Worcester 35CDi combi, intermittent starting.

[pangowen]

I have a Worcester 35CDi combi, about 10 years old, which has an intermittent fault.

About 35% of the time, in the morning and the evening the boiler fails to start when the central heating should come on.
It continually cycles through the same sequence, clicks repeatedly and what sounds like a fan spins up, but the burners do not light.
Sometimes the CH Demand is flashing slowly, but not always.
It can usually be fixed be either pressing and holding the reset button, or turning the boiler off completely and putting the central heating temperature control to max, or running the hot water, and usually a combination of all 3.
There is another issue in that it sometimes seems to cut off too early, in that the central heating comes on, but it cuts out and stops heating before the radiators get really hot.
The CH Demand light is on, since it is in the period when the boiler should be on, but the burners are not firing.

There is no issue with the hot water at any time, the boiler always fires to heat the hot water.

I have checked the pressure in the expansion tank and it seems OK, air comes out when I press the valve.


Any ideas what it can be ?

 

[Hardy1]

flow switch diverter valve, thermistors, its hard to say cause where not standing there(not being rude in anyway), get a gas safe registered engineer in, he'l sort it.

 

[diamondgas]

:iagree: gas safe engineer needed ... Intermittents are hard to pin down though, you've got to hope it'll happen wen the engineer is there! That isn't always the case though 🙂