Home Made Solar Heated Hot Tub for $350 (including the tub!)

This is a home made solar heated hot tub design submitted by Brian.

Thank You Brian!






Brian's Description:


Here's some info on my project.  I didn't take many pictures, but the construction is pretty basic.

I framed the collector box with 2"x6" lumber and 1/4" plywood, the size is about 26" x 8'.  The collector is a sheet of corrugated tin and 3/4" CPVC pipe.  The pipe lays down in the bottom of the corrugations.  I glued up all the pipe then applied a bead of flashing cement into each valley, then replaced the pipe assembly onto the tin and cinched it down into the valleys with some wire.  I painted it all flat black, but it might be better to leave the tin shiny so the "hills" could act as reflectors???  The glazing is just 4 mil plastic sheet, and there is a 1/2" thick sheet of the foil covered polyisocyanurate insulating foam board beneath the tin. The left over piece can be used as a reflector.  I later realized that I could have doubled the amount of piping in the collector if I would have ran it on both sides of the tin.  The back side wouldn't receive sunlight, but temps still reach 130-150 between the tin and insulation while running.

The pump came from Lowes and is the Garden Treasures MD170 fountain pump. Its Magnetic drive and rated at 150 GPH at 1' lift.  While testing the collector it spent several hours a day, for many days, immersed in 150-160 degree water.  The collector can easily and consistently heat a uninsulated bucket of water to 150 and a slightly insulated one to 160 degrees.  I then used an old 35 gal plastic trash can as a tank but the side split open at 120 degrees.   I was able to note a significant drop in heating, down to about 12 degrees per hour.  So far with the 150 gal tank it is about 3.5 degrees per hour, 4.5 with an improvised reflector and occasionally moving collector to track the sun.   The pump was meant to run submerged, so I had to modify it to run inline.  I epoxied on a short piece of  1/2" CPVC pipe for an inlet tube.

We are real happy with our choice of the 150 gallon Rubbermaid stock tank for our tub.  Just the right size for us, (yes we did go to Tractor Supply and sit in a few) also it seems to be a good fit with the collector.  The dimensions (being less than 48" wide) also make it easier and cheaper to build the enclosure.  The tank has a 1 1/4" threaded drain near the bottom, I used CPVC fittings to add a tee and a valve.  I draw the water from the bottom of tank and pump it though the collector and back to a thru hull fitting higher up in the side of tank.  The framing for the tank is also pretty simple.  I did cover the plywood top with some epoxy and glass cloth left over from other projects.  The skirting is from an old fence I replaced.

I am using 3 sheets of the 3/4" blue Styrofoam for the tank cover.  The left over pieces were used to build forms around the tank and I then poured in a 2 part urethane foam for insulation.  Did this from the bottom.  I used 2 gallons.  Would have been a lot easier to have it sprayed on... also will add a forth sheet of insulation to the top.

I did spend about $400 on this project, but did already have some of the materials.  The costs of some of the items:

Tank               $120
Pump                  30
Urethane ins        90 (with shipping)
sheet insulation    50
CPVC                75

I had most of the wood already, used sections of a old water hose between tank and collector, with pool noodles for insulation.

I'm sure I left out some details, so feel free to ask questions.  I live in SE Georgia...



And, the this note on performance from Brian:

Was in the hot tub last night at 110 deg, still 106 tonight without running it today.   It was about 80 degrees outside today with overnight lows in the 50's.  It does take 2 days to get the water up to temp if drained and refilled, hope to do it in one day when I get the batch water set up.



Full Size Pictures:






A couple small comments:


Brian lives in SE Georgia -- a fairly warm and sunny climate (I think).  So, if you live in a colder climate, you will probably need more collector area.   But, the collector does not cost much per square foot, so just build it larger.



The question on whether to leave the corrugated metal bright to reflect well, or paint it black is a good one.  My guess would be that painting it black is probably best.


It would be best to use polycarbonate glazing rather than other plastics, as it will hold up well to the high temperatures. 


Note the good insulation levels that Brian used -- I think this is a big key to success in heating hot tubs.


The collector design is a bit unconventional, but seems to be working well.   It will be interesting to see what the life of the CPVC piping is -- I think it may do well.

Here is a little more on a CPVC collector that I did a while back as an experiment.  It is really nice stuff to work with -- cuts and glues quite easily.


If the collector could be placed below the tub, then thermosyphon circulation might be used to eliminate the pump.  If this were done, it would be better to have a header style collector rather than the serpentine style.


Some Further Updates   --  Feb 2012:

- There has been some feedback from people building similar systems that the collector size is marginal even for warm sunny climates.  I think it would be a good idea to up the collector area -- it does not take much extra effort to build the collector larger.


- I would also seriously consider a collector construction similar to the one Tom used with grooved alum fins that fit over PEX tubing.  I believe that this is a more efficient design because of the better heat transfer between the metal fins and the tubing/water. 




Gary 3/12/07, Feb 11, 2012