ASK THE EXPERTS: Designing a Solar-Heated Spa Tub


I live in Washington State and want to install a solar water heater to directly heat a spa tub. Can you guide me to some solutions?

Fernando Lopez • via

Soaking in a solar-heated spa tub is a great way to enjoy the sun and water together. You can relax, while using nonpolluting energy from the sun.

In most climates, the best design to serve a hot tub is a drainback solar water heating (SWH) system because drainback systems have freeze and overheat protection built into the design. Drainback systems are protected from frost damage by draining the heat-transfer fluid out of the exposed collectors and lines when the sun isn’t heating it. All the lines must be sloped so the heat-transfer fluid can drain into the freeze-protected area once the solar pump is off. In the summer, a drainback system with flat-plate collectors and copper heat-transfer lines will withstand the excess heat that builds up when your hot tub has reached its maximum temperature.

For spa tubs, the storage is the tub itself, which will lose heat when temperatures outside the tub are lower than the tub water temperature. To slow the heat loss, use a well-insulated tub with an insulated cover.

If you are OK with the tub’s temperature fluctuating depending on the amount of sun and the season, then one 4-by-8-foot solar collector is a good start. In moderate conditions, a typical hot tub might require 300 kWh per month (10 kWh per day) to heat. With lower temperatures, such as those encountered during the winter, it could require double that amount.

You can look up the estimated energy produced by a solar thermal collector at the Solar Rating and Certification Corporation’s website ( One typical 4-by-8-foot flat-plate collector could produce about 4 to 15 kWh of heat on a sunny day, depending on the collector, time of year, and ambient temperature. Where you live in Washington, during fall, winter, and spring months (and depending on how well-insulated your tub is and what temperature you desire), you will likely need a second collector, or a source of backup heat could be added.

It is important that the water pH (acidity) not be allowed to drop below 7, or copper can be dissolved and this can result in green stains (even make your hair turn green—really!). A filter is needed to protect the pump and collector from tub debris. Use copper lines at the collector, since PEX lines will not survive the high temperatures coming out of the collector.

A key component of any SWH system is a differential temperature controller—common manufacturers include Goldline, Resol, Steca, and ArtTec (for PV-direct pumping). The controller will compare the solar collector’s temperature to the tub water’s temperature. When the temperature at the collector is higher than the tub water, the solar controller will turn on the circulation pump. Once the tub water reaches the set-point temperature, the controller will turn the pump off. This also is a protective measure to prevent someone from being scalded.

Evacuated-tube solar collectors might be seen as an attractive solution because they have lower heat loss in winter. However, installing them as part of a drainback system may void their warranty. That means more complexity and expense—you’d need a heat exchanger, glycol fluid, and a heat dump (this could be as simple as leaving the spa cover off) to keep the tubes and tub from overheating.

Steve Dyck • Guelph Solar Mechanical

Comments (2)

Doug Kalmer's picture

Copper tubing is not compatible with typical hot tub chemicals such as chlorine and bromine.

Fred Golden's picture

Because you 'should' be draining all the water once a month, to reduce total chlorides, you will want to oversize the solar system. Then you can drain on Monday, it will be 105F by Wednesday.

Spa will evaporate lots of chlorinated water, leaving behind salts and excess chlorine. Check with a pool supply store about chemicals required, draining schedule, and how to test for total chloides.

Problem is when below 55F air temperature, flat solar panels release a lot of heat to the air, and would be difficult to go over 110F on a cold day. Evacuated panels can make 150F water, even on a cold day. I would have a 80 gallon tank with the top below the spa to drain back. Then you can draw 8 gallons per minute for ten minutes. This is 64 pounds per minute, warmed by 40F would be 2400 btu's per minute.

Then the evacuated tube collectors can reheat the water tomorrow.

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