Combi boiler water goes hot and cold

[theferret]

Combi bolier potterton performa 24 eco e - when hot water runs the flame will burn for a few minutes then cut out only to reignite within a few seconds (long enough for shower to go cold and the missus to moan at me a lot). British Gas serviced it just before it started happening. Have turned the thermostat up high but still does the same thing. No lights on the front panel to suggest flame failure and always starts first time.

Any ideas? Dicky thermostat ... guess it is not the safety cut out as then it would not start again so quickly.

cheers
Graham

 

[bellairaphon]

possible diaphragm. take the case off and open a hot tap. in the left hand bottom corner a rod will pretrude from the divertor manifold behind a microswitch watch the rod and see what its doing staying where it is or moving back in breaking the switch. u can test by moving the little lever on the mirco switch whilst the tap is open and seein if the boiler stays on. also check with the hot tap open that the ch flow pipe dosent get hot.

 

[AlanE]

Blocked secondary heat exchanger.

 

[namsag]

Usually a partially blocked plate heat exchanger .
But if the low gas rate is not set right and is too high the boiler will not modulate down correctly and cause it to cycle which maybe the case if it happened straight after a service