Can I Heat my Water with an Air-Based Solar Thermal System?

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The simple answer to this question is "No", but the reality is a little more complicated. It is actually possible to heat a domestic water supply with an air-based solar thermal system... it's just a very bad idea.

Historically, many attempts have been made at creating a reliable air-based hot water installation. This was particularly true in the 1980s, when a system was tried with an air-to-liquid heat exchanger installed in the hot air duct. The exchanger passed heat to water pumped from a storage tank by means of a circulating pump that triggered whenever the blower came on.

Theoretically, the blower pushed hot air down the ducting, the air heated the exchanger and that heat was passed to the hot water supply. In reality, the system failed because it was incredibly inefficient – the air in the duct was never very hot, so there wasn't much heat to pass to the water tank, and the expense of running the blower, circulating pump and dampers pretty much wiped out any financial benefits the installation could offer.

On top of that, the setup has no freeze protection. The heat exchanger has to be on the warm side of the damper, so if the damper fails in cold weather, the exchanger can freeze. This happens surprisingly often – just about every system ever installed has frozen at least once. So what happens when the exchanger freezes? Worst case, it bursts. Then, when the system thaws out, you end up with water spraying out of a broken heat exchanger, which is attached to your mains supply so it never stops spraying water. If you're not there when the burst exchanger thaws out, you end up with a house full of water and a very large repair bill.

The only system design that could possibly work would have a closed heat exchanger loop filled with an antifreeze glycol mix. You'd need a second pump with an external heat exchanger, though you could use a single pump if the exchanger is inside the storage tank. And the system will still be horribly inefficient.

Lesson to learn: do not try to heat your domestic water supply with an air-based solar thermal system.