Biasi M90F 28S No CH

[logo5961]

Hi
Need some advice i have a Biasi M90F 28 combi boiler. Arrived home today to find CH not working.

The boiler trys to go through the start up procedure for a few seconds then shuts down. The green operation light flashes at its normal speed about once every 3 to 4 seconds and the lock out light does not come on.

The DHW works fine.

Has anybody got any ideas. All help very much appreiciated

 

[Agile]

There are several possibilities!

Most may be beyond you.

You might be able to:-

Visually check operation of system flow switch on CH.

Activate the diagnostic status ( see book ) and see what boiler thinks is wrong.

Tony

 

[Slugbabydotcom]

I'm sure its the rectangular time clock on these. If it is, unplug it from the pcb and the boiler will usually fire when you turn the power back on.
IIRC there's about 8 screws to undo to get the PCB cover off.
If its the earlier round type then its usually the case that the clock is powered through 1&2 and the circuit is switched on either 3&5 or 3&4. Linking the switch wires should turn the boiler on if the timer is faulty.
I'd say in Haynes manual terms its a 2 spanner job where 5 spanners is an expert rating.

 

[Agile]

No its not the digi clock faulty in this case Slug!

The boiler shows a demand and goes to the startup procedure so its not the clock.

Pretty much the only difference is the hydraulic circuit which may not be operating the flow SW on CH.

It might be the CH NTC and this will be shown on the diagnostics if he can do that. Unfortunately the behaviour with a faulty CH NTC seems to vary for some reason so thats not a good indicator.

So we await the results of the tests above if he is able to do them.

Tony

 

[logo5961]

Tony you are probably right regarding most of the possible faults being beyond me. I have tried to get it in diagnostic mode following the service manual instructions (not sure if i have done it right) but those attempts do not show any code other than boiler operating normally. On the primary circuit switch when the boiler turns on the pin comes out and presses the micro switch in.

Slugbabydotcom it is the model with the rectangular timer, but the boiler is being run in manual mode via a room stat when the stat clicks in the boiler trys to start then stops. I have disconnected the timer as you said but still no CH.

The service manual fault finder chart seems to indicate the likeliest faults as the diverter valve or possible PCB fault. Anybody got any other thoughts

Many thanks guys any more suggestions to what to look at

 

[Agile]

It could well be the diverter valve motor!

But the operation is then not exactly what you have described.

It would run in CH mode for 15-30 seconds and heat up the boiler and then come on again every 15-30 minutes to keep it hot.

The hot water will come out very hot initially.

Its easy to check by removing the clip from the motor and taking it out and seeing if the nose is wet.

If so then the gland is leaking and has spoilt the motor! both have to be changed.

If its not wet then see if it moves between CH and DHW. If not its either faulty or the PCB relay coil has failed. It rests in the DHW position with the relay de-energised.

Pushing the diverter valve pin down should give you heating !

Tony Glazier

 

[logo5961]

Thanks again Tony for the quick reply, removed motor its dry the actuator pin did not seem to fire just sort of slides loosely in the motor body. Also pressed the diverter valve pin and started the boiler but still no CH. Now starting to look like an expensive fault

 

[Agile]

Sorry I have just re-read your original posting and seen that after going through the start up it then it goes back to the resting position with no demand indicated and no fault codes shown.

Your later posting made me think that it had stayed in CH demand mode.

This is a very unusual fault but I am sure in front of the unit it would be immediately clear whats wrong.

You could remove the clock plug on the PCB and see what that does.

However, in front of the boiler there are many automatic checks I do as I scan over it visually before touching anything!

One of these is to see if the knob shaft is broken. You would not see the break but you can see that turning the knob does not rotate the pot on the PCB!

Tony

 

[logo5961]

Boiler now working thermistor faulty fixed by engineer. Thanks for all advice given

 

[Agile]

That should have been identified by the built in diagnostics if it was open circuit ( the most common fault! ).

Tony