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?

A conventional hot tub will cost you at least 3,000 dollars to buy, and there will be the fitting costs on top of that.  After that there will be the constant heating costs. But you can build a solar hot tub for less than 400 dollars, and the heating bills are zero.  Zilch! 

It is very easy to build a solar hot tub.  The result will be the centerpiece of your backyard and will work in all climates.  These systems get so hot that you will need a safety valve to prevent steam building up!

Start 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 panel
  • Stock tub
  • Inline filter
  • Pump (optional, depending on the site)
  • Various CPVC plumbing parts
  • Safety, inlet and outlet valves

A sheet of corrugated tin laid in a shallow wooden box is the basis of the solar collector.  The box base can be made of ¼ inch or ½ inch ply sheet, 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 so that cold water enters at the lowest point and exits at the highest. CPVC pipe is best, incidentally, because it has better heat tolerance then PVC. Paint the inside of the solar collector box and the pipes flat black, and close the top with a sheet of polycarbonate glazing.  

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.

Short pipe runs with no sharp bends are best.  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.

Once you have tested the solar hot tub for leaks insulate as much as possible – external pipes, sides of the tub and the tub cover (very important, to prevent heat loss at night).

Because the tub is designed to stand alone, any surrounding structure that you decide to add will be purely for convenient access and aesthetics, such as covering up the plumbing.

Proper positioning of the solar collector is very important.  In northern latitudes place it facing due south, or within 10 degrees of it, and sloping at between 15 and 30 degrees from the horizontal. Also you should slightly tilt the solar collector lengthwise to promote the upwards flow of water through the collector pipes.

Will heating the water in the solar hot tub take a long time?  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. Then you will need to keep the water temperature down rather than up!

It is essential, however, that you plan in thorough detail before starting to build your solar hot tub.  Decide on the size of tub and where to put it.  Then the best, and safest, bet is to get some good professional advice.




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