many thanks namsag and dal5band for your superb explanations
yes dal5band - the water is heating (as long as the central heating is on) so I think it has to be the microswitch which is not operating
namsag - from what you are saying it sounds as though the valve may not be opening fully...
Thanks for your quick reply namsag - would this be an electrical fault or a mechanical fault?
Would a fault in this motorised valve also be what is preventing the pump starting when the hot water is switched 'on'?
The motorised valves I have are Honeywell 8901 - are they easy to replace?
I have a fully pumped gas-fired system with motorised valves controlling hot water and heating. Hot water is stored in an indirect cylinder with a thermostatic control (which appears to be working correctly). Until recently it was all working correctly.
Now, if I try to heat the hot water...