Biasi 24s - elecrtical buzz from PCB and intermittent DHW/CH

[BENRIBBANS]

Hi all,

Another problem with our biasi 24s.......

When switching to CH there is an electrical buzz from the PCB. This lasts for a few minutes then stops and the boiler will fire up. Sometimes the buzz will return but the CH stays on. CH will stay on for a while but then tends to stop firing up, the boiler is still humming as if it is ready but the ignition cycle does not start. When in this state DHW will not work either, hot tap runs cold and ignition cycle does not start. Sometimes switching the boiler off for a while then back on works, but not always.

The only regular 'fix' seems to be to switch the boiler off at the mains switch (in the bedroom), wait for 10 mins or so then switch it back on and everything is fine for a day or so then the same thing happens. I am guessing there is a fault with the PCB but not sure? Is there anyway I can check it, the manual doesnt give a great deal of info other than how to replace it. I am happy to do this but would rather be sure of the problem before spending +£100 on a new PCB!

Thanks
Ben

 

[BENRIBBANS]

Anyone any thoughts?

 

[Agile]

I expect it may be the gas valve green mod coil buzzing.

If so it will do this when the boiler modulates back from being on at full power. However what you describe is not exactly that.

If this is the case the smoothing capacitor on the PCB is mostly failed. This is C1 and is 1000 µF 35v.

However the noise might also be coming from one of the two relays on the PCB although the cause will probably be the same.

It could also be coming from the main solenoid on thegas valve as well!

Several different things to check.

A mechanical stethoscope helps! So would an oscilloscope to measure the ripple on the 24v supply rail.

Tony

 

[kevplumb]

yup and everybody that owns a boiler should own a scope

howay tony

 

[BENRIBBANS]

Yes, and a hammer so you can beat the thing to death when it isnt working properly!

I will see if I can identify where the buzz is coming from. It usually happens when you first switch the boiler on, or first switch central heating on, and is definately the PCB as I have removed it to check for noises behind the PCB and there are none except the PCB itself. Sounds as though it is near the CH thermostat but I will check.

Thanks
Ben