Want To Build A Solar Hot Tub?

Dreaming of a long soak in a hot tub in your garden under a starry sky – maybe with a few good friends and a bottle or two of good wine?

Buying a hot tub will set you back at least 3,000 dollars and probably a lot more.  Then come the heating bills. But you can build a solar hot tub for less than 400 dollars, and the heating bills are zero.  Nothing! 

A diy hot tub is very straightforward.  The result will be the centerpiece of your backyard and will work in all climates.  The water will actually get so hot that you will need to fit a pressure relief valve for safety!

Begin with a good set of diy plans.  These should include clear step-by-step instructions, a materials list, schematics and diagrams to show you exactly how to build the solar hot tub.

These are the main parts of a solar hot tub:

  • Solar collector box
  • Stock tub
  • Filter
  • Pump (option, site-dependent)
  • Various CPVC plumbing parts
  • Pressure relief and control valves

A simple solar collector starts with a sheet of corrugated tin laid in an open wooden box.  Make the box bottom from a sheet of ½ inch plywood, edged with 2 x 6 inch lumber (or whatever you happen to have in your shed), and then line it with insulating foam sheet.

CPVC pipes (3/4 inch will do) are laid and fastened along the corrugations and connected in series causing water to enter at the lowest point of the solar collector and leave from the highest. CPVC pipe is best, incidentally, because it has better heat tolerance then PVC. The whole inside of the box and the pipes are painted flat black, and a sheet of ¼ inch (or thereabouts) polycarbonate glazing is attached to the top of the box to close it.  

The tub can be any suitable stand-alone stock tank, from 150 gallons capacity (for 1 or 2 people) to 500 gallons or more.

If the solar collector can be mounted one or two feet below the tub (perhaps by having the tub on a slight mound), it is possible to circulate the water through the heating system by thermosyphon action.  If this is not possible you will need a pump rated at about 150 GPH fitted at the outlet from the tub.

Pipe runs should be as short as possible, and avoid sharp bends.  Fitting an inline filter near the tub water outlet will avoid the task of having to clean out the tub from time to time. Finally you should fit inlet and outlet flow control valves as well as a safety valve to relieve excess pressure in the system.

Test the hot tub system for leaks, then fit insulation everywhere – any exposed pipes, the external walls of the tub, and the tub cover (essential to lower nighttime heat loss).

The tub will not need any external support so you will be free to build any structure around it you want, to hide the plumbing or provide an access platform, for example.

The solar collector must be sited correctly.  It should face south as nearly as possible in northern latitudes, and be inclined between 15 and 30 degrees from horizontal. It should also be slightly tilted along its length to promote the upwards flow of water through the collector pipes.

What time can you expect to take for heating the solar hot tub water?  It depends on where you live, but it could take up to a couple of days to reach a comfortable temperature of about 110 degrees the first time. Once there, though, you will find your main problem will be cooling the water rather than heating it!

Please keep in mind the absolute need to plan properly before starting to build your solar hot tub.  Determine the size of tub you want, and where you want to locate it.  Then the best, and safest, bet is to get some good professional advice.




Posted under Build Your Own Solar Panels, Renewable Energy, Solar Water Heaters

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