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Old 07-08-2011, 10:26 AM
 
2,996 posts, read 4,918,703 times
Reputation: 1793

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Some background : I live alone and want to totally eliminate the need
to run my 40 gallon electric water heater, and since i live in sunny
Florida I believe I can do so given my low requirement for hot water.
Im going to be building my solar collector very soon when all the
materials are gathered. My requirement for hot water usage is to take
a relatively quick shower of 5-10 minutes using 100 f. hot water and
believe the following plan will handle at least that :

I want to use 3/4"x300' coiled red Pex Tubing (USING CAPACITY CONTROL SO THE PEX DOESNT MELT) which measures 32"
diameter x 7" high . I will set this in a box which i will construct
out of wooden 2x12's with a 1/2" thick plywood on the backside
(treated) for a 36"x36" od Box. . I will lay 4" of dense pink
Foamboard in the bottom . A 32" square sheetmetal plate painted with
flat high temp. low gloss Black paint (primed first) will be layed on
top of the bottom Foamboard . The coiled Pex tubing will be wrapped in
heavy duty tin foil sheet to prevent UV light from seeing the Pex ,
and this foil will be pulled tight around the coil of Pex and also
painted high temp flat black. ... then the coil will sit inside the
box in direct contact with the sheetmetal plate. A double pane window
in frame measuring 36" sq. od (without any inside bars) will be
fastened on top of the box for a greenhouse effect. The box will sit
against the southside of the house on the ground at a 30 degree angle
from vertical in a location where where it will recieve full sun. I
will cut the hot water CPVC pipe above the water heater and route to
the Solar Panel using 3/4" CPVC (insulated) connecting to the 3/4" Pex
tubing stubbed out of the side of the Box using Pex compression
fittings . The return CPVC pipe (insulated) will run back to the top
of the water heaters hot water supply pipe. (Since my hot water
reqirements are very minimal, I opted not to heat the entire water
heater) .

Since stagnation temps/pressure in the box are of concern, i plan on
securing a section of shiny sheetmetal over the face of the Box for
capacity control during the summer months.

Specs on the Pex are 100 psi at 200 f. Specs on the CPVC pipe are 100
psi at 180 f. May aim is to deliver hot water in the 100-120 f. range
thru experimenting with how much to cover the front of the Box with
the shiny sheetmetal plate .

Total estimated cost for this project is $300 ($100 for the Window ,
$120 for the coiled Pex, and $80 for the wood and CPVC piping with
fittings .

If you have tips, considerations, concerns, etc... I would love to
hear them. Thanks. Dave
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Old 07-08-2011, 11:11 AM
 
5,090 posts, read 9,847,209 times
Reputation: 3955
I would post this on fieldlines.com.

Pretty good crew over there regarding all sorts of Renewable Energy Home-Builds.

Maybe look through their other on-line designs for additional ideas, as well.
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Old 07-08-2011, 11:33 AM
 
Location: Somewhere in northern Alabama
17,858 posts, read 54,158,070 times
Reputation: 30343
It may work, but you are creating thermal barriers by wrapping the pex. You don't mention a pump, so be aware that thermosyphoning is not as straightforward as it sounds. Personally, I'd look for an old water heater on Craigslist, rip off the cover and insulation, paint the tank flat black, stick it in a box lined with foil, and face it with an old patio door. Plumb it so you can choose between hot water from it or the electric one.

The normal tendency we all have is to over-engineer and under-think. Using waste materials is far more "green", less costly, and often less prone to fail.

BTW the tank in a box concept is called a solar box heater or some such. You might find plans online for various permutations. Look at them for gotchas and ideas, then roll yer own.
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Old 07-08-2011, 01:59 PM
 
2,996 posts, read 4,918,703 times
Reputation: 1793
Quote:
Originally Posted by harry chickpea View Post
It may work, but you are creating thermal barriers by wrapping the pex. You don't mention a pump, so be aware that thermosyphoning is not as straightforward as it sounds. Personally, I'd look for an old water heater on Craigslist, rip off the cover and insulation, paint the tank flat black, stick it in a box lined with foil, and face it with an old patio door. Plumb it so you can choose between hot water from it or the electric one.

The normal tendency we all have is to over-engineer and under-think. Using waste materials is far more "green", less costly, and often less prone to fail.

BTW the tank in a box concept is called a solar box heater or some such. You might find plans online for various permutations. Look at them for gotchas and ideas, then roll yer own.
I have read about the tank in a box idea (called Batch Heater) but i simply dont need all that hot water...my requirement for hot water is only for showers at the rate of about 6 gallons of 100-120 f. water per day. m not Thermosyphioning , and, I dont require a pump with my idea because water is forced thru the 300' of pex tubing whenever the hot water faucet in the house is opened -- im feeding the solar heated water directly back into the hot water supply pipe. This way, im eliminating a need for a pump, additional solar tank, controls, and valves ...i figure the return on investment of my system to be approx. 1.4 years (spending $300 total) . Thanks for the link Phillip above, ill try it.
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