Potterton 100 not turning off

[DavidE70]

The expansion vessel behind the boiler.
The boilers energy is going into the radiators...the burner will only modulate down once the target temperature set on the front panel is approached.
I'm not convinced yet you have a problem.
You do have a room thermostat in an area with a radiator with fixed valves ie. you don't adjust them.

Its the 100e so only servers CH at 1 temp. I take your point about the expansion vessel.

I guess I maybe jumping the gun on the temperature. What would be a quick way to test it? Shut all the rads down and power on the boiler until it turns off, overheat gets triggered or pressure gets tripped?

 

[Gasguru]

Shut some rads down and the burner should modulate down...look through the sight glass...the flames will get smaller.
Flow temp will be up around 80 to 85 degrees...at that temp it feels almost boiling.

 

[DavidE70]

Shut some rads down and the burner should modulate down...look through the sight glass...the flames will get smaller.
Flow temp will be up around 80 to 85 degrees...at that temp it feels almost boiling.

Really appreciate your help on this. I shut down most of the rads and changed the PCB back to the old one. I checked the thermistor resistance last night and it was working. Works normally now. PCB was new so dont know why it was duff.