ASK THE EXPERTS: Solar Hot Tubbing

Bob and Barbara Owens’ solar-heated hot tub
Bob and Barbara Owens’ solar-heated hot tub was featured in HP104. Heated water from the roof-mounted collector is piped directly into the tub. The SWH system pump and controller are housed in the box that also serves as a step.

Aloha! I want to build a solar-powered hot tub, and wonder how to keep the water at a constant temperature of about 103°F? Years ago, I made my own domestic solar hot water system. It worked well, but there was a wide variation in temperature depending on the sun conditions and the level of hot water usage.

Karim Wingedheart • via email

I assume from your greeting that you are from Hawaii. The combination of plentiful sunshine and the highest energy costs in the United States would make investing in a solar water heating (SWH) system for your hot tub a smart idea.

But don’t expect the SWH system to do it all. In HP104, Floridian Bob Owens describes his solar-heated hot tub, which needs help from the electric heater at times. Hot tubs are known to lose heat quickly when the jets are on, which is why many tubs are heated with a hardwired 240-volt, 50-amp dedicated circuit. (A 120 VAC resistance tub heater with a 20 A circuit has only 25% of the heating capability of the 240 VAC heater.)

The Owens’ single-collector SWH system heats his 360-gallon hot tub to 104°F just as an AC heater does, but it won’t keep up with the heat loss when the jets are on—losing 0.5°F to 1°F per minute. A 360-gallon tub losing 1°F per minute needs about 3,000 Btu of heat added per minute to overcome this loss and keep the tub temperature above 100°F. The heat loss will vary with the ambient temperature.

Any solar energy system capable of meeting this intermittent load would be so large that its size and cost would be prohibitive. The only solution is storage. In the case of a SWH system similar to the one described in HP104, a larger storage tank and more collector surface area would be required. A possible alternative is a grid-tied PV system—with the grid “supplying” the “storage.” In any case, the heat loss, size of the tub, and amount of time used per day are needed to know how to design an effective system.

If we stick with the 360-gallon hot tub and assume a loss of 1°F a minute for an average of 30 minutes a day, we have a daily loss of 90,000 Btu per day, or an electrical equivalent of 26.4 kWh per day. In Hawaii, an SWH system would require about three 4- by 8-foot collectors and a 120-gallon tank to keep up with the load. The tank water would be heated daily to 140°F to 160°F. To keep up with the heat loss, a pump of about 10 to 15 gpm (a medium-head pump) would be needed to transfer the heat from the storage tank to the tub.

In Hawaii, a batteryless grid-tied PV system that could supply the 26.4 kWh per day would need to be about 6 kW (DC). At an installed cost of $5 per watt, this system would cost approximately $30,000 (prior to incentives). The SWH system would cost about half that. This would change depending on applicable tax incentives and does not address the significantly reduced energy required to maintain the hot tub temperature with its cover on.

Please keep in mind this is a hypothetical scenario. Changes in heat loss, usage per day, installation location, and time of the year will all affect system size and performance. For example, an outdoor tub installation in Phoenix on a summer day won’t have any heat loss, while the same tub in November in Seattle will.

Chuck MarkenHome Power solar heating editor

Comments (5)

wallabygeorge's picture

Hey Ragoosus, You are probably overthinking this one. Best to divert excess hot water to the pool. Or use a tempering valve. Solar hot tub heaters mostly work when it is already hot. If you want heat when it is not hot the best way is solar AC. DC will work, but, watch out for relays and switches not rated for DC.

Second best and very workable are the refrigerant filled glass tubes. They will make steam in scotland. You will need a titanium heat exchanger. The hot water from the roof can then also be used for domestic hot water. The pool and spa chemicals will eat the solar panels if you do not use an exchanger.

Ragoosus's picture

I am also looking to heat my new hot tub/swimming pool combo. I am planning to build a 10x 10 array on my shed roof and not tie into the pool pumps. What is envisioned is a cold water intake from the pool and the hot water out flow into the hot tub. The hot tub overflow is a small waterfall back to the pool. I am looking for the electronics that would run a variable speed pump that would react to the grid temperature differential from 103 degrees. It would pump more water through the system if over 103 to cool it down and slow the flow of water to heat it up. Does the hardware for doing this exist any where?

Fred Golden's picture


I used to think the same way, heat the hot tub and let the excess hot water flow into a swimming pool. However it will not work well, and I only found that out by working at a commercial building with a hot tub and pool, on separate water systems, with separate pool heaters. You can not mix the waters in a commercial setting, or you would need to dump the pool water (26,000 gallons in my case) every 2 weeks by state regulations. We drain our hot tub every 2 weeks, probably could stretch that to 3 or 4 weeks and still comply with the state regulations, but don't want that much chlorine in the spa water. Dumping 200 or 300 gallons of spa water is no big deal, and we have a 100,000 Btu natural gas water heater to warm the water from 45 to 103 in about 8 hours.

The spa water will evaporate a lot of water each time you run it for about 30 minutes, and the jets are on. Each pound of water that you lose into the air will also "Consume" 1087 Btu's of heat, as that water will boil into steam, just like in a hot water boiler, or boiling a kettle of water on the stove. You are also heating the air that is leaving the spa (that was pumped in - in our case we have a 250 CFM air pump) so you are losing a small amount of heat to warm that 65F air to 103F. Warming 250 CFM of air from 65 to 103F will only take about 5,000 BtuH.

The spa water will leave behind a lot of solids as only pure water is evaporating from the spa, leaving behind all the solids, and salts. Once the concentrations of salts are to high, state law requires us to drain the spa. We also drain our pool twice a year as a precaution, to prevent injury and just keep it in top condition.

Your idea of a water fall is wonderful, and would look great backlit. However it will consume a tremendous amount of heat. Thousands of Btu's per hour that it is flowing. It is the same concept as our water tower. We have to get rid of about 2,000,000 Btu's per hour from our closed loop system - part of our heating and cooling system in the main building. We have water source heat pumps taking heat from this loop in the winter (with a 2,000,000 Btu make up boiler) and in the summer dumping heat into this water loop. By having a 5 HP pump flow water from the bottom of the water tower to the top, much of it evaporates, all of it flows across the tubing containing the closed loop water, and drops back into the sump, pumped back to the top again. Your small water fall will basically do the same thing. Each pound of water evaporated will drop 1,000 Btus of heat from the water, cooling it quickly.

As for heating the water, I will be using a set of evacuated tube solar water heaters. It will get stored in a tank, then copper tubing wrapped around that tank will be used to heat my domestic hot water. Seening that I might need a rather expensive titanium heat exchanger to run pool water through it, I might as well just use copper tubing wrapped around the 800 gallon tank to warm my spa. I know that copper tubing will not break down quickly, and will last for years under a normal pool.

Good Luck!

Fred Golden's picture

A heat exchanger could be used to warm the spa water, and transfer heat from the potable domestic water tank into the spa water as it is being pumped by the spa jet pump. A 90,000 Btu heat exchanger is pretty large, think about 1' diameter and 3' long. To store 90,000 Btu's in 150F heat that can only be cooled to about 120F and still be heating the 105F hot tub, you would need 3,000 pounds of water. 3,000 pounds X 30F is 90,000 Btu's. Also 3,000/8 pounds per gallon is a lot of water, or almost 400 gallons.

I don't know how accurate the 90,000 Btu heat loss per day is though. It would mean adding 90 pounds of replacement water per day, or about 11 gallons lost to evaporation.

By using dedicated tank and spa water in that tank, no heat exchanger would be required, and you can lower the tank size and temperature differential requirements. So a un-pressurized 200 gallon tank warmed to 150F and pumped slowly into the spa to maintain a spa temp of 105F will work, with the overflow from the spa returning to the (hopefully bottom) of the 200 gallon storage tank. This can still enter the top, but be directed to the bottom (cooler) part of the tank with some 1-1/2" diameter PVC piping. That will keep the hottest water at the top of the tank, and the cooler stuff at the bottom, stratified to allow more hot water to transfer int the spa each day.

Or heat it with wood fire and a small copper tubing heat exchanger?

wallabygeorge's picture

Hi There, The way to heat a spa with solar is to use a heat exchanger. Titanium is preferred. It is important to make sure that the potable water in the hot water system cannot be contaminated. The pressure in the potable system must be higher than the spa side. The usual circ pumps and valves are required. The normal spa temp controller can be used. Switch over the heater element output to the valves and circ pumps.

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