How the Solar Hot Tub Kit Works

The Sunbank Solar Hot Tub Kit is the least expensive way to heat a hot tub, but how does it work?

It’s pretty simple, really. The Sunbank flat plate collector traps the heat from the sun, and copper pipes running through the collector allow the hot tub water to circulate through the collector, via a solar powered pump, transferring that heat into the tub. And just like that, you’re heating your hot tub with the sun instead of fossil fuels.

What to Expect

The solar kit can be added to any hot tub, but it’s good to know how it works and what to expect.

The solar hot tub kit will start heating the tub in the morning, raising the temperature above your electric or gas thermostat setpoint during the day. Without the solar hot tub kit, the tub’s thermostat would kick on multiple times an hour to keep the tub at temp. But with the kit, on a sunny day, the backup heater will stay off. This is how you save energy. Depending on the weather, the volume of the tub, how well insulated it is, how many collectors you have installed, etc., the tub may heat from a 102F setpoint (for example) up to 104F, 108F or higher during the day. This is all free energy. You can also turn down the thermostat setpoint to allow the solar to get the tub up to temp and save more electricity. If you don’t use the tub that day, the tub will slowly cool down after sunset back to 102F and will be back on electricity for the evening. If you decide to have a sunset soak, you can easily dissipate any excess heat by turning on the jets. With the insulated cover removed, spa jets can dissipate heat at up to one degree per minute with no heating element turned on.

Some people who are either off-grid or want to run their spa completely from solar, will need to install two or more collectors depending on the size of their hot tub, the local climate, etc. Using the system like this works well for sunset soaks, as the system is typically at it’s hottest at the end of the day, cools overnight, and heats again the next day. Using the solar hot tub kit like this means, of course, that you will only have a hot tub on sunny days.

Plumbing Integration

Our redesigned kit runs the hot water directly through the collector. There is no problem running chlorinated water through a copper pipe, as long as the water stay between 7.4 – 7.8 pH and 0.5 – 5 parts per million (ppm) of chlorine.

There are two primary options for how you will get your hot tub water into the collector(s). The first option is to connect the solar pump to the hot tub’s drain and pipe the return water from the collector into the hot tub. The second option is to integrate with the hot tub’s plumbing. Hot tubs typically have PVC plumbing, and you will cut a section of this piping out and add in two tees, a supply and return to the collector. The pump will be mounted inside of the hot tub housing, where it is dry and safe from freezing. If mounted outdoors, the pump will need to be housed in a weatherproof box. The pump will turn on either when sun hits the solar panel mounted next to the collector, or when the controller turns it on, according to your settings. Hot tub piping varies, and so these pipe and fittings are not provided in the kit. Check with your spa manufacturer if you are concerned about voiding your warranty. 

Sizing

How many collectors you can use depends on a number of factors, including: how much space you have for collectors, your local climate, collector shading, how well insulated your tub is, the size of your tub, how often you use your tub, whether or not you use jets (they quickly dissipate heat), etc. There are three methods to see how big of a system you need.

1. The rule of thumb for hot tubs is that you want the solar collector area to be 100% of the surface are of the water in the hot tub. Our solar collectors are 22 sq ft, so if you have full sun and your hot tub is 7′ x 3′ (21 sq ft), you’d go with one collector. And if your tub is 8′ x 8′ (64 sq ft), you’d install up to three collectors. 
2. If you can isolate the energy use from your spa or hot tub heater, either by looking at your utility bill (which is difficult unless it is the only appliance that uses gas, for example) or by metering the electricity consumption.
3. Heat your hot tub up to temp and then turn off the heater. Open the cover and turn on the jets for the amount of time that you would normally use the tub. After 24 hours, measure the temperature of the tub. If you know the temperature difference and the volume of the tub then you know how much heat input it requires in a day. This is not as precise as metering, but is a close approximation.

If you have a functioning tub, it’s probably best to start with one collector. You can always add more later. If you are using solar as your only heat source, you should start with at least two collectors.

Freezing Climates

If your hot tub is located in a place that gets hard freezes or you need to mount your collectors further from your hot tub, like on a roof, then a “closed-loop” system may be better for you. A closed loop system uses non-toxic antifreeze, known as glycol, in a pressurized loop between the collector and the heat exchange with the hot tub. In non-freezing climates, distilled water can be used in the closed-loop in order to pump the fluid to a nearby roof that would require a much more powerful pump in an open-loop or drainback scenario. Closed-loop systems are more complex and need to be in use as long as the sun in shining. Contact us for more information if you think this system is right for your location.

In Summary

The hot tub kit may be a good fit for you if:

1. You want to use less electricity or gas to heat your hot tub.
2. You feel like you have a good understanding of how the system works and are confident doing basic plumbing work.
3. You have a good location to mount the collector(s).
4. You are fine with slightly overheating your tub in order to save energy.
5. You understand how to properly freeze protect your system, if necessary in your climate.