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 selective coating of the solar collector is really good at absorbing sunlight and turning it into heat. The sun shines through the tempered glass, hitting the selective coating, and trapping the heat. The copper pipes are soldered to the selective coating and the solar powered pump circulates the water from the hot tub through the copper tubes in the collector, pulling the heat off of the collector and putting it into the hot 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.

For an already functional tub, set to 102F, the solar hot tub kit will start heating it in the morning, raising the temperature above 102 during the day as the sun heats the tub. Whereas, without the hot tub kit the tub’s thermostat would kick on multiple times an hour to keep the tub at temp, with the kit on a sunny day the backup heater will stay off. This is how you save energy. Depending on the weather, the size and insulation of the tub, how many collectors you have installed, etc., the tub may heat up to 104, 108 or higher during the day. This is all free energy. If you don’t use the tub that day, the tub will slowly cool down after sunset back to 102 and will be back on electric energy 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, jets can dissipate heat at 1 degree per minute with no heating element turned on.

Some people may want to install more collectors and turn their backup electricity off. 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. This will also work for people who are off grid or don’t have their hot tub hooked up to a conventional heat source. But, of course, you are at the whim of the weather at this point.

Plumbing Intregration

Our redesigned kit runs the hot water directly through the collector. There is no problem running chlorinated water through a copper pipe. In fact, copper has a disinfecting property and could allow you to use slightly less chlorine while keeping your water clear. There are two options for how you will get your hot tub water into the collector(s).

The first 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 here, 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.

The second option, is to run the pipe or hoses from the collector directly into the hot tub water. In this scenario, the pump will be mounted in a weatherproof box. The piping you use can be copper, PVC, or PEX. The latter two must be protected from UV. Flexible hoses can be a good option for the last step into the hot tub. Make sure that the supply pipe/hose always stays below the water line. And for those with freezing concerns, you will want to follow solar thermal installation best practices, one of which is to be sure that the lines are sloped to drain and have no places where water can accumulate and freeze.

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 two methods to see how big of a system you need.

1. 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.
2. 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 pretty 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. 

Placement

If your hot tub is located in a place that gets a lot of sunlight then you can attach your hot tub kit directly to the south side of the hot tub. If the roof above or the yard next to the hot tub is the best place, then let us know the distance between the tub and collector so that we can build your kit correctly. Because it is a pressurized closed loop, the pump can overcome the distance to a third floor roof or a collector mounted on the other side of the yard.

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.