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Sorry, I get the feeling there is a sub text in there for some reason but it's too subtle for me.

Thanks for the link, but it's not clear that it's dealing with comparable flows. I've now had an answer from Worcester-Bosch to the same question: "The NRV prevents the formation of a "pocket" of stagnant water at the top of the cylinder by guaranteeing flow out of the top of the cylinder only...

Have you got a reference for that? It's surprising that the starting load would be significantly affected by even a moderate reverse flow.

.. but also? (this is starting to remind me of Peter Cook and Dudley Moore's "not only but also" shows, such as: ) ;)

OK, but the NRV prevents flow out of the circulation return tapping on the cylinder. For water to convect that way wouldn't it be necessary for the temperature at the tapping to be higher than at the top of the cylinder?

I understand convection but don't see how that would cause a problem without an NRV. Any chance of a few more words?

My secondary hot water circulation (i.e. returning the water from the furthest tap to a tapping about a quarter of the way down the cylinder) is quite noisy and I suspect the NRV might be rattling - leads me to wonder why there needs to be an NRV in the return line, before the pump ...

The F13 faults (main PCB connections, according to the manual) started a couple of months ago. All the connections seem OK and cleaning them with switch cleaner has made no difference. Switching off for a while usually clears the fault, occasionally it will clear by itself. Any suggestions...