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re: need ventilation info
30 apr 1999
d lesperance wrote:
>i've done my homework and figured out how many btu's i need to keep the
>house warm. but that's assuming i'm never exchanging any air.
stuffy...
>for the life of me i can't find anywhere how much energy is required to heat
>a cubic foot of air.
not much. air has about half the specific heat of water, which is about
4,000 times denser. one btu can heat about 55 cubic feet of air 1 f. so
you only need to heat up the air in your perfectly airtight house once
in the winter, assuming you close the windows :-)
a 32x32x16' tall house contains 16k ft^3 of air, and heating the air inside
(once) from, say 30 to 70 f takes (70-30)16k/55 = 11.6k btu, about the same
as a tenth of a gallon of oil.
>lots of stuff about airchangesperhour and cubicfeetperpersonperhour...
that's because it's hard to make an airtight house, and people need to
breathe. a very tight house might have 0.1 air changes per hour, ie the
indoor air leaks out and is replaced by outdoor air every 10 hours, which
needs to be warmed up and humidified. the house above needs 1.16k btu/h
because of the air leaks. think of this as another contribution to the
thermal conductance of the house (that sum of areas divided by r-values),
which turns out to be about the same as the air exchange rate in cfm
(since 55 is about the same as 60.)
people need 5-10 cfm per occupant of fresh air, which might come from
an air-air heat exchanger, eg a double-walled woodstove chimney with a
humidistat that opens a damper or turns on a small blower to move outdoor
air down the outside of the chimney. the house above leaks 1600 ft^3/h,
ie 27 cfm, enough for about 5 people, on a typical day.
with real r20 walls and ceiling and 80ft^2 of r4 windows, its thermal
conductance would be 80ft^2/r4 = 20 btu/h for the windows plus about
3000ft^2/r20 = 150 for the walls and ceiling plus 27 for air leaks, a
total of 200 btu/h, so on a 30 f day it needs about (70-30)200 = 8k btu/h
to stay warm, three occupants might contribute 1000 btu/h when they are
present. the rest might come from 7000/3.41 = 2 kw of electrical power
used inside the house, ie an unfrugal electrical energy consumption of
at least 2kwx24h/dx30d/m = 1400 kwh/month.
nick
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