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Thread: Need constructive critique on my idea for a solar hot water panel , please

  1. #1
    DIY Junior Member TheycallMeDave's Avatar
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    Default Need constructive critique on my idea for a solar hot water panel , please

    This looks like a great forum.

    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 120 f. hot water and
    believe the following plan will handle at least that :

    I want to use 3/4"x300' coiled red Pex Tubing 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 .

    Notes: No pump is required since the line pressure will force water thru the Collector once a hot water faucet is opened in the house. A deep freeze is not a concern in my locale (Florida) . UV light from the sun hitting the PEx is not a concern since the Pex coil will be wrapped in heavy duty tin foil and painted flat black as well.

    QUESTIONS :

    1. Will condensation form inside the box during the winter ... enough to inhibit operation ?
    2 . How will the dense pink styrofoam stand up to heat over the years ?



    If you have tips, considerations, concerns, etc... I would love to
    hear them. Thanks. Dave.

  2. #2
    In the trades Dana's Avatar
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    Default

    120F is scalding hot. Most people who like very hot showers never see exit temps above 110F.

    The operating temp of pink foam (=XPS==eXtruded expanded PolyStyrene) ore EPS (bead board) is too low for use in solar collectors of any type, and it breaks down with UV exposure as well. Use polyisocyanurate (often foil faced, eg DOW Thermax), or rock wool batting.

    300' of 3/4" PEX is only 7-8 gallons, which isn't much thermal mass- you're very likely to exceed the operating temp of the PEX with that approach, and it won't support a 10 minute shower even at 180F.

    To get good heat transfer from your painted foil to your PEX it needs to be wrapped tightly to the tubing with no insulating air-gap. (A tough job, on 300' of PEX.)

    A better approach is to submerge a coil of PEX (it doesn't need to be 300', but probably at least 100') in a 30-80 gallon black painted barrel, using the PEX as a heat exchanger rather than the collector & storage. The water in the barrel can be kept at/near atmospheric pressure and should probably be topped off seasonally to replace any evaporation. The up side is that you have more heat stored, the down side is it takes longer to heat up once depleted. By keeping the heat-storage water and potable water separated, there is no chance of long-term water stagnation health issues, which is a signficant improvement over this design. Painting the entire barrel black and using foil-faced iso for insulation you get a fairly good performance boost out of it too, since then the majority of the light/heat is absorbed by the tank, an nearly none is absorbed by the frame or insulation.

    If you google [batch hot water heater] you can probably find lots of tips & refinements. Gary Reysa's Build It Solar site is also a good resource for DIY solar water heating info (from the very simple to fairly complex.)

  3. #3
    DIY Junior Member TheycallMeDave's Avatar
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    Default

    Quote Originally Posted by Dana View Post
    120F is scalding hot. Most people who like very hot showers never see exit temps above 110F.

    The operating temp of pink foam (=XPS==eXtruded expanded PolyStyrene) ore EPS (bead board) is too low for use in solar collectors of any type, and it breaks down with UV exposure as well. Use polyisocyanurate (often foil faced, eg DOW Thermax), or rock wool batting.

    300' of 3/4" PEX is only 7-8 gallons, which isn't much thermal mass- you're very likely to exceed the operating temp of the PEX with that approach, and it won't support a 10 minute shower even at 180F.

    To get good heat transfer from your painted foil to your PEX it needs to be wrapped tightly to the tubing with no insulating air-gap. (A tough job, on 300' of PEX.)

    A better approach is to submerge a coil of PEX (it doesn't need to be 300', but probably at least 100') in a 30-80 gallon black painted barrel, using the PEX as a heat exchanger rather than the collector & storage. The water in the barrel can be kept at/near atmospheric pressure and should probably be topped off seasonally to replace any evaporation. The up side is that you have more heat stored, the down side is it takes longer to heat up once depleted. By keeping the heat-storage water and potable water separated, there is no chance of long-term water stagnation health issues, which is a signficant improvement over this design. Painting the entire barrel black and using foil-faced iso for insulation you get a fairly good performance boost out of it too, since then the majority of the light/heat is absorbed by the tank, an nearly none is absorbed by the frame or insulation.

    If you google [batch hot water heater] you can probably find lots of tips & refinements. Gary Reysa's Build It Solar site is also a good resource for DIY solar water heating info (from the very simple to fairly complex.)
    Thanks for the tips on the styrofoam. I just might change the project to your idea. Thanks alot.

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