No hot water - Grant combi boiler

[RKB]

Looking for ideas on this....
Jumped in the shower this morning to discover it's ice cold...
Checked boiler pressure - OK at 1 bar. Switched on radiator heating - all OK, rads getting hot. Discovered that if I turned the hot water tap on fully in the kitchen or bathroom, hot water appeared. However, anything less than full on, and the water ran cold.
Do you think this is the divertre valve in the boiler? I can see a small piston activate a microswitch when I turn the taps on, so I think it's detecting the demand, but maybe the valve has failed internally?
Fortunately, the bath works (22mm pipe), so I won't pong too much wjile I work out what's wrong.
Thanks

 

[croydoncorgi]

If the boiler is firing OK and heating water at high flow rates, are you SURE its firing at all when the shower is used? If you are getting hot water at the bath tap AND the boiler is firing with only the shower on AND the shower's still cold - sorry, that's virtually impossible. The heat has to be going somewhere.

If the boiler is not firing at low-flow, chances are that the diaphragm / pressure switch that senses hot water demand is not working properly. Don't know the internals of the Grant Combi, I'm afraid.

 

[oilman]

It's the diaphram in the diverter valve. There is likely to be some problem with the blending valve too as it has probably never been cleaned.