk

This is starting to remind me of that airplane on a treadmill thread...

I think the place you've been mislead may have been from the marketing fluff for your hyper-efficient heat-pump. Those systems have one small flaw, in that they can take a very long time to warm up a house (compared with other methods). So I imagine the salesfolks might feed lines like "no need to turn down the thermostat".

Even with that slight "flaw", ground source heat pumps are probably the very best way to go, when circumstances permit.

Where we live here, people with systems like yours need a back-up heat source for REALLY COLD times. A few days a year, nothing more.

Cheers

This could be. Floor heating is indeed a 'slow system' when it comes to heating up a house. On the other hand, I find the whole 'not having to worry about the thermostat' thing very comforting.

It's not that I don't want to believe you, but I'll certainly look further into it, this is interesting stuff.

I would be interested in more data on that thermostat. Last time I looked into it, most were of questionable calibre.

I wonder what is available now (for North American FAG heating system).

It's nothing fancy - a Venstar thermostat on an Insteon home automation rig. It works adequately.

-jk

EcoBee which has been brought up before, seems to offer control of more HVAC options, including multi-stage heating and cooling, HRV and central Humidifiers and Dehumidifiers.

I was actually thinking about this earlier. Dealing with a forced air system myself, I wonder if the nest (or another thermostat) could learn to adjust for this effect in a forced air system. Since starting and stopping the heating elements and fan over and over takes energy and is also annoying, it seems like it might be worth doing. Caveat: I'm not really familiar with the space, so there may be more standard products out there that already do this...

If you're doing a 2-wire system (ie just 2 wires going to your transformer and relay), you need to be careful as to the relay you use as nest trickles current through the relay coil, without closing the relay, to charge the internal battery.

If you're supplying a setup to anyone, the best idea is to ensure you have a common wire (ie, both sides of the transformer come out to the thermostat end, as well as the one wire that goes through the relay back to the transformer) - ie run at least 3 wires vs just 2. This is far by the better solution if you have the choice.

Let me know my PM if you want any more specific advice

Yes. Obviously they have a penchant for secrecy (which has worked quite well, given the amount of publicity they got mostly for free), so I can't tell you details but I did the initial designs, devboards, bringup, etc.

Good to see that it's bought some attention to an unloved area of the home!

Any hope that Nest will build a third-party hacker-friendly ecosystem around itself?

I guess that's a wait-and-see...