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re: furnace size
18 dec 1999
pondered insufficiently, and wrote:
>...you propose to heat 1700 sq in mo at -20...
you have two opportunities to disagree here :-) foistly,
does the house described require about 40k btu/h at -20f?
>with a car-radiator sized coil supplied by 150 degree
>( at peak ) water. have you ever taken a step back
>from your wonderful calculations ( which i have enjoyed
>thoroughly over the years, btw ), and taken a good hard
>reality check ?
sure. here's two good hard reality checks:
1. a car that idles while burning 1 gallon of gasoline per hour on
a hot day (a total of 120k btu/h) is doing something like that, and
2. magicaire's $150 all-copper 2'x2' shw 2347 duct heat exchanger
transfers 45k btu/hour between 125 f water and 68 f air at 1400 cfm
with a 0.1" h20 pressure drop. that's a thermal conductance of about
800 btu/h-f, or (150f-68f)x800btu/h-f = 66k btu/h with 150 f water.
>>some ashrae-standard 5.41 pound 58.53 btu/h bunnies or
>>6.61 pound 68.02 btu/h cats could help as well.
> i have the mass equivalent of maybe 5 ashrae standard cats,
>contained in 3 ashrae standard cat-containers.
poor critters. funny, you don't look chinese...
>even given the lessening of radiative cat surface / mass,
>i should probably have to leave my windows open all winter
>just to cool the place down, huh :-)
no. three cats are not enough. we've already done that calculation.
three horses or cows could help a lot more. swiss people help heat
their houses with cows downstairs and a vapor barrier (but not much
insulation) above them.
> do you get better heat transfer if you shave them ? :-)
of course. the heat transfer rate is higher, and so is the heat
generation rate per pound, since cats are mammals with approx.
constant body temp.
consider the heat production of mature maintenance-fed sheep in
btu/h per pound of body weight, as described in table 2 on
page 9.8 of the 1993 ashrae hof:
fleece length at 46 f at 68 f at 90 f
(in.) total latent total latent total latent
shorn 4.0 8% 2.7 12% 2.0 38% [sheep sweat]
1.2" 2.2 29% 2.0 28% 2.0 65%
2.4" 2.0 23% 1.9 43% 1.9 76%
lambs,
1 to 14 days 10.5 -- 8.0 -- -- --
nick
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