How fast should interior temperature drop?

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.

Reply to
kevins_news
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One more comment. 1700 sq feet plus basement is the size.

Reply to
kevins_news

I drop 3 degree F an hour at 15F outside, I have alot more insulation than you 2-3x.

Reply to
m Ransley

In my house I get a drop of about 5 to 8 degrees F. Or you would end up at

15C Ed
Reply to
Edwin Pawlowski

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.

Reply to
donald girod

kevins_news wrote in news: snipped-for-privacy@4ax.com:

OK. Here's my thoughts... Fist, one needs to know the "U" factor of the entire composite of the walls and ceilings in order to compute any heat loss. Second, wind speed around the house, and third (in the old days anyway) crack sizes anywhere in the house. I've seldom seen two houses that will leak heat at a given equal rate. There's always one or two things governing the heat loss of a building. You could actually try some night when the time is right but to "suggest" a guideline would be impractical. Lastly (and just a friendly jab at our wonderful Canadian neighbors whom I dearly love and respect) heat losses and BTU calculations are actually figured using the Farenheit scale simply because it's more accurate. 0 to 100 = 100 degrees but 32 to 212 = 180 degrees. It's more exact. Now I'll shut my mouth but I love you folks up north! Hell, you're only a mile from my house and I can hear your cows mooing at night!

Doug

Reply to
Doug

Best way is to try it one night and chart the thermo.

Reply to
Joseph Meehan

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.

Reply to
George E. Cawthon

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.
Reply to
George E. Cawthon

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.

Reply to
m Ransley

Your just going to have to try it for yourself. In my center unit townhouse I have nobody living next door, but I can turn off my heat even on a very cold night and it never gets below 50 degrees. Actually, the only time I really run the heat is first thing in the morning, so I can step out of the shower into a warm room for a few minutes. And my house has no insulation in it. I'm sure that having a basement helps.

Reply to
Childfree Scott

When the house was built they provided numbers for what they were using in the walls and ceiling. I checked up on ontario code and saw that's what they were using. Reputable builder so i don't suspect them of using less. And -15C was just referring to last night. A cold snap would be consisten -25C. Still nothing like when i lived in Winnipeg Manitoba though. Weeks of -30-35C were normal. (-22-31F)

We do have a setback thermostat. We have it go up to 66F in the mornings, evenings, and on weekends when we are home (we like it colder than most it seems). I set it back to 59F at night and during the day while we're at work. But i have no idea how fast it drops and how often the furnace turns on during the day to maintain that 59F since i'm not home.

The reason i asked was to get some sort of guideline. Assume i do the test and find out that at night the temperature drops from 66F to 59F in half an hour and my furnace spends a good portion of the night on to maintain 59. Without any outside opinions i might assume that's completely normal. But thanks to what you all have said, I would suspect that something was wrong.

I'll turn the thermostat off and get myself some numbers.

Kevin

Reply to
kevins_news

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.
Reply to
m Ransley

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

Reply to
Bob

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...

Reply to
Robert Barr

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.

Reply to
George E. Cawthon

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 c>

Reply to
George E. Cawthon

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.

Reply to
George E. Cawthon

It wont drop that fast unless the outside is well below 0 and you have incredibly poor insulation. Many electronic thermostats put the run time in memory. Mine is kind of cheap, so it only shows run time for today, yesterday, and since the thermostat was set to heat. None of that means much though since a big unit would naturally run longer than a small unit while providing the same amount of energy. Mine runs about 5 hours a day set on 72 during the daytime and on 65 for about 8 hours at night with temp highs of 35 and lows of 20. With your low thermostat settings I would suspect your furnace runtimes would be much less.

Reply to
George E. Cawthon

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
Reply to
m Ransley

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