Why You Should Build A Solar Hot Tub

Do you dream of a hot tub in the garden, 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 to buy, and there will be the fitting costs on top of that.  After that there will be the constant heating costs. But a solar hot tub can be made for about $400, and the heating bills will be zero.  Zilch! 

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.  These systems get so hot that you will need a safety valve to prevent steam building up!

Begin with a good set of diy plans.  Included should be a full list of materials needed, clear step-by-step instructions and diagrams, plumbing schematics and setting-up details.

These are the main parts of a solar hot tub:

• Solar collector panel
• Stock tub
• Filter
• Pump (option, site-dependent)
• CPVC pipes, connectors, elbows, etc.
• Safety, inlet and outlet valves

A sheet of corrugated tin laid in a shallow wooden box is the basis of the solar collector.  Make the box bottom from a sheet of ½ inch plywood, with edges made from 6 x 2 planks or similar, 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 allowing cold water to enter the solar collector at the bottom and exit at the top. Do not use PVC for the pipes, it has a lower heat threshold than CPVC. 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.  Otherwise a suitable pump of about 120 to 150 GPH rating will need to be fitted in the outlet pipe run.

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.

Test the hot tub system for leaks, then fit insulation everywhere – 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. Also you should slightly tilt the solar collector lengthwise 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?  Depending on where you live it could take as much as two days to reach a comfortable 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.  Know what size of tub you want and where you want to site it.  Then seek out some good professional advice – it really will be a good investment.