Bengasman was referring to the plate-to-plate heat exchanger, which is the secondary heat exchanger. You are presumably looking at the main or primary heat exchanger. Check the manual carefully before tinkering.
Well if the outside of the water-to-water heat exchanger seems to be clean then it can't be that. Does any part of the boiler look like it might need a good hoovering?
What does the manual / user guide say about your exact symptoms?
If in time a reduction in hot water occurs, the likley causes may be impurities caught in the domesic hot water flow switch fliter or lime scale deposited in the domestic hot water heat exchanger. it is advisable to have the applience cleaned by a authorised sevice engineer.
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