Noise from Boiler and pressure fluctuation

[mysteryman]

I go along with everything DP said, but I was referring to the water-to-water or plate heat exchanger. It is probably this which is obstructed on the primary, or boiler side, as distinct from the secondary, or domestic side. This is why you are getting satisfactory central heating, but not hot water. The waterways in the plate heat ex are narrow and more liable to blockage.

By all means try chemical cleansing, but be prepared for a powerflush if that doesn't work.