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re: radiant floor heat water heater ?
6 dec 2003
iowa883  wrote:

>i am building a new shop and would like opinions on radiant floor heat vs.
>conventional heating. i also heard that some people use a normal hot water
>heater and a circulating pump . is this a good method ?

sure. why not, if it has enough capacity? then again, you might make a
2-layer plastic film greenhouse roof over steel pipes and fill it with
air during the day and foam it with soap bubbles at night for daylighting
and simple solar heating with no fuel nor pipes nor pumps. i think bob
quist in toronto is now doing this with venlo glass greenhouses. 

nrel data indicate december is the worst month for solar heating in des
moines, when 930 btu/ft^2 of sun falls on south walls and 520 falls on the
ground on an average 24.4 f day. a 20'x30' shop with 10' walls could gain
0.8x20x30x520 = 250k btu/day of roof sun and 0.8x10x30x930 = 223k btu of south
sun on an average day, a total of 473k btu, or more, with snow on the ground. 

with r20 insulation (eg 6" of 1/16" soap bubbles at a mean 50 f at night),
thermal conductance would be 30 for roof plus 50 for walls, so you could
store heat for 5 cloudy days with rc = -120/ln((60-24.4)/(70-24.4)) = 485
hours in c = 80x485 = 39k btu/f of thermal mass cooling from 70 to 60 f. 

as an alternative, on an average day, you heat the shop to 70 f for 24 hours
and maintain overhead water at a temp t, where 

473k = 6h((t+70)/2-24.4)10x30/r1  [daytime south wall]
     +18h(70-24.4)10x30/r20       [nighttime south wall] 
     + 6h(70-24.4)20x30/r1        [daytime roof]
     +18h(70-24.4)20x30/r20       [nighttime roof]
     +24h(70-24.4)10x70/r20       [rest of shop],

which makes t = 193 f, if i did that right, but that seems high, given
radiation loss. if the shop needs 24h(65-24.4)80 = 78k btu on a cloudy
24.4 f day or 390k btu for 5 cloudy days, it needs 390k/(130-70) = 6496
pounds of overhead water cooling from 130 to 70 f, ie a 10.8 psf or 2"
layer in a plastic film duct over mylar film over welded wire fencing,
with another layer of film above that and a thermostat and slow ceiling
fan below to bring down warm air on a cloudy day. more mass in the shop
would reduce the overhead mass requirement, as would a night setback.
overhead mass allows more effective night setbacks, vs a radiant floor. 

nick




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