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
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.
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