Hot Tub

Do you dream of a hot tub in your backyard, luxuriating under the stars after a hard day’s work – perhaps with some friends to share it with?

Buying and installing a conventional hot tub will cost you upwards of 4,000 dollars.  After that there will be the constant heating costs. But you can build a solar hot tub for under 400 dollars, with no heating costs at all.  Free!

A diy hot tub is very straightforward.  You will get a great addition to your garden 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.

The basic components of a solar hot tub are:

• Solar collector box

• Stand-alone tub

• Inline filter

• Pump (option, site-dependent)

• 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.  Make the box bottom from a sheet of ½ inch plywood, 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 causing water to enter at the lowest point of the solar collector and leave from 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 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.  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.

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 (essential to minimize nighttime heat losses).

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.  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.

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. After that your main problem will be keeping the temperature down!

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 invest in some good professional advice – you will not regret it.