We have a Vaillant VCW GB 221h combi (hybrid) boiler. It's slowly developed a problem with the CH. The hot water works fine, but whenever the CH is on and the room thermostat clicks on, the boiler only fires for about 5-10 seconds and then the main burner cuts out.
The pilot light stays lit without problems.
It appears to cut out once the temperature has gone over about 50c (the thermometer shows higher than that a short time after it's cut out, which I just put down to a bit of a delay in the change) but then takes some time before it kicks in again (once the temp has reached about 35-40 deg and then cuts off shortly afterwards.
If I turn the numbered knob on the front of the main switch panel up to a higher number (where you have the CH switch and the main switch with the light on - it cuts out at the 50ish temp when it's on 3.5-4 which is right for the hot water) it stays on for a bit longer, but then the temperature ends up going to about 60-70. It still cuts out after about 30 seconds though.
I have replaced the thermostat control circuit, thinking it was that which was causing the problem - luckily I got a new one for £30 off ebay at the time - but it didn't fix it. I've tested various thermostats (the little ones on the pipes) to see if they cause the problems, but am I right in assuming that if one fails, it generally means the heating won't light at all as a safety precaution?
I'm unsure where to look next - the box of things on the right with three 'pins' and microswitches that stick out seems to move in and out ok when the hot water is used and the valve at the front with the black notched cam seems to turn ok when hot water is drawn as well.
There's a problem with the T shaped pipe where you let extra water into the system to top the radiators up - I think the valve has gone and it's letting water into the system slowly, meaning we have to occasionally drain a bit out of the radiators to keep the pressure down until that's fixed. Could it be that increased pressure buildup has damaged something in the boiler at some point? Maybe a diaphragm in one of the valves? The pump appears to run normally.