On Dec 21, 1:01*pm, Wayne Whitney wrote:
On 2008-12-21, wrote:

Last night, the temp here in Iowa dipped down to around -10 with a -35
degree windchill. *This morning I noticed the temp was about 4 degrees
off of where I had the thermostat set (It was set for 68, but the
heater was running continuously to keep it at 64). *Right now, the
temp has risen to about -5 and the heater is running continuously to
keep the house at about 66-67 degrees.

When you look at the windchill, I think this is probably one of the
coldest days since I moved in 2 years ago.. Does this sound normal, or
could there be an air flow problem? *The amount of hot air coming out
the vents seems to be normal compared to warmer days.

A heating system should not be designed to maintain the set point 100%
of the time. *Instead it should maintain the set point, say, 99.5% of
the time. Otherwise, that extra capacity that you installed to get
from 99.5% to 100% would only be used 0.5% of the time, which is not
cost-effective.

So if your heater is only outpaced once every two years, there's no
problem with the sizing. *Others have mentioned ways to reduce your
house's heat loss, which would decrease the frequency that your
current heater is outpaced by the cold weather.

Cheers, Wayne


Exactly what I was thinking. If it rarely gets down to -10 with
high wind, then having the furnace sized to handle a max 74 deg temp
differential sounds reasonable. At my house that would equate to
keeping it 70 inside, with it -4 outside. Here, I haven't seen that
low of a temp in decades.

If it does get that cold frequently and you want it to be warmer, you
probably need a larger furnace. But with that comes increased cost,
both upfront and potentially operating cost as well. Before I worried
about that, I'd carefully check insulation, ducting, widnows/doors, etc