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low temp radiators
1 dec 2002
news  wrote:

>"rob"  wrote in message

>> no hydronic heat - i don't like the tubes embedded in the concrete floor.
 
warmboard is another alternative, thick plywood with a heat-spreading layer
of aluminum on top and grooves for hydronic tubing. a 4x8 70 pound(?) sheet
costs $149 plus shipping (without the tubing) and is said to move 800 btu/h
into 70 f room air using 100 f water, ie 800/(100f-70f) = 26.7 btu/h-f.

a concrete hydronic floor might cost $5/ft^2 and need 160 f water for
25 btu/h-ft^2. then again, it can provide desirable overnight heat storage. 

>> they are also much more expensive compared to a forced air system.
 
grainger sells 8' lengths of heatrim-american 3/4" fin tube pipe for $52.27.
they move 330 btu/h-f-ft with 140 f water at 4 gpm, and 310 at 1 gpm with
a pressure drop of 0.047 "h20/ft. the thermal conductance is 310/(140f-70f)
= 4.4 btu/h-f-ft. 

so we might a) move 800 btu/h with 100 f water using a $149 sheet of
warmboard (which also serves as a subfloor), or b) move 800 btu/h with
100 f water with 800/(4.4(100-70)) = 6' of fin-tube pipe that costs
6/8x52.27 = $39. what's the cost of a subfloor... $15 for two sheets
of plywood?

the fin-tube heat can be locally regulated, and it is easier to repair,
if damaged. if it moves 310 btu/h-ft in slow-moving air, how much more
can it move if it's mounted in an air inlet at the base of an 8' closet
or wall cavity, with an equal-sized vent at the top?

nick




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