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re: low temperature fan coil performance?
15 feb 2003
iain mcclatchie wrote:
>the solar design firms that i've talked with recently have said that
>radiant floor ("hydronic") heating is a much better fit to solar thermal
>than forced air.
it's nicer because it uses less electrical energy.
>hydronic heating typically runs with water temperatures around 100 - 110
>degrees, and fan coils typically run with water temps around 150 - 180
>degrees... so hydronic heating is more compatible with low cost plastic
>solar collectors.
a well-insulated home might use lower temp water in either case. another
option is low-temp baseboard radiators which might be installed invisibly
inside the lower parts of hollow interior walls, with slots for airflow
near the wall tops and bottoms. a $2 foot of fin-tube moves about 4 btu/h,
so 800 btu/h-f costs about $200, with no fan power or noise.
>1) what temperature water can i reasonably expect to get out of a large,
> low cost plastic pool-type collector with polycarbonate glazing over it?
ohm's law for heatflow, but radiation limits the upper temp. then again,
pool collectors under glazing might melt or crack in summertime. you might
talk with steve baer at (505) 242-5354 about zomeworks "skymats" and pipes
near the ceiling.
>2) will fan coils deliver enough btus/hr?
sure, if you use enough of them.
>i'm no longer comfortable assuming 800 or even 400 btu/hr/degree f for the
>fan coils, since the ratio of deltat in and deltat out is fairly large,
>and i suspect there is some sort of exponential in the math here. anyone
>want to supply a better formula, and maybe better numbers to assume here?
magic-aire's 2'x2' shw 2347 duct heat exchanger can move 45k btu/hour from
125 f water to 68 f air at 1400 cfm, with a 0.1" h20 air pressure drop, ie
its conductance is 45k/(125-68) = 789 btu/h-f. i'd guess that stays the same
at lower temp diffs if the water and air heat capacity flow rates in btu/h-f
are still balanced, with z = cmin/cmax = 1. if not, you might estimate the
ntu of the heat exchanger from the data sheet and correct for unbalanced
flow rates using the formula e = (1-e^-ntu(1-z))/(1-ze^-ntu(1-z)) or look
at the data sheet tables for outputs at various air and water flow rates.
nick
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