ASK THE EXPERTS: Solar Pool-Heating System

Pool collectors are relatively inexpensive and designed to operate at lower temperatures. They can be drained for freeze protection during the non-swimming season.
Flat-plate collectors are designed to operate at the higher temperatures required of a domestic water heating (DWH) system. Using them for DWH and pool heating may add unnecessary complexity and cost to the system.

I’d like to install an evacuated-tube solar heating system for my house and for my 40-by-20-foot swimming pool. What kind of system do you recommend, and how much would it cost? I will need permission from my homeowners association and a township permit to install it. Also, will the system be eligible for the 30% federal tax credit?

Sami Poykko • Cherry Hill, New Jersey

Good solar water heating system design depends upon several factors, including the targeted delivery temperatures, the heating demand, the climate, and the auxiliary fuel being used.

The delivery temperatures for a swimming pool and for domestic hot water commonly result in two different solutions if the systems are separate. Since pool water is typically heated within 20 to 25°F of the outdoor temperature, polypropylene solar collectors are usually the most cost-effective. Medium temperature collectors, such as flat-plate and evacuated-tube types, are commonly used in domestic water heating because their insulation helps the absorber retain the collected heat—even when outdoor temperatures are below freezing.

To heat both a pool and domestic water, a glazed flat-plate collector is most suitable. Evacuated tubes are less efficient for applications like pool heating due to collection inefficiencies. They also will be more prone to overheating in the winter, since a large system will be required to meet a swimming pool’s heating demand, and you will only be using that hot water for a portion of the year.

Unless your domestic hot water use is heavy, the solar collector’s size will be driven by the pool’s heating demand. Since evaporation is responsible for roughly 70% of a pool’s heat loss, solar pool-heating systems are sized based on the pool’s surface area. In New Jersey and given a reasonable solar orientation, collectors equal to 50% to 60% of the pool’s surface area should be sufficient. For a 20-by-40-foot pool, that would equal roughly 400 square feet of collectors. You might add one or two additional collectors depending upon your domestic hot water needs during pool heating season.

Since you are in New Jersey, the system needs freeze protection. A standard pool-heating system uses the pool water to collect heat by diverting the water through the collectors after it is filtered. Freeze protection is provided when the system is drained for the winter. Since your system would also heat domestic water, you will need to install heat exchangers—one for the pool loop and one for the water heater. The heat exchangers will transfer energy from the solar loop, which will need to be filled with propylene glycol or configured as a drainback system. If the system is drainback, this would also resolve overheating issues. Finally, how the system integrates with your water heating system will depend upon how you currently heat your water. Integrating a system with a traditional gas-fired water heater will require a different solution than one that uses an electric element.

Ultimately, combining these systems may not be the best idea. You will be paying substantially more for heating your pool than if using a standard solar-pool heating system, and, when the pool is not in operation, the system will be oversized for heating your domestic water. Using one system for two purposes could cost between $14,000 and $16,000 in parts, will require a complex design, and could easily take 100 hours to install.

The federal tax code excludes systems used to heat pools from the 30% tax credit. You’d need to discuss with your tax preparer whether any of your expenses related to heating domestic water would qualify. If you separate these systems, the parts could be $8,000 to $9,000, the labor could be reduced to 70 to 80 hours, and the solar domestic water heating of the system will qualify for the tax credit.

Vaughan Woodruff • Insource Renewables

Comments (3)

Peter Gruendeman_2's picture

Sunshine is a relatively dilute resource and equipment for collecting solar heat is expensive enough that one needs to design a system or systems for the intended goal. That's why the earlier commenter recommended to using one system for heating a swimming pool. Pool heaters are cheap, simple and durable--very much a DIY project.

There can be more of an overlap between solar thermal(ST) for space heating ST for domestic hot water. ST for space heat competes against the same dollars spent on air sealing and more insulation, which is typically being a better use of money than renewables unless one's heating bill calculates out to less than about 4 BTU per square foot per heating degree day. Others have written about this or you contact me off line. Mild NJ winters may not leave you with much of an economic incentive for using solar thermal for space heat once your home is well sealed and insulated. I used to live in NJ, near 7A on the turnpike.

In regards to solar thermal for domestic hot water(DHW), this older technology is now more costly than using solar electric for DHW. Older readers will howl at this idea; younger readers read the spec sheets and talk with people who have installed both, and yup, that's how it is, unless you have a considerable need for DHW an little space for collectors.

Long story made short, one grand, unified system to provide for all your various heating needs will be an expensive undertaking that will not likely make economic sense. Two or three systems (cheap pool heaters, better insulation and PV for DHW) are likely to provide much better return on investment.
Pete Gruendeman
La Crosse, WI's picture

Dour is my specialty ;)

A pool cover is always a great idea and a transparent one will let more passive solar heat reach the pool water. There is significant complexity to integrating domestic water heater with pool heating, and the pool heating demand is likely going to be significantly higher than that for domestic water heating. I recommend being very careful if you are going to try to use a domestic hot water system with electric backup to heat a pool as well. The energy use and cost could be extraordinary.

Carl Wagner_2's picture

Such dour advice, but don't shoot the messenger I guess. What about a black solar cover for the pool and sizing the system for domestic hot water only. After it's built add a loop to the pool with electric background p and see how it goes?

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