On a venerable Ravenheat 85ET...
If the heating is running, and radiators all warmed up, then the hot water comes out fine (v. hot).
If heating is off, then the hot tap does start the boiler burning, but water is tepid at best. The radiators, though, do start to warm up. (Haven't run long enough to see if they get fully hot, but they're getting there).
I've taken off the diverter valve. Diaphragm doesn't have any obvious holes or tears, though it is looking old. Testing over the sink, just dribbling in water to the connections, in heating-only mode, the valve seems to work as I'd expect i.e. closes off one of the pathways between the inlets/outputs. In hot water mode, though, it doesn't seem to completely close the path to the heating supply i.e. all three inlet/outlet connections are still, to some extent, open. Is that right? If not, I'm guessing that means the internal valves/seals inside the diverter have failed (or are gritty)?
I don't quite understand why heating on gives me hot water, but water-only doesn't.. I might expect that to work, too, but possibly heat the rads. as well?
Am currently planning to replace the entire diverter, I think, (not just diaphragm). Any other ideas before I shell out?