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I had a new Combi boiler (a Vaillant EcoTec exclusive green IQ 843) installed a few days ago replacing a previous system one. My flat has 2 bathrooms on two floors with 2 showers and 1 bath.

Heating works, and hot water mostly works as well but there's something that kicks in when we shower that is not good.

After a few minutes using the shower (say, 4-5) a vibration/buzzing noise starts being audible through the pipes, and the boiler goes from outputting water at the configured 65C down to a value that constantly swings between 48C and 54C, resulting in perceivable changes of the water temperature coming down the shower.

At some point in one case the boiler completely stopped outputting hot water, and the display showed 17C with the tap icon constantly on (meaning "draw-off mode with no burner") as far as I can see.

I've noticed that the boiler live monitor constantly reports state S40, which is "comfort protection active", and according to the maintenance manual ( ... 413944.pdf) should be a temporary mode (but for us it's constantly on).

Our installers suggested that there might be a TMV "somewhere" that might be malfunctioning, but we couldn't find one and before starting to open floorboards and break walls/ceilings I would want to make sure there is not a more plausible explanation. Someone advised me to turn the cold feed to the boiler off and try to open the hot water taps, saying if I got water out it would mean I have a TMV, but none of the taps/showers actually get any water when opening the HW tap in that case.

Does anybody have any idea what might be involved here?
what is the system pressure reading on the expansion bottle?

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