Solar Hot tub

Discussion in 'Solar and Geothermal Water Heating Forum' started by Scuba_Dave, May 2, 2010.

  1. Scuba_Dave

    Scuba_Dave Extreme DIY Homeowner

    Messages:
    885
    Location:
    South of Boston, MA
    I picked up (2) 4x12' HW panels in December
    I was undecided as to how to use them...partially heat house/Sping & Fall, garage, basement ?
    Or set them up to provide HW for the house
    Well yesterday I decided to set one up & test it out

    I connected a small 350gph pump & set it up to circulate the water in my hot tub
    Approx 7 degree rise with the panel 1/3 shaded - late in the day about 3:30p
    So today I decided I would set the 2nd one up
    The 1st one was working & raised the 450g in the hot tub ~7 degrees in 2 hours
    It was about 12:30 by the time I had the 2nd one connected & turned the pump on
    Initial water coming out (due to shut off) was 140

    After running for a while that dropped to 94/7 due to partial cloudy day
    At times with more cloudes it dropped to 82/7
    Over the entire 6 hour period (10a-4p) it raised the Temp just over 19 degrees to ~75.6
    Not bad...2 possible cloudy days coming up then some sunny days
    So hopefully by the of tomorrow it should be over 80...nice for a warm dip

    I was hooking it up today & my son wanted to go in
    Hot tub to him means hot...I had him feel the water - it was only 60 with the 1st panel running
    Now I just need enough panels to heat the pool & HW for the house
  2. Scuba_Dave

    Scuba_Dave Extreme DIY Homeowner

    Messages:
    885
    Location:
    South of Boston, MA
    Into the 2nd day of heating, tub is closing in on 90
    Panel output is just over 112
    Not bad results for $20 in parts
    This will probably save $20-30 a month in electric cost

    I will probably disconnect 1 panel in the summer & use that to preheat water going to the WH
    I need to build a system to heat water now
  3. Dana

    Dana In the trades

    Messages:
    2,933
    Location:
    01609
    7F is a very low delta-T, indicating that you have 3-4x as much pump as you need. Most systems run higher delta-Ts, balancing a slighly lower collection efficiency against power costs for driving the pump.

    Hopefully the pump your using is oxygenated-water tolerant. Iron pumps won't last very long in unpressurized open systems like that.
  4. Scuba_Dave

    Scuba_Dave Extreme DIY Homeowner

    Messages:
    885
    Location:
    South of Boston, MA
    1st pump was not moving that much water
    New pump moves more water & in full sunshine can raise the temp 12 degrees an hour
    In the morning with low sun I am getting only 3-5 degrees an hour
    And in cloudey weather also low output which is expected
    Now that the tub is at temp it doesn't take much to keep it over 100
  5. leejosepho

    leejosepho DIY scratch-pad engineer

    Having just moved to Louisiana, I plan to do the same kind of thing for my 1000-gallon pool and to warm the supply for the water heater for the house.

    I suspect thermostatic control of flow would be best. A car has a thermostat to keep the water in the radiator long enough to cool, and now we need to keep it there for long enough to get it hot.
  6. Scuba_Dave

    Scuba_Dave Extreme DIY Homeowner

    Messages:
    885
    Location:
    South of Boston, MA
    Not a problem keeping the water in the solar heater long enough to heat up
    I am seeing over a 26 degree difference between input temp & output
    And its still Spring & only 50 out at times
  7. Dana

    Dana In the trades

    Messages:
    2,933
    Location:
    01609
    I think I'd misread your earlier post- a 26F delta-T is a quite reasonable operating point. (Somehow I'd inferred "Approx 7 degree rise with the panel..." to mean a 7F delta-T for input/output numbers when you were actually talking about the increase in tub temp over time.)
  8. Scuba_Dave

    Scuba_Dave Extreme DIY Homeowner

    Messages:
    885
    Location:
    South of Boston, MA
    It was actually both, poor explanation on my part as I jumped from one to the other
    With one panel connected & 1/3 of the panel shaded late in the day I was only getting a 7 degree increase from input to output
    Then the 2nd day in full sunshine 1 panel raised the entire tub temp 7 degrees in 2 hours
    After that the 2nd panel was connected & I saw a much larger increase in the delta T
    With clouds I do see it dip down to 5-7 degree delta T for brief periods

    But with a sunny day it really cranks out the heat
  9. Scuba_Dave

    Scuba_Dave Extreme DIY Homeowner

    Messages:
    885
    Location:
    South of Boston, MA
    Working great, shut it down @1pm as the hot tub reached 106 - fully circulated tub water
    Also recd the electric bill - lowest in 4 years !!
    35% lower then last month, and also 35% lower then same month last year

    Now I need to setup a loop to preheat water for the house
    Last edited: May 15, 2010
  10. Scuba_Dave

    Scuba_Dave Extreme DIY Homeowner

    Messages:
    885
    Location:
    South of Boston, MA
    Solar Hot Tub Part II

    I just picked up 4 more collectors - 3x6 but better quality
    These are higher quality then the ones I have, but only 3x6
    Much better glazing & heavy duty....they look nice

    So I'm going to take one 4x12 off the current hot tub setup & add a 3x6
    Then take the 2nd 4x12 & another 3x6 & connect them to the hot tub by the pool
    Then the last (2) 3x6's for HW for the house

    The hot tub by the pool I will also circulate water to the pool to keep the hot tub below ~101
    And this will help heat the pool too
  11. Scuba_Dave

    Scuba_Dave Extreme DIY Homeowner

    Messages:
    885
    Location:
    South of Boston, MA
    The 1st hot tub works good...TOO good !!
    It heats up from 96 to 106+ in 2 hours

    So I am pulling one 4x12 panel off & adding that to the hot tub by the pool
    I've cleaned the tub out & connected the hose to the panel
    Input 55, output 78-86 depending upon the sun/clouds
    I'll be able to cycle water from the pool to cool it off if it gets too hot
    This will help heat the pool too
  12. Scuba_Dave

    Scuba_Dave Extreme DIY Homeowner

    Messages:
    885
    Location:
    South of Boston, MA
    1st tub is still Maxing out around 106-108, cools down to 104 when I go in at night
    2nd tub is only getting up to 98 or so, with the solar panel on the pool cabana roof there isn't as much flow from the pump
    But its the perfect Temp to go in with my son
    The pool was up to 76 on Memorial Day
    Today the surface was up to 82, have to see what it is tomorrow w/filter & pump going
    Maybe I can get the last 2 solar panels connected for the pool
  13. Scuba_Dave

    Scuba_Dave Extreme DIY Homeowner

    Messages:
    885
    Location:
    South of Boston, MA
    I'm actually only running the solar heater on the hot tub every other day now
    The warm tub runs every day - It was about 102 today - if it gets too hot I cycle water to the pool
    The inground pool - ~19,000g - topped out at 88 degrees during a hot spell
    It has cooled off to just over 84 degrees now...perfect
    We are using the poool more this year then any other year

    I'm extending the roof from the pool caban over the bar area
    Once that is done I am adding 2 more 2x20' solar panels ot the cabana roof for the pool
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