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krw[_3_]

In article ,
says...
Fri, 04 Jul 2008 22:17:02 -0700 from Tim Smith reply_in_group@mouse-
potato.com:
In article ,
Stan Brown wrote:
My understanding is that the furnace uses less fuel overall to keep
the water in the heating pipes at a constant temperature than to let
it cool down by 10 or 15 degrees in the day time (when I'm at work)
and the night (when I'm in bed) and then reheat it.

My understanding is that this is true for hot-water heat but not for
forced-air, since it takes much less energy to heat air than water.

In general, if you have something that you need to be above ambient
temperature at time 1 and at time 2, it takes more energy to keep it at
that temperature than it does to let it cool after time 1, and heat it
back to the desired temperature at time 2.
[snip logical explanation]

Yes, that makes perfect sense -- basic thermodynamics.

The problem, and the difference between physics and engineering :-)
is that no device is not 100% efficient. What I don't know is how
that affects things.

"Efficency" of the mass doesn't matter. The heat used is
proportional to the difference in temperature between inside and
outside only. Any energy you put into latent heat will come back
later. Though as Stan points out that a longer running fire would
likely be more efficient.

I also wonder about effects on the house structure of letting the
indoor temperature fluctuate 10 or 15 degrees twice a day. And I
wonder about mold too -- maybe I misremembered what my mold guy said
and it's just that he said to keep the temp above 65 to prevent mold
formation -- though presumably that's more of an issue in spring and
fall since winter humidity is too low for mold.

As long as you don't get condensation mold shouldn't be an issue.
The issue here would be the minimum temperature not the difference.
However, there are things in the house that might not like the
constant temperature and humidity variation. IOW, there may be
other reasons to not go wild here.

It's not a simple question. The answer could well turn out to be,
"yes, setting back the temperature uses less energy, but it's worse
because ..."

Ok, find the "..." and we'll listen. So far, the cost argument
isn't cutting it. ;-)

--
Keith