Worcester Bosch 400 No CH after repair

[Railman]

Our Combi Highflow 400, was recently repaired by Bosch engineers. They were extremely professional and replaced various faulty parts (pressure vessel, thermostat) and left it working fine.
The next morning the boiler failed to fire for CH, but HW was fine. The pump was running, and if CH thermostat turned up, it would fire for about 5 seconds and then die down again.
As we were going away for a few days, I turned the whole lot off.
When we came back it was fine again.
WB engineer came anyway, and suggested it was an airlock in the boiler, which had dissipated while it was turned off.
Three days later, it has happened again.
WB engineer coming tomorrow.
If it was air in the boiler that was introduced during the original repair, why has it happened again?
How can I get rid of it for good, if I have bled all the rads?

Thanks in anticipation.

Dave

 

[dan the man]

Ive had exactly the same problem and it was doing my head in.

Hot water ok and heating fires up but goes off on temp after a few seconds.
I thought it was the diverter valve not moving over,
#

But it turns out at the back of the diverter valve is a small pipe that creates a pressure from the pump and helps the diverter valve shift over when it needs to for the heating. This was blocked as very small it easily happens..

ask him to check its clear as as soon as i unblocked it it worked

 

[Railman]

For all interested:

As dan the man suggested, this pipe was checked and found to be ok with no sludge.
The actual problem was a failing diverter valve. It appears that it can't be checked, but by replacing it, everything worked fine.
He also replaced the diverter solenoid, but this made no difference.

Dave