In message , Jon
writes
For both h/w and c/h the boiler will fire up for 3 seconds then stop
for two seconds, fire up for 3 seconds then stop.. constantly. This
appears to have occurred overnight. It is a Biasi Riva, five years
old.

I replaced the h/w heat exchanger a year ago due to limescale, but
even before then the c/h was fine and the problem was gradual.

Any ideas very welcome. Please post to group, I do not check that
email address.

Have you checked a) the temperature sensor, and b) that you don't have a
blockage restricting the flow, or your pump has failed????
--
geoff