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A flashing dhw led means that the preheat function is trying to come on, I'm guessing it is an instant model with the preheat ? However this should also be the same in just dhw mode. If the flow switch makes on the domestic side then does the pump run ? If so try lifting the primary flow...

I can't understand if you mean the pressure relief valve is passing, or the expansion vessel is leaking. The pressure relief is a bitch to get to, but I can do it in about 90mins to 2hrs, it's just some thing else will leak during the drain down and refill, but ideal still do the hydro/gasket...

Ok having just given a recent brief to fellow engineers on this matter, I might be able to help explain not only what an interlock is, but how to answer your question (I said might) An interlock is by definition either, 'safety' or 'efficiency' A safety interlock can be an air pressure...