Worcester 240 BF shutdown !!!

[brk1]

Having problems with my 240 BF boiler , it will run all day with Central Heating on, but when we run a bath from it, withing one minute of finishing, it trips the gas valve and cuts the flame, it doesn't trip halfway through the bath being poured, always within about a minute of stopping. From what I can see from the fault finding flow chart ,the nearest and best match is the overheat protection section, it matches the water to heat exchanger getting too hot and causes the overheat thermostat to trip, then it also matches exactly the "gas valve shuts down but pump and indicator lights remain on(if demand still exists) i.e the CH is still on.

Can any experts confirm this please. Hot water seems to be extremely hot from the kitchen tap (right next to boiler) this might be O.K anyway?

When you go to re-light the pilot it takes a long while to get the gas valve to stay in (maybe due the the thermostat tripping it ? ) a lot longer than a normal lighting procedure would take. You can feel whatever holds the gas valve in only just catching the button, the more time you push it in and out, the more you can feel it grabbing, until it finally stays in and boiler remains lit.

According to the manual the pump overrun thermostat sits on top of the overheat thermostat, are these separate parts or a combined unit ? Only from the short parts list the only part listed is a "Limit Thermostat" complete with leads, is this all that is required to change the parts or would I need a new pump overun thermostat as well ? Don't know how they are joined together ? But pump seems to run normally anyway, didn't know if you would recomend changing both at the same time or not, thanks if anyone can help.

 

[Netdragon]

turn the hot water control knob on the user control facia down to lowest and operate boiler if it still gets very hot you have problem with the stat or the pcb/gas valve may not be modulating to a low flame (sticking).

Try the first method if the hot water runs cold then you know the stat is detecting something, it could of gone out of calibration and is jst a replace task.

the only other thing is the heat exchanger to the hw is scaled but it would give from your hot water taps this type of thing "Hot water for a few seconds and then runs cold then hot again, cold, hot and so on) if this is the case the hw heat exchanger is blocked up and needs a new one

 

[brk1]

HW heat exchanger is clear, had that soaking for 12 hrs last year so that should be clear, gave the whole system an overhaul last summer, new diverter valve, system flush/descale, topped up with inhibitor , new AAV etc so system was running good. Just tried to turn down the water temp to min on the front panel , it still gets really hot,I work outside and don't have lily white hands but I can't keep my hands under it when it reaches full temp even on the lowest setting. Which pcb are you talking about ,the main mobo or is there another one hiding in there as well? to stop the gas valve modulating to a low flame. It seems strange that it doesn't trip till after you turn the hot water off, I would have thought if it was just overheat ,it would shut down in the middle of filling the bath up as well ?

 

[Netdragon]

What happens with hot water is the boiler ignits and goes to HIGH FLAME the omg i need to shove a shed load of heat to the hw circit and as it gets nice and hot it should reduce to a low flame and control the heat to the hot water by altering the burner pressure.

this is normaly done by the PCB main one that has a chip with the software setup to adjust the volts dc to the gas valve.

this is the time to grab your manual and read for what voltages and resistance you should be getting on your hot water thermistor and compare with the fault finding charts in the manual (if u dont have go to the manufacts website) as most of the stats now days work of resistance wear and tear can make them a bit out of wack.

But as a rule if the Central heating dont trip the over heat then its not the overheat stat and pump over run is after demand to cool boiler as long as pump runs during CH / HW demand its ok and a few mins afer the pump over run is ok. quick simple little check.

 

[brk1]

Thanks for these answers,I'll have to get my meter out this weekend and do a little checking, still got the manual, I also have a spare main PCB that came off another working boiler, so can always change whole board if neccesary.Will let you know whats going on

Limit stat was at fault, and what an ass is was to get out and back in again, still, all working fine again now.