Worcester Bosch overheat problem

[burlebob]

Hi I've got a Worcester Bosch 280RSF with an overheat problem - boiler boils and reset button pops out after drawing HW.
Having read other posts on this forum previously I recently replaced the W-W exchanger thinking this was the most likely cause. The existing exchanger was reasonably clean, but I replaced anyway and, surpisingly given there was nothing wrong with the original this appeared to solve the prob. I concluded that there must have been air in the system and sat back and soaked up the praise from the missus for fixing it
Now the problem has started again, i'm assuming there is air in the system again but no amount of bleeding/running the system seems to clear it. Could it be the AAV is faulty/letting air into the system somehow? Assuming it is air, what's the best way of getting rid of it, or am i barking up the wrong tree?

Thanks in advance

 

[cider]

Hi I've got a Worcester Bosch 280RSF with an overheat problem - boiler boils and reset button pops out after drawing HW.
Having read other posts on this forum previously I recently replaced the W-W exchanger thinking this was the most likely cause. The existing exchanger was reasonably clean, but I replaced anyway and, surpisingly given there was nothing wrong with the original this appeared to solve the prob. I concluded that there must have been air in the system and sat back and soaked up the praise from the missus for fixing it
Now the problem has started again, i'm assuming there is air in the system again but no amount of bleeding/running the system seems to clear it. Could it be the AAV is faulty/letting air into the system somehow? Assuming it is air, what's the best way of getting rid of it, or am i barking up the wrong tree?


Thanks in advance

aavs generally let in air if on suction side of pump. cant remember if your boiler modulates but if it does the low bunner pressure setting might be too high and causing ovht on low flow (showers etc). found this once on boiler that had been looked at by endless british gas engineers that hadnt done basics told owner wtr to wtr heatexchanger needed changing but wasnt covered by contract.i changed it cheeper then them only to find it clean as whistle and then checked bp to find it way too high. will need corgi person to check it pulling wires of gas valve and all that stuff.

 

[pannierstan]

Another classic on these is the pump over run stat if this model has one I cannot remember.
If its shot the residual heat in the boiler causes it to trip after the tap is turned off especially after a long run like a bath or shower.

Stan

 

[superdupergasman]

Is the DHW flow switch disengaging after drawing water??

Does the burner still fire after water tap is closed??

 

[gasandoilman]

Have you wired the circulation pump direct to the boiler to allow for pump over run
I had this problem on a w/b and thats what it was the installer hadnt wired in properly..

 

[pannierstan]

Have you wired the circulation pump direct to the boiler to allow for pump over run
I had this problem on a w/b and thats what it was the installer hadnt wired in properly..

Its a Combi old boy! I dont think he needs to check that

Stan

 

[burlebob]

cider - it does have a modureg which got me thinking. I stuck a meter across it to check the volts, it doesn't seem to change from around 21volts even if i reduce the water flow, suggesting there's not a lot of regulating going on.

I then swapped the DHW and CH thermistors over, thinking that maybe the CH one could be faulty, but the problem's still there.

I'm thinking that maybe it's the control board - do these things go wrong very often?

superdupergasman - yes flow switch moves back and burner goes off

gasandoilman - the CH pump will run to cool the boiler, more ofen than not though it doesn't get there in time and the reset switch pops, or if it does run the boiler makes some pretty scary noises

 

[tonybhoy]

Perhaps the pump is on it's way out