Recirculation Solar Thermal Systems

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A recirculation solar hot water system is almost exactly the same as a flooded open-loop installation. However, it has one significant advantage: in times of very cold weather, it can recirculate warm water from the storage tank through the pipes and collector array to prevent freeze damage.

A typical layout for a recirculation solar thermal systemTo do this, the solar thermal installation must have a microprocessor controller which can switch on the system pump when temperatures fall to dangerous levels. Many differential temperature controllers come with a recirculation option, so it's not hard to source an appropriate model. Unfortunately, this requirement also negates the use of a PV-powered pump.

The recirculation option only pumps warm water from the storage tank through the pipes and collector. It doesn't heat the water in any way, so it's not guaranteed insurance against a long freeze or extremely cold weather – once the warm water in the tank has cooled, the solar hot water installation is just as susceptible to freeze damage as a flooded open-loop setup.

Of course, the water being recirculated is also the same water that the system heats, so cold weather badly affects efficiency and raises grid-tied operating costs as the backup water heater does more work. You'll still need a water softener as well, since the system is just as prone to clogging from hard water as a flooded open-loop setup.

Recirculation solar thermal systems really just provide a bit of emergency cover in locations where freezes are rare, mild and short-lived.