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
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.
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.
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
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
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
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,
I'm confused though, you have lots more insulation than I do
and your house loses heat faster than mine?
m Ransley 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...
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.
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
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
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.
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.
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.
HomeOwnersHub.com is a website for homeowners and building and maintenance pros. It is not affiliated with any of the manufacturers or service providers discussed here.
All logos and trade names are the property of their respective owners.