always worth changing the diaphragm ,if it is indicating overheating then the heat exchangers might be restricting the flow of water due to sludge or limescale, remove the domestic heat exchanger and have a look down the inlet and outlet it doesnt need much to slow the flow down but cleaning them properly is a tricky job ,you really need to pump warm descaling acid around the heat exchanger,if your not sure you can tackle this job calling a plumber could actually save you money
roy(service engineer)
October 2006
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