Tripping Boiler

[mrfixit]

I have a Baxi Bahama 100 combi boiler with the Honeywell controls and a remote programmable roomstat which was installed in March 2000 and has been very reliable requiring the ususal flowswitches (4 HW & 2 CH), diverter valve cartridge (spindle leak), air psw (microswitch mounting) and the annual inspection/drain/flush etc; however recently with the selector switch set to HW/CH the boiler frequently trips on overheat and during the recent warmer months I set the selector to HW only but it still persistently trips on overheat e.g. soon after starting to fill the sink for washing up.
I have replaced the control thermistor, overheat thermostat and control pcb but I am now unsure if the cause is likely to be the control pcb, diverter valve, pump or some other problem and I realise that at 11 years old the spares situation could be a problem, but it is in very good nick and I am not really in a financial position to replace it. I am an engineer competent with multimeter etc. and any further tests or advice will be very gratefully received.
R

 

[Boilerman2]

Scaled or sludged hot water heat exchanger??

 

[mrfixit]

Hi Boilerman,

Thanks for the advice, the primary issue with water in my area is manganese deposits in the mains riser, so as a first step what would you recommend I use to clean the HW heat exchanger without poisoning everybody or damaging the boiler even though the HW flow is good.

This boiler is drained, cleaned, flushed and Fernoxed annually on the CH side and has been tripping on both CH and HW, however when selected to HW only it still trips even though the boiler temperature selectors are set at minimum and the HW flow is normal.

On CH or HW do you have any idea why the boiler does not modulate the burner and keep the pump running to maintain the heat input below the overheat limit.

R

 

[picasso]

the pump on the bahama is a standard 15/60 grundfos head, sounds like lack of circulation so the pump might be the problem.

 

[mrfixit]

Hi Picasso,

Thanks for your advice, I know I can easily get a replacement pump so I will give that a go and see what happens.

R