John Rumm wrote:

On 18/05/2021 12:01, 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.

Yup, there are a number of different ways that they can detect the DHW
flow. Some use pressure drop valves or diaphragm switches, some direct
flow detection switches etc. Hard water scale can make many of them less
sensitive, as could diaphragm deterioration etc. What make and model of
boiler is it?

It's a Baxi 105 HE.

I realise I have actually replaced the diaphram before. It seems to move
smartly enough, it's not sticky, it just seems to move only when there's
more pressure than it should need.

Daniele