Harryfox121
To answer your point I will try to explain how the boiler works.
1, There are two heat exchangers above the flames at all times.
2, on call for hot water the DHW flow switch is closed the diverter valve moves to circulate the CH water round the boiler tripping the CH flowswitch
3, flames come on heat hot water to taps.
4, flames also heat water in CH heat EX which is pumped round the boiler to cool it down (as the CH is not required). before the water goes through the heat EX again.
5, The above takes place with the CH turned on or off as you do not get heating when hot water is being called for.
6, Due to the above if the pump is not working the water in the CH loop will over heat and the O/H stat will trip out the boiler.
7, If the boiler is set for CH. and the flow switch is stuck in the ON position the boiler thinks water is flowing and the system lights up, and the heating will appear to work as it should (it working under the control of boiler stat room stat ect.
8, if the pump is not working the boiler is alight but the heated water is not getting away from the heat EX so the over heat trips out the boiler.
9, are you still with me
10 I rest my case