Much like seasonal swimming pools, spas and hot tubs can profit enormously from solar heating systems. This is especially true if you're installing a solar thermal system for your domestic hot water supply or space heating because the tub or spa can act as a shunt load, using excess output from the household system.
Tubs and spas use the most heat when they are initially filled. They are generally very good at retaining heat, especially if they are well-insulated, so a shunt load is often enough to keep them fully heated in the summer months.
There are two main ways of heating a spa or hot tub: with a dedicated solar thermal setup or as part of the home's overall solar hot water installation.
Dedicated solar thermal for spas and tubs
A dedicated solar installation can only be used with wood-stave hot tubs (the ones that look like a chopped-open barrel). Plastic tubs and spas cannot use a dedicated setup for two reasons: there's nowhere to put the heat exchanger and there's no way to dump any excess heat. The only way you could install a dedicated system for a plastic tub would be to heat a reservoir of water and use that to keep the tub topped off, which is inefficient.
For a wood-stave tub, a simple closed-loop system is best. A single standard collector (4 feet by 8 feet) and a PV panel are enough to generate the heat and power for most tubs. The pipes (made of 1-inch Pex, for example) are run from the collector into the tub, where they are coiled around the circumference, under the wraparound seat, then out again. A DC pump is enough to push the heated solar fluid around and keep everything hot and luxurious.
As part of a solar hot water installation
This is the more common approach to hot tubs and spas: they are a luxury addition to the normal domestic solar hot water installation. There are some additional considerations but the setup is fairly standard, well-tested and works with any kind of tub or spa – even the molded plastic models.
The first thing to remember is that most tub or spa water is treated in the same way as pool water: there are chemicals in there (usually chlorine) which will corrode copper, so all piping that comes into contact with this water should be schedule-40 PVC solid core pipe.
There's also the problem of the heat exchanger, for the same reason: that will have to be made of cuprous nickel or marine-grade stainless steel to avoid corrosion.
Most tubs and spas are well-insulated, so a single standard collector works for this setup, too (4 feet by 8 feet) – in fact, if you add a panel to your domestic hot water collector array and figure about 30 extra gallons (135 liters) of tank space, you're most of the way to a complete system!
Apart from the specialized heat exchanger, the tub/spa system will need a 120V AC pump for circulation and two aquastats.
The heat exchanger goes into the