Ed,

thank you very much for your reply.

Ed Sirett wrote:

It could be a combination of scaling in the DHW heat echanger and/or the
modulation failing.

how can scaling cause this ?
I saw some "descaler" products being sold -- do they really work ?

It can be quite hard to see the modulation on the gas flames the real
test is to measure the burner pressure and see it varies within the limits
specified in the book.

afaiu, I need a way of measuring the actual burner pressure.

Check the gas rate (see the Gas Fitting FAQ) all these test don't require
very expensive tools.

thank you, I'll read up on this.
is it common for gas valves to fail to modulate properly ?

I checked overheating thermostat -- it does not seem to be causing the
problem. the same for air pressure switch. voltage drop on the DHW
sensor seems to vary according to temperature.

I am completely lost now. I ordered the sensor though.

Why do you think that the fitter will recommend a pump - especially as the
pump has zero to do with the DHW on a Ferroli Moderna?

perhaps because he happened to have one in his boot ? 8) replacing a £5
thermistor is not that attractive as replacing a £200 pump, especially
if he got it from another unsuspecting customer. 8)

of course pump has nothing to do with DHW circuit, I was just trying to
make an example with something completely irrelevant.

/max