On Tue, 18 May 2021 12:01:08 +0100,
(D.M. Procida) wrote:

What's a typical cause when a boiler starts to require increasingly high
demands for hot water before it decides that it should light up the
burner to deliver it?

My guess is that there's a pressure drop valve, and that it has become
stiff or its seals have become sticky.

On our multipoint gas water heater there is a water pressure detector
valve that is linked to a gas control valve via a small stainless
steel push rod and on my first MPWH a slight leak there caused
corrosion and so to stick (and eventually stuck) the interaction
between the two.

I removed the water side, released and cleaned the pin, replaced the O
ring and re assembled it with some suitable silicone grease and it was
fine for ages after that (till neighbour got rid of his later one and
we upgraded ours). ;-)

No idea if that's how they work in combi boilers (might be in the
older ones)?

Cheers, T i m