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  1. #1
    Join Date
    Apr 2011
    Location
    North Florida
    Posts
    32

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

    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 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.

  2. #2
    Join Date
    Apr 2011
    Location
    North Florida
    Posts
    32
    Before someone tells me that the Pex will melt...please read the writeup which describes how i will implement capacity control of the collector so the Pex doesnt melt or see too high of pressures . Thanks.

  3. #3
    Join Date
    Jun 2011
    Location
    Virginia Beach VA
    Posts
    46
    So how does this work without a pump?

  4. #4
    Join Date
    Mar 2011
    Location
    Louisiana
    Posts
    119
    Just so I understand, you are planning to leave the tubing in the shipping coil and not laid out single layer in the box?

    J

  5. #5
    Join Date
    Apr 2011
    Location
    North Florida
    Posts
    32
    Quote Originally Posted by terawatt View Post
    So how does this work without a pump?
    No pump required ; once the hot water faucet is opened in the house, the line pressure forces water thru the Pex Coil and into the hot water supply line . Simply, no controls, no valves, no thermosyphon (although i may convert it to thermosyphon as the collector is on the ground and the water heater is 8' higher) .

    It will work fine for just myself cause all i require each time is 5-6 gallons of hot water .

  6. #6
    Join Date
    Apr 2011
    Location
    North Florida
    Posts
    32
    Quote Originally Posted by Jarreau View Post
    Just so I understand, you are planning to leave the tubing in the shipping coil and not laid out single layer in the box?

    J
    J, Correct...it is a 300' coil of 3/4" i.d. Pex which measures approx. 28" in diameter x 10" high. Im leaving a couple inches on all sides of the box in case the Pex expands . The Pex coil will be wrapped in h.d. tin foil and painted flat black so the suns UV light doesnt hit the Pex (which would destroy it in time) .

  7. #7
    Join Date
    Mar 2011
    Location
    Louisiana
    Posts
    119
    This may work. You're going to have almost 8 gallons stored in the loop which given some time should heat up.

    Once you have a feel for what the loop temperature is going to be, to figure your resulting shower head temperature us the following formula:
    =((x*t1)+(y*t2))/(x+y)

    x= flow rate of your hot water (GPM)
    y= flow rate of your make up water supply or cold water side of your shower mixing valve.
    t1 = temperature of the water off of your solar system
    t2 = temperature of the water from your make-up supply

    For example (in a perfect world)
    140° off of the solar system
    65° for your cold water temp
    1.5 GPM flow for both HW & CW streams
    results 102.5° out of the shower head
    lasting approx 5 minutes


    J

  8. #8
    Join Date
    Apr 2011
    Location
    North Florida
    Posts
    32
    Quote Originally Posted by Jarreau View Post
    This may work. You're going to have almost 8 gallons stored in the loop which given some time should heat up.

    Once you have a feel for what the loop temperature is going to be, to figure your resulting shower head temperature us the following formula:
    =((x*t1)+(y*t2))/(x+y)

    x= flow rate of your hot water (GPM)
    y= flow rate of your make up water supply or cold water side of your shower mixing valve.
    t1 = temperature of the water off of your solar system
    t2 = temperature of the water from your make-up supply

    For example (in a perfect world)
    140° off of the solar system
    65° for your cold water temp
    1.5 GPM flow for both HW & CW streams
    results 102.5° out of the shower head
    lasting approx 5 minutes


    J
    Thanks for the formula.

    In all reality, I believe I will get 160 f. + water off the Collector if i want to during FLorida summer months as the sun shines here almost non stop. , so, that will mean that I wont be needing as much volume of hot water at the shower since it will be so hot (and will need mixing with cold) . I saw a YouTUbe Video of a guy who openly layed 100' of black poly coiled tubing on his carport roof on a 85 f. day and within a few hours it was putting out 160 f. water ... and that wasnt even using a Collector Box with double pane windows . So, i expect to be pleasantly surprised. 8 gallons should do me fine each time ; if i required more hot water then id go the storage tank route and thermosyphon since my collector is going to be below the water heater . We shall see ...it will be easy to convert if i want to.

  9. #9
    Join Date
    Apr 2008
    Location
    Pa.
    Posts
    1,077
    id forget about the pex and wraping it all up....why not just use a coil of well pipe?

    Id like to try taking an old refridge or freezer and setting a tank inside painted black then a sliding glass door on top...

  10. #10
    Join Date
    Jun 2011
    Location
    Virginia Beach VA
    Posts
    46
    >once the hot water faucet is opened in the house, the line pressure forces water thru the Pex Coil

    Ok so if the water heater tank is operating, a demand will bring already-hot water from the top of tank into the PEX which will reduce or eliminate the opportunity for solar collection. May even cause an energy loss.

    If the water heater is not operating, and you need hot water on a cloudy day, at night etc. there won't be any.

    What am I missing?

  11. #11
    Join Date
    Apr 2011
    Location
    North Florida
    Posts
    32
    Quote Originally Posted by farbeondriven View Post
    id forget about the pex and wraping it all up....why not just use a coil of well pipe?
    Because it doesnt have a high temp. capability. , wont stand up to freezing like Pex does, and wont take expansion like Pex.

  12. #12
    Join Date
    Apr 2011
    Location
    North Florida
    Posts
    32
    Quote Originally Posted by terawatt View Post
    >once the hot water faucet is opened in the house, the line pressure forces water thru the Pex Coil

    Ok so if the water heater tank is operating, a demand will bring already-hot water from the top of tank into the PEX which will reduce or eliminate the opportunity for solar collection. May even cause an energy loss.

    If the water heater is not operating, and you need hot water on a cloudy day, at night etc. there won't be any.

    What am I missing?
    My aim is not to use the water heater , and just have the Collector Water fed directly into the hot water supply line to the house. I tested the system over the weekend and on a 90 f. day that was mostly sunny , by filling the Pex Coil with 58 f. Well water and let is sit for 5 hours ... after 5 hours the Box temp. was 161 f. and the exiting hot water was 158 f. Even if the Collector Water only got up to 105 f., that is plenty hot enough for a shower , and i take my daily showers at roughly 5-6 pm so the Collector Water will be at its hottest before cooling off for the evening.

  13. #13
    Join Date
    Feb 2010
    Location
    Ontario Canada
    Posts
    1,341
    When I built my house I stayed in a old cabin on my land. I ran a garden hose on the ground between the new house (where the water supply from the new well was) and the cabin. The water would get so hot you could scald yourself in you were not careful. This was with just 75 feet of black garden hose. I think I would be wanting to put a pump in the system and use the existing hot water tank as storage.
    Do not underestimate the power of the sun.
    Never give up; Never surrender!

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