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.
What can be wrong?