Ed Sirett wrote:

The heat transfer is so restricted by the chalk that the boiler cylces
between minimum and off gas rates.

don't understand. surely this means that the heat is just lost into the
flue and that water temperature in the exchanger would be lower ? I see
something quite different -- the water is quite hot.

The problem is to get the descaler into the pipework.

how is this generally done ?

IME this fault is not common and would be at least as likely to be PCB
driving it as the gas valve itself.

yes, I'd think so. I'll try to check the driving signal.

That's why you need help.

yeah, maybe it is time to call for professional. 8)

Why?

the old sensors looked dodgy. the DHW one is fit loosely in it's casing
and I suspect the internal contact may be bad. I bent the pins and
rotated the internal plastic part -- it helped. it does not cycle that
often anymore.

for the parts. Selling parts that are not required at prices that are
highly inflated is not a good strategy for building up a base of loyal
customers.

of course you are right. perhaps I watch TV too much. 8)

...on a Ferroli but on most other makes it has a lot to do with it.

to maintain DHW's constant flow ? seems unnecessary for common water
mains supply, where pressure is always quite high.

thanks for your explanations, Ed.

/max