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Newbie Solar Pool heating Question (long).

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Posted by Mark on December 4, 2003, 3:20 am

What I have is an 11 x 24 fiberglass pool in a 20x30 greenhouse type
structure located in central Massachusetts. The pool is about 7000 gallons.
The pool is currently heated as a zone on an oil boiler with a heat
exchanger. I keep the pool covered with a solar blanket when not in use to
minimize heat loss and evaporation. The greenhouse structure has a bronze
tinted twinwall polycarbonate  roof so although the room is warmer during
the day it's not hot in there by any means. Night time it can be quite cold
and the last few nights where we've gotten down into the 20's the room temp
with the pool cover off and in use was about 58 or so. I also have a
supplemental fan forced hot water unit heater for heating the room at night
but it's a little underpowered and only raises the room temp a few degrees
at best on real cold nights.

Currently with the outside temps averaging in the 30's I'm experiencing
about a 4 degree temperature drop in water temp at night, I just have a
cheapy solar cover and going to get a better thermal cover which may help
minimize the temp drop. My biggest concern is, I like the water warm, 86 to
88 degrees f, with the temps dropping I can just about watch my oil tank
gauge falling and can use 60+ gallons in a weekend bringing the water up to
temp (I turn it down to 72 or so when I don't plan on using it for a few

So being a solar newbie, I'm wondering if even during the winter months with
somedays only reaching the low 20's if solar heating would still work as a
supplement to oil heat for heating the pool water. I'd like to use a system
with a heat exchanger and fill it with glycol, controlling it with a pc and
turning on a circulator pump only if certain conditions are met (collector
temp > than pool temp) I'm very new to the idea of solar but very familiar
with pc's, software and automation controls.

I'd have an area of about 15x20 on the roof of the pool area for collectors,
this gets sun most of the day in winter and summer.

This is my first winter with the pool and although I like the idea of
swimming when there's snow on the ground, I don't like the idea of swimming
without being able to afford a six pack to drink while I'm doing it. The oil
cost is going to kill me. Of course I can close it down until spring but
what's the fun in that ?

In your experienced opinions, would it be possible to supplement the heating
of the pool in winter time with solar ?

Thanks for any suggestions,


Posted by Nick Pine on December 5, 2003, 1:42 pm

What's the solar transmission? You might replace the bronze with clear.

So it's losing 4x7000x8 = 224K Btu at night, the heat equivalent of
about 2 gallons of oil, With a water to air thermal conductance of
about 224K/(87-30) = 3930 Btu/h-F, like 10 houses. Wow.

I'd get a non-tinted clear cover.

OK. Insulate the walls of your pool and greenhouse and cover the twinwall
with two layers of greenhouse polyethylene plastic film with 10 gallons of
10% detergent solution and 2" holey PVC pipe in film troughs at the eaves,
and turn on a shop vac blower to fill the space between films with soap
bubble foam at night. Put an air return at the ridge and use a microswitch
and some window screen in the return to turn off the vac when bubbles begin
to push on the screen.

NREL says 480 Btu/ft^2 of sun (270 diffuse) falls on the ground on
an average 27.4 F December day with an average daily max of 34.7 in
Worcester, MA. If the modified roof has 50% solar transmission and
you unfoam it for an hour a day on average, the pool might collect
11x24x0.5x210 = 28K Btu and the rest of the greenhouse might collect
20x30x0.5x210-28K = 35K Btu. An R1 roof for an hour a day and average
R20 insulation at night (6" of 1/16" bubbles at a mean 50 F) makes
28K+35K = (T-27.4)20x30(1h/R1+23h/R20), ie T = 76 F greenhouse air.
How much insulation does the pool need to be 87 F?

As a loyal member of the order of HVAC criminals, I am required
to bill you $2,621.87 for this consultation. Please remit.


Posted by Tim on December 5, 2003, 9:51 pm
 Thanks for the reply Nick, have some additonal questions though:


Why is that Bad ? This is a seperate boiler from the house furnace which is
forced hot air. The boiler was installed for the pool and to replace
electric hot water with another zone from the boiler.

From the specs on the polycarb, light transmission is 70%, uv protected so
probably not too much uv, the room does heat up on warmer days though.
Unfortunately this is my first season with this, bronze was choice to keep
heat down in summer.

Pool is a inground with cement deck, too late to insulate the sides, heard
that fiberglass is a poor conductor of heat so possibly not losing too much
through the sides although it definately is losing some.

Maybe it's time to close it down for the winter or find an alternate heat
source that's less expensive.

Check is not in the mail.. Thanks for the help...

Posted by Nick Pine on December 5, 2003, 10:05 pm

Fine, at $2,621.87 each :-)

Our US 5% of the world's population uses 26% of the world's energy...

Not too bad...

You can do that with foam during the day.

That's not very important, inside the greenhouse, but you might slip some
foamboard under the liner or make an 11x24' double wall transparent cover
that lifts off the pool with two ropes and a counterweight when you swim.

Consider the sun.


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