My boiler (an Ariston Microgenus 27) has developed an intermitent fault. Sometimes I come home expecting a nice warm house to find the place freezing, but the boiler whirring away quite happily to itself, the burners coming on for a few seconds then going off again. When it's in this state the flow pipe to the rads is hot, but the return is cold. The LEDs on the boiler show that the heat exchanger has got up to temperature, which presumably is why the burners keep cutting off. I've found one way of solving this is to turn the radiator circuit temperature control on the boiler right up. This seems to waken it from it's little trance, it starts to heat the rads and then I can turn the knob back down and all is well (until next time this happpens a few days later).
I think this might be a failed (or failing) bypass valve (which is an integral part of this boiler). There's no legitimate reason why the bypass should have kicked in - the rads are all on, the TRVs are demanding heat because the place is like Siberia, and anyway there's one rad with no TRV. What do people think of this diagnosis? The only reason I'm doubtful is that I'm not sure that the flow pipe should be getting so hot. Surely if the bypass has kicked in the water should just be circulating within the boiler? And why would turning the temperature knob up solve the problem?