What to consider when choosing a solar hot water circulating pump based on the pumps head:
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The head rating on a pump is a static rating, and applies to the maximum head in an open loop system, such as pumping up to a roof tank. In the case of circulation through a roof panel and back down a return line, once the system is primed the weight of water falling down the return line is equal to the weight of water in the feed line, and essentially the pump only needs to overcome the total pipe friction in the system. When you take this into account the limitation is more to do with pipe length than the height.
When replacing the pump, it is also a good idea to replace the non return valve with an AVG solar rated non return valve . Most manufacturers require the non return valve to protect the circulating pump for several reasons.