Boiler keeps cycling

[Jody emmott]

I have a bosch greenstar 30 si which keeps cycling every 30 seconds only on ch. All rads get warm but not hot 14 in total. The pressure in the system holds about 1.7 bar .The system has been flushed all thermostatic valves work and are fully open. I have checked the pump it's fine. I have changed the pcb and overheat temp sender. The boiler stat is set to max and the room stat is calling for heat. The strange thing is if you put the boiler in service mode all rads get hot. If you then exit service mode the cycling stops until the burner cuts out when the stats reach temp ? Any ideas would be appreciated. This has only happened in the last few weeks.

 

[lightning]

Why would you change the pcb?????

 

[Jody emmott]

I have the the same boiler in another property. I changed the whole control panel. As process of elimination

 

[45yearsagasman]

Expensive way of repairing a boiler. Call a pro.

 

[MeldrewsMate]

Service mode (chimney sweep?) will turn the burner to maximum until the safety cut-out operates.
1. Check the boiler temperature hasn't been set low (i.e. to the temp of the rads that are 'warm, not hot')
2. Check if weather compensation has been fitted, this will cause rads to be cool if the outside temp is warm.
3. Check that the output power has not been limited to a low value, again overridden by chimney sweep mode.

MM

 

[MeldrewsMate]

All these posts about heating. Anyone would think there's been a Bank Holiday already!...

...and a shortage of jumpers.

Come on wimps, save a bit of gas for my kids please.

 

[MeldrewsMate]

 

[Jody emmott]

Thanks everyone. Bought a kane 455 flue gas analyser. Set the gas valve now all good. Was running too lean on minimum hence cut out and cycled. Boiler as good as new. Anyone want to buy a calibrated analyser?