Indoor swimming pools have exactly the same heat load throughout the year. A solar thermal setup can provide 50% of the load in winter and 100% in summer – the amount varies because there's less solar radiation in winter to generate heat. In general, an indoor pool solar thermal installation is sized to generate the right amount of heat on the best possible day: this ensures that the system never overheats.

Indoor pool heating systems have a longer return on investment than their simpler, plastic, seasonal siblings: there's more equipment, it's more expensive and the systems have to be under-sized to avoid overheating problems in summer.

However, you can get a bit more out of them by mounting the collector array at the ideal angle for the winter sun, so that you maximize output during low-radiation months and lower the summer input at the same time. There's a trade-off there – between a larger array and more balanced year-round output – that requires careful analysis and calculation.

A solar thermal system for indoor pool heating is almost exactly like a standard pressurized or drainback installation. It includes all the usual equipment, from collectors to piping, gauges, controllers and everything else. The one big difference is the heat exchanger: since the water being heated is chlorinated, a copper heat exchanger would corrode and become useless.

Special heat exchangers are required in solar pool heating installations. There are two excellent options available, depending on pool type:

• Large Pools with surge tanks are designed to handle several people entering the water quickly at the same time – in other words, a bunch of people jumping in! The surge tank sometimes contains the water filter and is a great place to install a coil of Pex tubing to act as a heat exchanger.
• Pools without surge tanks usually place the heat exchanger directly in the filter piping, just after the filter.

If you opt for a pressurized system, you can run the entire setup with a PV-powered pump: only one pump is needed (for the solar loop), which can be controlled by the surge tank (if there is one) or on the same daytime timer as the filtering system if there's no surge tank.