I have a system boiler with zone valves, giving independent hot water/central heating or both. Recently when the hot water comes on, the central heating stops or goes very low. When hot water is switched off, the heating comes back on, although it never gets to full heat. I've changed out the controller with an old one, and the problem remains. There is good pressure and no leaks and the radiators are bled. I'm guess one of the zone valves is going out, possibly the pump not producing enough power/flow? But how to check?
Any suggestions appreciated.