re: pool heating... random thought... techie answer
10 oct 2003
william p.n. smith wrote:
>...a well insulated pool would continue to heat up day after day,
>is that your point?
my main point is that the final temp has nothing to do with thermal mass.
it would be the same if the pool were full of air or rocks or feathers.
>>if an ac provides 48k btu/h and a shaded pool has a thermal conductance of,
>>say, 1200 btu/h-f (eg a 30'x40' r1 cover), its final temp will be 48k/1200
>>= 40 f warmer than the outdoors, no matter how much water is in the pool...
>_if_ the a/c runs 24x7, and the pool walls have no thermal conductance,
>and the cover never comes off, and there's no evaporation or makeup water.
>back here in the real world, a pool might be an interesting heatsink for
>a central a/c system.
pools mainly lose heat from the top, and covers prevent evaporation, even
with miserable insulation, r1 at best. in the real world, pools seem unlikely
to need heat when ac is needed. hot tubs might, and acs can heat water for
showers, but higher temp heat sinks decrease ac efficiency.
>on the other tentacle, nick's point is worth thinking about, as even a
>slow rise in temperature has to stabilize somewhere. if water is
>free, and you leave the cover off to let the heat out of your pool,
>and we have some idea of the duty cycle of the a/c, there's probably a
>way to figure this out... [cooling degree days probably drive the a/c
>duty cycle and effect the cooling capacity of the pool, though hot
>days and cool nights will just make things harder to analyze.]
ashrae says an uncovered pool loses 100(pw-pa) btu/h of heat. nrel says
boston has 264 (f) cdd in july, with w = 0.0117, which makes pa = 0.552 "hg.
pw = e^(17.863-9621/(460+tp(f)), approximately...