Hi . could be thermostat fault and shutting off too soon or system sludged and cutting out on the overheat stat. Could PCB fault ( some boards heat up , expand and break contacts but most likely first two.
When the boiler stops does the fan keep running ?
Also check the heating pipes are they red hot? When it shuts down?
Hi .arevyou there?
Ok . did you try this on hot water. Can you please try the heating, does it do the same?
Sorry , got 3 conversations going AT the mo .
If your pipes are hot it must have fired up?
Could be the fan .it may be running but not at full strength .when the fan runs it switches the air pressure switch . this then opens the gas valve and sparks ng starts. Temperature is regulated by the stats. If you change the fan changevthevair pressure switch at the same time
I'm just wondering whether its going out on the overheat stat. Could be getting too hot and switching off. This could be caused by week pump, sludged heat exchanger,or fault stats. If the pipe ere getting too hot near to boiling then could that . if you have a multi Meter the ohms for the pump and fan can be checked