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?

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.  Then you have to face the heating bills. But you can build a solar hot tub for less than 400 dollars, and the heating bills are zero.  Free! 

It is very easy to build a solar hot tub.  You will get a great addition to your backyard and you will be able to enjoy it in all weathers.  You will need to fit a safety pressure valve to prevent steam build-up – the water will really get that hot!

Begin 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 panel

• Stand-alone tub

• Inline filter

• Pump (optional, depending on the site)

• Various CPVC plumbing parts

• Safety, inlet and outlet 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, with edges made from 6 x 2 planks or similar, 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 allowing cold water to enter the solar collector at the bottom and exit at the top. CPVC pipe is best, incidentally, because it has better heat tolerance then PVC. Use flat black paint for the pipes and box interior, and glaze the top with a ¼ inch polycarbonate sheet.  

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 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.  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.  Fit an inline filter at the tub outlet to avoid the chore of regular water replacement. Finally you should fit inlet and outlet flow control valves as well as a safety valve to relieve excess pressure in the system.

After testing the solar hot tub for leaks fit as much insulation as possible – all outside plumbing, the sides of the tub and the tub cover (vital 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.

Proper positioning of the solar collector is very important.  In the northern hemisphere it should face due south, or no more than plus or minus 10 degrees from it, and it should slope between 15 and 30 degrees from horizontal. It also needs to have a slight tilt lengthwise to encourage better water flow through it.

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!

Remember, proper planning is essential before you start to build the solar hot tub.  Know what size of tub you want and where you want to locate it.  Then invest in some good professional advice – you will not regret it.

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