Gloworm 18hxi working but not firing for long

[jamessouthampton]

Hi, bit of a strange one this the boiler fires ok and seems to run fine with no fault codes etc but it fires heats and the temperature on the display starts increasing until it reaches the target (which takes about 20 seconds) and then it cuts out. About a minuute later it repeats this process. The radiators get warm but not hot.
The pump appears to be working by the water tank upstairs but is there another pump in the boiler? Just thinking that it seems like the water is heating up too quickly in the boiler and perhaps it is not being moved round the system fast enough perhaps?

Any advice appreciated.

Many thanks

 

[mickyg]

it is not being moved round the system fast enough perhaps?

yes
faulty pump, blockage, air lock, dry header tank/no pressure. Something along those lines. I would expect the boiler to be a bit noisy as it gets up to temperature as well

 

[jamessouthampton]

Thanks for replying. Bizarrely it was the same first thing this morning until I went in the kitchen and watched it for 10 minutes where it seemed to be operating normally again, gets up to temperature then backs off the gas a little then waits for it to go down a bit then increases the gas again and repeats. Not sure if fixed for good or just happened to work while I was watching!
If it was air in the system is it possible this could have moved round and escaped perhaps?