My 280 rsf overheat thermostat trips when DHW demand is switched off after it has been on for a long time, ie after running a bath etc. I have tested the thermisters and they seem ok. In the last year the pump, expansion vessel and PRV have been changed and the AAV was changed this week as it sounded like air was trapped in the boiler just before it trips. The pump does operate after demand is switched off and so does the diverter valve. It sounds like there is air trapped in the boiler pipework bubbling then it trips. I have also removed both heat exchangers and they appear to be fairly clean. Any ideas??

Andy Brown, October 2010

hi Dan

When I say thermister, this is what you refer to as the pump over run thermostat. There is one on the DHW flow pipe and one on the pipe that comes out of the main heat exchanger above the burner assembly. They are both the same, you can check they are working correctly by checking there rsistance with an ammeter, the resistance is in the boiler manual. The one on the water to water heat exchanger only regulates the temperature the hot water comes out of the tap, the one that controls the pump over run is the thermister on the main heat exchanger because when the pump runs on after DHW demand is switched off the boiler circulates the excess heat around the radiators. My problem in the end was scale build up in the main heat exchanger so the heat couldn't be removed quick enough. I ended up descaling my system and boiler which fixed my problem. But because it was so severely scaled I used a powerflush acid cleaner and circulated it in my system for two hours using the boiler pump and I fitted a magnaclean.

Andy, November 2010

Hi im not a boiler expert. but i had a over heat problem with my worcester 240 rsf 3years ago with it over heating so i changed the pump overrun thermostat what coast £9.00, and i fixed it. switch power off then remove two small screws and two wires that pull off then new one goes on, and i've not had a problem after i did that. just check your lay out first as yours is a 280. i've had a problem with my heating last week. i see you said you checked the thermisters is that the central heating sensor? if so i changed that, and heating is working fine now but i would like to know where the thermisters is as it might be worth me checking as well. hope this helps you

Dan, October 2010