Combi boiler problems

[AKMild]

I have a Ravenheat combi boiler that's on the blink. I have called the plumber but the problem is intermittent and for my own peace of mind I'd like to know if there's an expert ABer out there who can think what the problem might be.

From time to time, the boiler will not fire up, even though the heating is switched on and the thermostat is not too high, or if I am running the hot water tap. What happens is that the boiler clicks, as if to light, but the burner doesn't ignite. The pilot light is on, and like I said, the problem is intermittent, so I don't think it's a gas supply problem. When it does work for the shower, at a time of day when the heating is off, the rads warm up, which can't be right. Also, the system pressure seems to fluctuate between 0.75 bar and 2.0 bar which I've not noticed before.

Someone suggested a 'diverter valve' problem, which means nothing to me.

Any ideas?

[The Builder]

Sounds like a job for the boiler engineer rather than the plumber AK.
Something is clearly amiss!
Diverter valve just sends hot water to HW or CH .......... depending on which is "calling" at the time.
Could be pressure vessel, or one of several causes.
Whatever, it does need to be looked at.

[terence7556]

when you say the rads heat up when you have a shower, do you mean the rad in the bathroom or more than one rad ?if more than one rad the diverter valve is letting by, it may have a split diaphram, if your valve contains one.
this is nothing to do with your ignition problem and you should do as the builder suggested and call in a gas safe engineer
most boilers need morethan one bar pressure when cold and this will rise to 1.5 or 2.0 bar when the boiler is ignited, its when it gets to 3.0 bar, then it indicitive of a problem

[AlBags]

If the boiler doesn't light .. and you have a pilot lit after the click .. it may be the thermocouple.
Diverter valve wear or leak-by for the other problem.
System pressure cold .. radiators all bled .. 1bar approx. May rise when hot to 1.5-2. Air in system will give more variation in working pressure on the gauge.