# Enclosed Swimming Pool Needs Heat

• posted on May 13, 2004, 3:11 am
I just purchased a home which has a 16' x 33' oval inground pool. It is completely enclosed in a 45' x 24' building. the walls are 95% glass with a small base of brick. the only air flow is one wall of 24' x 4' of windows that will open and the screened door. The top is solid metal with no skylights. I live in Augusta, Georgia and would like to have a water tempature of 80 degree+. The current daytime temp here is 82 - 93 degrees with a night time temp of 60 degrees. My water temp has only reached 72 degrees. Can someone recommend a heating system that is not too costly. Please help I need to get this up and running very soon because I bought this house, with a pool, for physical therapy purposes, as my doctors have recommended. Thanks Loretta
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• posted on May 13, 2004, 2:15 pm

So the enclosure has about 1080 ft^2 of roof and 1104 ft^2 of walls and a thermal conductance of about (1080+1104)/R1 = 2184 Btu/h-F.

You might keep the windows closed most of the time to reduce evaporation loss from the pool and trickle some water over the roof and collect it in a gutter and run it back into the pool...

NREL says 1940 Btu/ft^2 of global and 820 Btu of diffuse sun falls on the ground on an average 70.7 F May day with an average daily max of 83.7, so on an average day, 1940-820 = 1120 Btu of beam sun falls on the ground in about 4 hours. The humidity ratio w = 0.0107 pounds of water per pound of dry air, so the vapor pressure of water in air Pa = 29.921/(1+0.62198/w) = 0.506 "Hg... Tw (F) air at 100% RH has Pw = e^17.863-9621/(460+Tw) "Hg.
If the roof collects E = 1120x1080 = 1,209,600 Btu on an average day and loses Ee = 4hx100x1080ft^2(Pw-Pa) by evaporation (according to one ASHRAE formula) and the enclosure loses Ec = 24h((Tw-460-70.7)2184 and E = Ee+Ec, Tw (R) = 558-8.242e^(17.863-9621/Tw). Plugging in Tw = 550 R on the right makes Tw = 546 on the left. Plugging in Tw = 546 on the left makes 547.5 on the right, then 547.0, then 547.2 R, ie 87.2 F, so it looks like you can make the pool 87.2 F in May, evaporating 4hx100(1.324-0.506)1080/1000 = 353 pounds or 42 gallons of water per day. Don't try this in Arizona. It might rile the locals, for one thing.

You might use the pool pump (Tom Lane says most pool pumps can work to a 60' head) with a \$10 130 F thermostat in a small box with a window to turn on the pump, and a pool thermostat to turn off the pump when the pool is warm enough.
For a longer season, you might replace the metal roof with a single layer of clear corrugated polycarbonate plastic ("Dynaglas" is one brand of this greenhouse roofing material.) This costs about \$1/ft^2 in 4'x12' sheets, and it has a 10-year 90% light transmission guarantee and it might last 20 years, with a simple pool cover to lower the indoor RH (polycarbonate ages quickly in the presence of warm water vapor.) This could keep the pool at least 80 F whenever the daily global horizontal sun H and average air temp Tah satisfy 1080x0.9H > 24h(80-Tah)2184, ie Tah < 80-0.1854H...
Month H Tah Ta(actual)
Feb 1110 59.5 47.4 F Mar 1430 53.5 55.5 It looks like Apr 1800 46.7 62.7 this could keep the pool May 1940 44.0 70.7 at least 80 F from March Jun 2010 42.7 77.5 through October. Jul 1930 44.2 80.8 You might use greenhouse Aug 1750 47.6 79.7 shadecloth to avoid Sep 1520 51.8 74.5 overheating Oct 1290 56.1 63.8 the pool. Nov 940 62.6 55.0
A double layer of greenhouse polyethylene instead of polycarbonate could keep the pool warm all year, if the space between the layers is filled with air during the day and soap bubble foam at night. This "replacement foam insulation" is used for greenhouses in cold climates like Alberta. The foam can also shade the pool during the day in summertime. Greenhouse film costs about 5 cents/ft^2 in folded rolls up to 40'x150'. It has a 4-year guarantee, and it's easy to replace, on a calm day.
Nick
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Join solar guru Steve Baer and PE Drew Gillett and PhD Rich Komp and me for an all-day workshop on solar house heating and natural cooling strategies ("HVAC Nonsense") on July 9 in Portland, OR--see page 25 of http://www.ases.org/conferences/2004_call_for_papers/SOLAR2004_prelim_program.pdf