Thermodynamics and the efficiency of heating a house

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wildgunman
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Thermodynamics and the efficiency of heating a house

Postby wildgunman » Thu Jun 02, 2011 2:48 pm UTC

Someone recently asked me a question about the efficiency of heating a house up to temperature. They said that people in the north will often leave the heat on for the day when they go out, because its wastes more energy turning the heat off, letting the room cool down, and throttling the heat all the way from 35 degrees back to room temperature than it does to just keep the house close to room temperature all day. This was not a question of letting the pipes freeze or anything, just a conjecture that for reasonably short periods of time (say 2-8 hours), it used less energy.

At some gut level this seems to make sense, but I couldn't quite puzzle out any scientific reason why it should be the case. If the system was homogeneous, then it should be rubbish. The heat into the system must match the heat flowing out of the system, so it's must be more efficient to turn the heat off.

If it was true, my only guess as to why would be that it has something to do with the diffusion of heat from the furnace. My first suggestion was that rapidly heating the house back to temperature required heavy use of the HVAC circulation system, whereas keeping the house in a rough thermodynamic equilibrium all day could just be achieved through passive heat diffusion. However, even if there was a difference in the strain on the circulation fans, I can't imagine that they would draw that much more energy. From what I can tell, climate control systems don't optimize the fans.

My second more convoluted suggestion was that, since heat is being lost near the walls and ceiling, rapidly heating the house involves creating large pockets of much hotter air near these bleed points. This is where my knowledge of thermo sadly gets a little fuzzy. Since these are substantially hotter than the center of the room before a rough equilibrium occurs, you get more heat loss to the outside as you try to speed up diffusion to the center of the room. My guess was that this causes more net energy to be lost from the system than allowing the system to slowly diffuse heat across the heat gradient. Is this intuition correct, and would it be rendered irrelevant if the hot air pockets began at the center of the room, roughly equidistant from the interface at the walls?

Has anyone heard a similar statement? Is there any other reason why it might be true, or is it just junk?

gorcee
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Re: Thermodynamics and the efficiency of heating a house

Postby gorcee » Thu Jun 02, 2011 3:49 pm UTC

There are a number of reasons for leaving the heat on, but the specifics often depend on the type of heating that you have, and what the cost efficiency is.

One of the most important reasons for not totally cycling off the heat (or A/C), is that heating (A/C) units are designed with specific duty cycles in mind. If you start operating them at duty cycles that they are not designed for, you wear them out faster. This has nothing to do with thermodynamics, really, just the material properties of the actual devices.

There's also other factors to consider, like during the day it's warmer out. So you bleed less heat / retain more warmth / get solar heat so you don't have to use as much fuel. If you get home at 5 or 6, it's already dark in the wintertime.

Also, it sucks driving home in the cold and coming home to a cold house.

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Re: Thermodynamics and the efficiency of heating a house

Postby crates » Thu Jun 02, 2011 6:02 pm UTC

Agreed, there is no thermodynamic reason per se but the reason is increased machine wear. For example, I worked a year in Fort McMurray, Alberta, Canada, which I consider to be practically the top of the globe and you run your car heat sometimes a full half hour before starting it in winter. The reason for it is again the machine wear. Plus, getting the machinery up to temperature again is a difficult task and thus, it is circumventing by just leaving it running all the time.

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Re: Thermodynamics and the efficiency of heating a house

Postby Moose Hole » Thu Jun 02, 2011 8:57 pm UTC

I have a heat pump that is more efficient than the auxiliary heating coils. However, the heat pump is only efficient if the outside temperature is over 35F. When the temperature drops below that point, the heating coils kick in. So if the heat was started during the day when the temperature is higher, it would be more likely that I could use the heat pump, which will save energy as long as my heat loss isn't too high.

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Re: Thermodynamics and the efficiency of heating a house

Postby Technical Ben » Thu Jun 02, 2011 9:38 pm UTC

Insulation would make a huge difference too. My house is very old (well, IMO, 1980s :D) and has useless insulation. Where as some newer houses could stay warm on the existing warmth without any additional heating. So YMMV.
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wildgunman
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Re: Thermodynamics and the efficiency of heating a house

Postby wildgunman » Thu Jun 02, 2011 10:52 pm UTC

Moose Hole wrote: So if the heat was started during the day when the temperature is higher, it would be more likely that I could use the heat pump, which will save energy as long as my heat loss isn't too high.


This explanation actually makes a ton of sense. Because of the problems with icing, heat pumps have serious problems reaching efficiency from a cold start in freezing weather. If you keep your heat pump running during the day, pushing small amounts of hot air into the house the heat pump runs at max efficiency. As you said, throttling it up from cold is not only less efficient, but it also necessitates turning the less efficient direct heat backup heat system on.

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Re: Thermodynamics and the efficiency of heating a house

Postby Mokele » Fri Jun 03, 2011 3:37 pm UTC

This is the reason bears don't truly hibernate. Yes, I know, you've heard they do, but that's wrong - they sleep through the winter with only a modest decrease in core body temperature, while in truly hibernating animals like mice, the body drops to near-ambient temperatures.

Basically, if you're a little mammal (or little house), you have such a high surface:volume ratio that you burn a LOT of energy just keeping yourself warm, even in ideal conditions. You also have very little mass to heat up. So if you turn down the body heat over winter, you save a LOT, and while you have to ramp up your body's heat generation extra-high to warm back up, the increase isn't enough to offset the savings.

For big mammals/houses, there's a low surface:volume ratio and lots of mass, so you don't have to spend as much to keep warm (per kg) and it takes a lot more time and metabolic effort to cool down and warm back up.

In general, if you look at mammals in cold climates, you'll find a size limitation on true hibernation - once you get above a certain size, it's no longer economically justifiable. The longer and colder the winter, the larger that maximum size is (with the caveat that big mammals are also more capable of migrations).

A lot of this is that heat isn't always produced at the same efficiency, and doubling a furnace's output (biological or mechanical) rarely requires only doubling the input. Real systems are lossy, non-linear, and complex.
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Re: Thermodynamics and the efficiency of heating a house

Postby Izawwlgood » Fri Jun 03, 2011 3:50 pm UTC

I perused this thread yesterday, and oddly enough, when looking at an apartment yesterday afternoon, was told this exact thing. I'm not sure if it's an old wives tale, but the landlady said that because of the type of furnace, steam generation is easier to change in smaller increments than in one massive spat, and that the heating costs would be significantly cheaper if you nudge it down to say 62 in the morning, and back to 68 in the afternoon, as opposed to cranking it down to 55 in the morning, and up to 73 in the afternoon.

So I dunno. I can envision a scenario like mentioned earlier where the gradual increase in heat is easier on the system and on the house.
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Re: Thermodynamics and the efficiency of heating a house

Postby Technical Ben » Fri Jun 03, 2011 5:53 pm UTC

Well, the only true way to find out is... SCIENCE!
(Test, repeat, rinse etc) :D
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Re: Thermodynamics and the efficiency of heating a house

Postby Izawwlgood » Fri Jun 03, 2011 6:02 pm UTC

I'm convinced my electric fee is just arbitrary numbers thrown onto a bill, given that it spiked in the winter (we have gas heat) when I had my computer shut off more than usual, and dropped by about 40% when I set up a 130 watt aquarium light and pump and started playing games on my computer more.

So, aside from looking at the rate of heating in a given space, I wouldn't trust my bill to be a trustworthy source of data :D
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Mokele
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Re: Thermodynamics and the efficiency of heating a house

Postby Mokele » Fri Jun 03, 2011 7:20 pm UTC

Izawwlgood wrote:I'm convinced my electric fee is just arbitrary numbers thrown onto a bill, given that it spiked in the winter (we have gas heat) when I had my computer shut off more than usual, and dropped by about 40% when I set up a 130 watt aquarium light and pump and started playing games on my computer more.

So, aside from looking at the rate of heating in a given space, I wouldn't trust my bill to be a trustworthy source of data :D


Something I've learned from scrutinizing my bills is that, at least here, they can and do vary rates at different times of year. Still, there should be something on your bill showing raw consumption of electricity or gas (kW-h or ft^3).
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Technical Ben
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Re: Thermodynamics and the efficiency of heating a house

Postby Technical Ben » Sat Jun 04, 2011 9:11 am UTC

You might find Izawwlgood that in winter you use more lighting? Or it could be fluctuations from bad prediction models of payment methods to the utility company. They use an "account" which they deposit payments into. Then, as you use the electric, they take the payments from the account. They can panic some times, and fill the account with loads of cash, then next month you use no electric. Later, like at winter, your using the electric, and your account gets low. The electric companies here in the UK are notorious for either leaving accounts with massive debts or with massive deposits. Then send a completely unexpected bill/refund when they realise your usage has changed.

I suppose it's not like a phone contract. A phone contract I pay £30 (for calls, not network use) front. If I use £10 worth of calls, I don't get £20 back. With a utility company, if you pay for a £30 tariff, and only use £10 electric, they tend to give you the £20 back. This can lead to some months of excess and some months of surplus. Over the cause of a year, you can drift wildly over or under your tariff.
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Re: Thermodynamics and the efficiency of heating a house

Postby Zamfir » Sun Jun 05, 2011 12:48 pm UTC

Something that might matter a lot: having a pleasant distribution of temperature around the house. It's relatively quick to heat up the air in house, but it takes far longer to heat up the walls and floors.

A house that is warmed through is more pleasant to live in, because it has an even, stable temperature. Air heating gives you above-average warm air at some places, and cold nooks and crannies where the heated air doesn't easily get to. So if you walk around the temperature around you changes, which is very noticable. And there are cold and warm drafts caused by the temperature differences and the HVAC. It also makes the heating react strongly on the exact position of the thermostat, which might be at a location that is heated sooner or later than other parts of the house.

So if you let your house cool down too much it'll take some time, easily hours, before the walls and other masses in the house are warmed through. If someone goes to work during the day and turns of the heat before leaving, they might spend most of their waking hours in the evening while the house is still warming up.

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Re: Thermodynamics and the efficiency of heating a house

Postby Izawwlgood » Sun Jun 05, 2011 1:54 pm UTC

Mokele wrote:omething I've learned from scrutinizing my bills is that, at least here, they can and do vary rates at different times of year. Still, there should be something on your bill showing raw consumption of electricity or gas (kW-h or ft^3).

Yeah, and the usage was up in the winter, when I had less electronics running, and up in the recent months, when I've had a large aquarium running and my desktop on more frequently. I doubt lights are a big consumer, as we use CFLs, and we're watching the same amount of TV.

The building is oddly subdivided, so I wonder if a couple outlets for our neighbors are on our meter. Although that said, we've got our own fusebox, so, who knows?

Technical Ben wrote:Then send a completely unexpected bill/refund when they realise your usage has changed.

That would be nice.
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Re: Thermodynamics and the efficiency of heating a house

Postby Velifer » Mon Jun 06, 2011 3:09 pm UTC

In my case, dropping the temp during the day more than a few degrees would be problematic. I have a heat pump, which only puts out air a few degrees higher than the set-point. When the outside temps drop too low for the heat pump to work, I also have a propane kicker to warm the place up. The system is sized to the square footage of the house, and can't really handle large swings.

The heat pump runs pretty cheap. The propane for the furnace is spendy.

The heat pump has long slow duty cycles to maintain the inside temps. They're designed to run that way. Dropping the temp then bringing it back up would cause the auxiliary propane furnace to kick on hard to bring the house temp back up. The heat pump alone couldn't do it (a lesson learned the hard way when the propane aux. went down one December). Fortunately, I also have a wood burner for ambiance and emergencies.

So for this setup, changing the thermostat set-point more than a handful of degrees over the day would burn quite a bit of money.
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Re: Thermodynamics and the efficiency of heating a house

Postby Moose Hole » Mon Jun 06, 2011 4:07 pm UTC

Izawwlgood wrote:it spiked in the winter (we have gas heat)
Maybe you used a bunch of electricity on running the rusted out fans for the heater or something. You could also have used more hot water than normal. Sometimes one heating element on the water heater goes out, and so the other one has to work very hard to keep up with the demand, and it ends up using a lot of energy.


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