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Does a house lose heat faster at colder temperatures?


VCH

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Here's a question for all you physics people.

 

Will a house maintained at for example 25C loose heat more rapidly than the same house maintained at 14C? Let's say the outside temperature is -30C.

 

I recently had a natural gas furnace serviced, and the technician said that it makes no difference whether the home's temperature is maintained at 14C or 25C, heat loss will be the same no matter what and the furnace will use the same amount of gas.

 

Heat moves from warmer to cooler objects, but does a larger temperature difference (between the cool and warm objects) mean faster heat loss?

 

My point here is will maintaining the home's internal temperature at 14C save energy as opposed to maintaining it at 25C?

 

Side question: What about the conventional wisdom that furnace thermostats should be turned down at night? Would turning the thermostat down 12C then up to 14C lead to more energy use bringing the temperature back to 14C, then would be used if a constant 14C was maintained?

 

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Originally Posted By: VCH
Side question: What about the conventional wisdom that furnace thermostats should be turned down at night? Would turning the thermostat down 12C then up to 14C lead to more energy use bringing the temperature back to 14C, then would be used if a constant 14C was maintained?


The furnace is turning itself on and off every so often to maintain the temperature near what you set the thermostat to anyway: effectively, it works in cycles of letting it cool down a little and then bringing the temperature back up. So leaving it at a lower setting for a while and then turning it back up will save energy.
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It takes energy to heat the air in the house so having to raise the temperature more means more energy is needed. Pure physics.

 

Now if you have a high thermal mass, lots of material inside the house heated to that higher temperature, then it takes longer for the house to cool down because the cooling air draws heat from the thermal mass. That's the same reason why they tell you to keep a refrigerator full of food.

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I just can't imagine any way that this technician's statement could possibly be right. It is conceivable that a modern house is so well insulated that the difference in fuel bills for maintaining 25 versus 18 is surprisingly small. But no difference at all?

 

Put it this way. If the exact same fuel could expenditure could just as easily heat a house to 25 as to 18, in -30, then it ought to work at outside temperature 18, too. But when it's 18 outside it takes no fuel at all to keep the inside at 18. Therefore if 25 takes no more fuel than 18, you must be able to keep your house at 25, when it's 18 outside, without burning any fuel. That's seven miraculous degrees of added warmth at zero cost. Handy in those nippy fall evenings. And if you can get 7 degrees of extra warmth for absolutely nothing, why not more? Why not 50?

 

The truth is, you can't. It takes more fuel to maintain 25 than to maintain 18.

 

Against -30 outside, the difference is a raise of 55 against a raise of 48, which is a much smaller proportional difference than the difference in the feeling of warmth you get from 18 versus 25. Try watching your gas meter for a few days at one setting, then a few days at the other, and see how much difference it actually makes. But record the outside temperature as well as you can, at least high and low to get a crude 24-hour average. See whether the gas burnt is proportional to the difference between outside and thermostat. It should be, at least roughly.

 

So turning the heat down at night does also save fuel, at least a bit. If the fuel cost is proportional to temperature difference, then a 5 degree drop out of a 50 degree inside-outside difference should save 10% of your nightly fuel, so a few percent of your 24-hour fuel expenditure. You can decide whether that's worth cold toes when you get up in the night.

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If we assume that all the heat loss in your house is from conduction through the walls, then the energy lost would be directly proportional to the temperature difference between your house and the outdoors. So 14 vs 25 degrees at -30 outdoor temp would save about 20% in energy costs.(I'm not sure how accurate this is, I suppose it depends on how good the insulation is in your house and whether you leave your doors open a lot or something.)

 

The technician doesn't know what he's talking about, because there's certainly no way that there's zero difference between 25 and 14 degrees.

 

Edit: Actually now that I think about it I'm a bit more confident about that 20% number. I can't think of any way that your house could be losing heat that isn't dependent on temperature difference, and the difference is always going to be 20%.

 

Edit2: Conduction not convection, although the statement still holds true. Niemand threw me off by bringing up Newton's law. tongue

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Thanks guys.

 

I'm not so sure about turning the furnace down at night though. I suppose it depends on how far and how cold it is outside. If it was turned down too far it might take more fuel to warm the house back up the next morning than to maintain that temperature through the night.

 

 

For example if you turned it down to 1C and then back up to 20C the furnace would likely run longer to get back up to 20C than it would to maintain 20C.

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Originally Posted By: VCH
Thanks guys.

I'm not so sure about turning the furnace down at night though. I suppose it depends on how far and how cold it is outside. If it was turned down too far it might take more fuel to warm the house back up the next morning than to maintain that temperature through the night.


For example if you turned it down to 1C and then back up to 20C the furnace would likely run longer to get back up to 20C than it would to maintain 20C.

I like sleeping in a cold house, especially with like a foot of blankets. It feels good, but I don;t want to wake up in the morning.
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Originally Posted By: VCH
Thanks guys.

I'm not so sure about turning the furnace down at night though. I suppose it depends on how far and how cold it is outside. If it was turned down too far it might take more fuel to warm the house back up the next morning than to maintain that temperature through the night.


For example if you turned it down to 1C and then back up to 20C the furnace would likely run longer to get back up to 20C than it would to maintain 20C.


Again, this is not how physics works. It will always, always, always take more energy to maintain your house at a higher temperature than at a lower temperature, because your house is losing heat energy at a greater rate when it's a higher temperature, and that energy has to be replaced somehow. However much fuel it takes to warm the house up the next morning when you let it get cold, it will take more fuel to stop it from getting cold in the first place.

Consider two possibilities:

1) You let the house cool down to 1 degree. Your heater then uses energy to heat the house up by 19 degrees, to 20 degrees, when you turn it back up.

2) You keep the thermostat at 20 degrees. This means that every time the temperature falls to 19 degrees, your heater has to use energy to heat it up by 1 degree. But this will happen more than 20 times throughout the night. (If it doesn't, then in scenario 1 the house would never have cooled down to 1 degree in the first place.) So the furnace has to effectively heat up the house by 30 or 40 degrees throughout the night, because the house is constantly losing energy.
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Originally Posted By: Thuryl
Originally Posted By: VCH
Thanks guys.

I'm not so sure about turning the furnace down at night though. I suppose it depends on how far and how cold it is outside. If it was turned down too far it might take more fuel to warm the house back up the next morning than to maintain that temperature through the night.


For example if you turned it down to 1C and then back up to 20C the furnace would likely run longer to get back up to 20C than it would to maintain 20C.


Again, this is not how physics works. It will always, always, always take more energy to maintain your house at a higher temperature than at a lower temperature, because your house is losing heat energy at a greater rate when it's a higher temperature, and that energy has to be replaced somehow. However much fuel it takes to warm the house up the next morning when you let it get cold, it will take more fuel to stop it from getting cold in the first place.

Consider two possibilities:

1) You let the house cool down to 1 degree. Your heater then uses energy to heat the house up by 19 degrees, to 20 degrees, when you turn it back up.

2) You keep the thermostat at 20 degrees. This means that every time the temperature falls to 19 degrees, your heater has to use energy to heat it up by 1 degree. But this will happen more than 20 times throughout the night. (If it doesn't, then in scenario 1 the house would never have cooled down to 1 degree in the first place.) So the furnace has to effectively heat up the house by 30 or 40 degrees throughout the night, because the house is constantly losing energy.


Not quite correct, since I believe that most furnaces work in a different manner. It will rarely reach the temperature you set- if you heat it to 20 degrees Celsius, it will initially heat up to about 22ish, and then shut off until it hits 18ish, leaving to a relatively narrow range of time where it is actually active. However, wthis does not really happen at night, since your houses are so incredibly well insulate that even if you shut off your thermostat entirely, it would probably take at least a day to get unbearably cold (presuming that you have windows close, blinds and curtains drawn, and decent windows and insulation. So, I just have my thermostat shut off entirely at 10, and then turn on at 5, giving it a whole hour to raise the temperature maybe, maybe 20 degrees (I'm mixing my metaphors here, I'm in Fahrenheit now), and making the house nice and toasty when I wake up. Plus, the fact that you are not running the furnace at all for 1/3 the day also cuts down drastically.
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Well the point wasn't really to demonstrate the workings of a thermostat. Anyway Thuryl is right of course; just imagine if you were to go on vacation for a week and turn off the heat while you were away. When you return you have to heat the house back up to 25 degrees or whatever temperature you previously had it set at, but obviously you saved energy in not running your heater for an entire week. The same applies when you turn it off for a single night.

 

Edit: Well that's what I get for taking a phone call while posting. Anyway.... Yay energy balances!

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The other thing about the night is that you aren't awake. You probably have blankets and can comfortably sleep in a house colder than what you'd prefer while awake. You don't want the temperature to plummet, but you can save a bit by turning it down. By the same token, if you're working nine to five and your thermostat permits, you might as well have a cold house from nine to, oh, four.

 

—Alorael, who has an easy way to understand why more energy is lost from a warmer house: infrared. If warmer things are brighter in the infrared spectrum, they are emitting more light. That light is energy. More energy is, therefore, lost.

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Originally Posted By: Perfectly Understandable (Alorael)
The other thing about the night is that you aren't awake. You probably have blankets and can comfortably sleep in a house colder than what you'd prefer while awake.
Which is probably one reason why cooler nighttime temperatures are sometimes referred to as "good sleeping weather."
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Originally Posted By: Lord Safey
Also having an insulated house saves a fortune on cooling or heating while it may not be worth it tearing up your house (spending lots of money to do so) to insulate your house, it is something that should defiantly be considered when buying one.
Definitely worth looking into, especially since we're eligible for government money now when we re-insulate our homes.

But ugh, not fun work. A couple of Saturdays ago, I was spraying insulation in a friend's attic. The old 'insulation' was several inches of wood shavings. Not fun getting slivers when you're crawling/rolling around in a low-clearance area.
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Originally Posted By: Dintiradan
Originally Posted By: Lord Safey
Also having an insulated house saves a fortune on cooling or heating while it may not be worth it tearing up your house (spending lots of money to do so) to insulate your house, it is something that should defiantly be considered when buying one.
Definitely worth looking into, especially since we're eligible for government money now when we re-insulate our homes.

But ugh, not fun work. A couple of Saturdays ago, I was spraying insulation in a friend's attic. The old 'insulation' was several inches of wood shavings. Not fun getting slivers when you're crawling/rolling around in a low-clearance area.



I know what you mean. I've been rolling around in a crawl space for the last week. Sharp rocks hurt! I think that government programme is over in February. But it was one of the more useful ones for sure.

The ecoENERGY Retrofit seems to be still running.

Frozen Feet how is this topic your area of expertise?
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  • 1 month later...

Lacking central heating isn't actually so terrible if you have adequate non-central heating. At the very least, you can be okay with some more layers.

 

Lacking air conditioning is rather area-dependent. And it's also very much a cultural thing. Israel, bafflingly, is largely AC-free, or was about ten years ago. Israel also has a whole lot of broiling desert.

 

—Alorael, who can only assume that it's dry heat and that any misery is clearly God giving everyone and anyone just deserts. In more ways than one.

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My dorm room at university has no heating or cooling other than plug in heaters or fans. However being a concrete block building it's unberably hot until the temperature falls below -15C. Anything higher than that and I have a fan going. Its ridiculous to have a fan going in January and have to walk around naked. The rooms are 12 by 8 feet, so that probably accounts for the heat buildup as much as anything. Add a hotplate computer and TV into that prison cell and you have a very hot room.

 

In my parents home I have to keep a jacket and toque on inside most of the winter because the inside temperature is normally 12C (cold).

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Originally Posted By: Alorael
The New York Times has an article about opting not to have heat.

Pure Madness. And this coming from a Wisconsinite.

Originally Posted By: Alorael
—Alorael, who can only assume that it's dry heat and that any misery is clearly God giving everyone and anyone just deserts. In more ways than one.

/facepalm
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