How fast should interior temperature drop?

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I know that question is too open ended to really answer. Each house will behave different. Each room will behave different depending on windows, exterior walls, etc. But I'm just looking for some sort of estimate.
Say it's -15C outside and +20C inside at midnight. If i left the furnace off (gas forced air) all night, what kind of temperature could be expected at 8am? I guess a "If it drops lower than X then you may have issues" is the kind of answer i'm looking for.
The reason i ask is because i'm an insanely over-critical person who wants to squeeze every bit of efficiency out of this house until the next one which i will have built properly. :-)
I'm in the middle unit of a 3 unit townhouse. So i share a wall with each nieghbour on the sides and only my front/back walls are exposed to the elements. However I have a 2nd story which neither of my neighbours do, so upstairs is exposed on all four sides. Because the vast majority of my main floor walls are shared with neighbours i figured that my heat loss should be significantly better than any detached house. But i have nothing to compare against. Buit new in 2001 according to ontario building code. I think that's r-34 in the attic and only r18 in the walls. All the windows are double paned. I can't feel any air leakage and no condensation or ice ever forms between panes but the bedroom windows ice up like hell every night along the bottom edge. (35% humidity inside at 19C with -15C outside).
I haven't done any tests to see how quickly the temperature drops but i was just looking for some sort of general numbers to compare against once i do.
any thoughts appreciated.
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On Mon, 26 Jan 2004 19:07:34 GMT, kevins_news

One more comment. 1700 sq feet plus basement is the size.
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kevins_news wrote:

You are right it is impossible to answer with any degree of accuracy. But a few questions, the wall are ONLY R18? Second, why don't you just turn the furnace off and see how far it drops? Third, why do you care? And none of those are smart-ass comments. I don't understand you point. Turn off the furnance and you find out. Anything below about 60 F (? 14 C?) would be too low for efficiency in reheating, so you just set your thermostat for 60 F for efficiency. Personally, I don't want my house going below 65 F even to say a little money.
Finally, even in my 1500 square foot ranch which is fully exposed and the walls are only R-11 insulated, the temp would not drop much below 60 F overnight with those outside temperatures. I would suspect that your house would not drop from 20 C to less than 14 C overnight unless you had a real strong wind blowing.
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message

I keep seeing claims like this. I've never seen any believable documentation to back it up. I know of no reason why it should take more energy to heat it back up than you save by turning it down farther. (With the possible exception of a heat pump system.)
Bob
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Bob wrote:

Sounds logical, and I don't know for sure, just what the utilities recommend. However, I'm not going to let my inside temp drop to 60 degrees when it is 15 outside, it just makes it too uncomfortable and too long a period to reheat.
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I drop 3 degree F an hour at 15F outside, I have alot more insulation than you 2-3x.
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m Ransley wrote:

Whoa! You need some insulation. 3 deg x 8 hours is 24 degrees would result in 48 degrees F if you started at 72 F. My garage which isn't heated and starts at a maximum of 48 degrees seldom gets below 40 degrees. In fact it never got below 38 degrees when the temp dropped to -1 degree last month.
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Well George at 1800 sq ft my gas bill was 50 last month , You do temp drop at 15 f . I have 3x the insulation you do.
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Much lower than mine, but then a direct comparison is not possible without giving all the parameters-- running temp, night temp, solar values, degree heating days, gas prices, etc.
I'm confused though, you have lots more insulation than I do and your house loses heat faster than mine?
m Ransley wrote:

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George E. Cawthon wrote:

Hmmm. Keep in mind that the rate of heat loss isn't going to be linear with time. It's going to be proportional to the temperature difference -- which itself is dropping with time.
Just to be argumentative...
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Robert Barr wrote:

Right, the drop per hour will become less and less. I assumed he meant an average of 3 degrees per hour over the night, but 3 degree drop in the first hour or two is possible for my house. Still there is no way my house will drop to 48 degrees in 8 hours at an outside temp of 15 degrees. Maybe with an outside temp of 0 or -10 degrees.
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In my house I get a drop of about 5 to 8 degrees F. Or you would end up at 15C Ed
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Well, it will drop as fast as it drops. "Should" doesn't enter in. You are talking about very modest low temps (my god, -15C is only like +5F), and your insulation is thoroughly adequate for that kind of cold, if indeed it meets code, which you are assuming. I doubt you have ripped a wall open.
There are only two considerations in my opinion:
1. Don't let it get so cold in the house that you get massive condensation on the windows 2. Don't let it get so cold that the furnace can't restore comfort in a reasonable length of time.
And if you are waiting till you get up to turn on the furnace, that time is probably shorter. In a well-insulated house (if yours is code, it is well-insulated), there is very little payoff in letting the place get way cold. A 5 degree setback day to night is plenty. You should buy yourself a setback thermostat and learn how to run it, and let it take care of things for you.

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My first test ,3 degree drop at 15 f was with wind, Last night at 20 F no wind it went down 1 F, 700 sq first floor 2 sliders , 1 7x5 tripane and 3 windows. Lot of glass.
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kevins snipped-for-privacy@rogers.com wrote:

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As i said It was windy on the first test only an hr test, second test no wind a 1 hr test 1 degree drop. I have r 100 - 110 in attic , apx r 30 walls, a fully insulated basement r-14 , and r7.2 Under, concrete floor. New windows and doors, dual - tri pane. For 1800 sq my gas bill was 50 total utilities 1 yr with central air 600. So Id say im doing alot of things right. Did your test run with any incandesant lights on , A 100 watt light bulb is a 90 watt heater putting out 10 watts of light, 10 bulbs 900 watt around 3000 btu hr. Cant have a fair test that way. Was it on a sunny day . Whatever. My redo cut my bills 75 -80 % and my temp drop is low as my bills prove
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m Ransley wrote:

You've got insulation that is way beyond most of us. And you confirmed what I was saying, "you have to know all the factors to make a comparison." Sorry I missed the wind in the first comment and it would sure account for a much more rapid heat loss. No lights on in my house at night, but the comment on my garage failed to note that I have a freezer that would provide some heat. Don't know how much. We heat a well insulated building about 8' by 8' with a 7' ceiling (one window and door) with a 100W bulb. In most cases it stays at 40-50 degrees with less than 12 hours of light time down to 5-10 degrees. Below 0 we usually run a small heater set at 45 degrees but it doesn't come on very often.
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On Thu, 29 Jan 2004 02:16:45 GMT, "George E. Cawthon"

I did a test the other night. Unfortunatly it got warmer outside than when i originally asked.
Pretty consistently 20F oustide all night. Not windy when we went to sleep but very windy in the morning. Furnace turned completely off.
Temp at midnight was about 66F in bedroom and 66F near thermostat. Temp at 8am was 60.5 in bedroom and 61.5 near thermostat.
Kevin
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At 20 you only lost 5 degrees in 8 hrs , Im loosing 1-1.5 at 20 no wind, but i also only tested one hr. Temp wont fall at a linear rate , but i do have alot of glass and a fireplace that is really an airconditioner as well as the stairway pours cold air downstairs. And I only tested the living room. My bills are real low but I see I have problems I havnt been able to fix. Old leaky houses can be a pain.
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