Saturday, May 12, 2012

How Large a Solar Collector Do You Need to Heat Your Hot Tub?

This is a pretty good method to determine how large a solar collector is needed to heat your hot tub. 

The size of the collector needed to heat a hot tub depends on a lot of things: volume of the tub, how well its insulated, how good the cover is, what your climate is like, how hot you keep it, ...   Its not easy to take all that into account to get a good idea what size collector you need.  But, I ran across the method detailed below in "More Other Homes and Garbage" that uses the temperature drop of the hot tub over 24 hours to determine the daily heat loss for your hot tub in your climate and for the way you use it -- it seems like a good approach to me.

By the way  "More Other Homes and Garbage" is a terrific book written the 80's, but full of good ideas, good analysis, and written in a style that won't put you asleep -- and, its only $8 used!  Better get one before they all disappear!

Given that hot tubs are the biggest energy users in most homes that have them, its not a bad idea to do the test on your hot tub to see how the heat loss looks, and even if you can't do a solar heater, things like more insulation and a good cover can make a lot of difference.



  1. Using solar power to heat your hot tub is a great idea! It will definitely contribute to energy conservation.

  2. Hi Karen,
    What kind of foam do you use in the covers?

    Wondering if you have considered offering a cover with (say) 4 inches of polyisocyanurate rigid foam -- this is R6.5 per inch, so it would give you an honest R26 cover. Also wondering whether it would make sense to offer a cover with insulated skirts that go down the side of the tube a ways to beef up the hot tub side insulation?

    The polyiso foam is readily available at about $1 per sqft for 2 inch thick (R13).

    The difference in heat loss for a 80 by 80 tub through an R10 cover vs an R26 cover with 102F tub temp and 42F average ambient temp is about 400KWH of electric heat and 800 lbs of CO2 a year for a cover used 22 hrs a day.


  3. Gary- great post. We have some clients try this. All covers are using EPS foam. We do insulate skirt material with a closed cell foam (same as used in hinge and floating blankets) and it does work well. I'll look into polyisocyanurate rigid foam. Good advice. Thanks

  4. A solar collector to warm up your hot tub water is a wonderful way to conserve energy. This will also bring your electricity bills down!

  5. Polyiso likes to get saturated near water, so XPS is the best choice, even though EPS is usually used.

    But I have definitely lifted some HEAVY saturated EPS covers, even though they are encased in a heavy vinyl envelope.

  6. Hi Kevin,
    My problem with XPS is that the blowing agent is still a very destructive greenhouse gas, so I avoid it whenever possible. I guess that leaves the EPS or well sealed Polyiso.


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