I have a Vaillant ecoTech Plus combi boiler. The central heating works fine. But the hot tap water became intermittent. If I got lucky and hot water would come from the tap, it never went cold as long as it was running. So I think it’s likely a flow switch because there were times when it simply did not seem to detect that hot water was on. I heard no boiler fan and the faucet icon did not appear on the display on the times it failed to produce hot water. The moments where it would work are now history. Now it’s cold water every time. So it was a gradual manifestation.

I popped the cover off and there’s some kind of scaling or buildup of something on the pump. The clip that holds something in place above the pump is corroded. So while the system behaves as if it does not detect water use, the pump does not look good. Would a faulty pump cause my issue?

About two years ago I had an intermittent hot water problem and it turned out the secondary heat exchanger was blocked by limescale. In that case the water would go cold mid-shower. The current intermittency problem is just with the initial switching on of hot water - no interruption once it was going. So I kind of doubt the heat exchanger is my issue.

Any theories?