We have a Alpha Combi HE CB33 boiler which has recently started to fail to heat the hot water. Sometimes you turn on the hot tap full and no hot water comes out even after running for 30 seconds. Sometimes it is hot as normal as soon as the boiler kicks in. On one occasion the hot water cut out mid-shower.
Probably a problem with the diverter, maybe the diaphragm is split or stretched.
Do you find if it only happens at certain times of the day, they tend not to work when there's a greater demand on the MCWS, first thing on a morning before when people are showering etc before going to work and again in the evening. As there's not enough pressure to make the connection with the microswitch.
My Gloworm Combi seems to have a similar problem. It is quite randomly deciding not to fire up and heat the hot water when the hot tap/shower is switched on.
It seems to be related to the flow rate on demand. At its worst - we have to turn on the kitchen tap (with the greatest flow) to get the boiler started, then quickly switch on the required tap/shower to keep it going.
It seems to need more flow rate to get it going than it used to?
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