I have Baxi Bermuda gas fire, with back boiler, which supplies central
heating and hot water to my home.
A few days ago the water heating failed. I checked the header tank
which was O.K. and, on the highest thermostat setting, the central
heating appeared to operate satisfactorily although, when at less than
maximum setting, it failed to operate the pump.
When the water heating was tested on the same maximum thermostat
setting, the boiler lit for just over a minute, rested for 3 mins then
reilluminated. I allowed it to operate in this way for over an hour
but, although the flow and return pipes behaved as expected, the water
temperature at the tap was unchanged i.e. cold.
It seems that no heat exchange is taking place between the primary and
secondary water supplies. I'm reasonably sure that the boiler
thermostat is faulty, but that does not explain this odd behaviour.
Can anyone offer any suggestions as to what I should explore next,
please, or am I wandering down a blind alley?
Thanks.

Tony.