Dreaming of a long soak in a hot tub in your garden under a starry sky – maybe with some friends to share it with?

Buying a hot tub will set you back at least 3,000 dollars and probably a lot more.  After that there will be the constant heating costs. But a solar hot tub can be made for about 400 dollars, and the heating bills will be zero.  Free! 

A diy hot tub is very straightforward.  You will get a great addition to your backyard and you will be able to enjoy it in all weathers.  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.

Basic parts of a solar hot tub are:

• Solar collector box

• Stock tub

• Inline filter

• Pump (optional, depending on the site)

• Various CPVC plumbing parts

• Safety, inlet and outlet valves

The solar collector can be a simple sheet of corrugated tin in a shallow wood 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 line it with a 1 inch sheet of insulating foam.

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. Use CPVC for the plumbing, rather than PVC, because of its better heat tolerance. Paint the inside of the solar collector box and the pipes flat black, and close the top with a sheet of polycarbonate glazing.  

You can try out various stand-alone tubs at your local hardware store and choose a suitable size, usually between 150 and 500 gallons depending on the number of people to use it at one time.

If you can mount the solar collector at least one foot below the tub to allow thermosyphoning, you should be able to avoid fitting a pump.  Otherwise a suitable pump of about 120 to 150 GPH rating will need to be fitted in the outlet pipe run.

Pipe runs should be as short as possible, and avoid sharp bends.  You should fit a filter near the cold water outlet, otherwise you will need to empty and refill the tub regularly, which is a chore. You will also need control valves at tub inlet and outlet, and a pressure relief valve for safety.

Once you have tested the solar hot tub for leaks insulate as much as possible – all outside plumbing, the sides of the tub and the tub cover (essential to minimize nighttime heat losses).

Any structure you decide to build around the tub will be for aesthetic or practical reasons such as access, since the tub is designed to hold its contents without external support.

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.

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. 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.  Determine the size of tub you want, and where you want to locate it.  Then seek out some good professional advice – it really will be a good investment.