Biasi 24S diverter problem?

[kaboom]

I try not to do breakdowns but I have to visit a Biasi with the following symptoms.
Works fine on CH alone. No boiler response on HW alone but water heats if CH on already. No previous breakdown history- HW just stopped.
Is that most likely the diaphragm? Would anyone be kind enough to talk me through me the basic diagnotic steps?
Grateful.

 

[Slugbabydotcom]

Generally speaking

The diaphragm is set up so that it can detect the difference between inlet and outlet pressures. The differing pressures cause the diapragm to push out a pin which then activates a microswitch the boiler will then light.

What you have to do is identify the water section containing this diaphragm and the pin which it pushes when a hot tap is opened. Sometimes it is the microswitch that is not functioning properly.
If the pin doesn't move and operate the switch it is likely that the diaphragm is the cause. They usually wear and perforate or tear

Biasi's usually have another diaphragm operated flow switch which detects when the pump is working so be careful you are checking the right one.

The trouble with the 24s is that there were many different versions made as the boiler has evolved. Its best to get the exact model and GC no then download the service instructions from the biasi website.

The GC number usually has a format of 2 digits a space 3 digits another space and then 2 digits eg. 47 970 06
The 24s is usually prefixed with another code eg. m90 m96 m110
Biasi changed a lot of things during production of each version so its also wise to get the serial number before you trot off to your heating spares supplier.

 

[Agile]

The hydraulic block has not changed ever since the first 24S boiler was imported during 1996 until they were superceeded by the M90 at the end of 2002.

Tony