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 by pressing the hot water 'on' button on the programmer, nothing happens. But as soon as I press the central heating 'on' button, everything bursts into life and hot water starts circulating both through the heating pipes and through the hot water cylinder. But if I then switch the heating off again at the programmer (leaving the hot water 'on') the pump stops and everything goes off.
At first I thought the programmer might have developed a fault, so I installed a new one. The same problem persists.
From my own experience of a similar problem last year. I think you will find that the motorised valve has either stuck or has got a fault.
The alternative is there is a problem with the tank thermostat.
I am no expert, but I managed to deal with this last year, by replacing the motorised valve.
However word of warnning, I now have a similar problem again but one that means I cannot turn on the hotwater at all. which is why I was looking at the forum and saw your note (I think this is the tank stat)
hi broglet sounds like its your hot water zone valve thats faulty probably the actuator you can test it by setting your cylinder stat to max .and turn your room stat to min.then set your programmer to hot water only this will start your boiler and pump running wait about 3mins and check the pipework either side of your valve if the pipes are still cold then its a zone valve/actuator fault