Do you dream of a hot tub in your backyard, luxuriating under the stars after a hard day’s work – 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 a solar hot tub can be made for about 400 dollars, and the heating bills will be 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.  You will need full schematics, clear building diagrams, comprehensive lists of materials and professional advice.

Basic parts of a solar hot tub are:

• Solar collector box

• Stock tub

• Filter

• Pump (optional, depending on the site)

• CPVC pipes, connectors, elbows, etc.

• Pressure relief and control valves

A simple solar collector starts with a sheet of corrugated tin laid in an open wooden box.  The box base can be made of ¼ inch or ½ inch ply sheet, with edges from 6 x 2 lumber or whatever comes to hand, and finished with an insulating foam sheet as a liner.

The water is heated in a series of ¾ inch CPVC pipes, laid and glued along each groove of the corrugated sheet in a serpentine pattern 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. 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.  

Use a stand-alone tub of between 150 and 500 gallons, depending on how many will be in it.

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.  Otherwise a suitable pump of about 120 to 150 GPH rating will need to be fitted in the outlet pipe run.

Try to arrange the shortest possible pipe runs and to avoid 90 degree 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. Inlet and outlet valves (to control water flow) and a pressure relief (safety) valve should also be fitted.

Once you have tested the solar hot tub for leaks insulate as much as possible – any exposed pipes, the external walls of the tub, and the tub cover (vital to lower nighttime heat loss).

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.

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 also needs to have a slight tilt lengthwise to encourage better water flow through it.

How long will it take for the water to heat up in the solar hot tub?  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. After that your main problem will be keeping the temperature down!

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.