Biasi CH not working but DHW ok

[Gregsta]

Hi

Had a search of forums but found nothing like... can anyone help?

I have a Biasi 24S combi boiler and the CH is no longer working. The DHW is still working fine and firing up the bolier ok.

The diverter valve is working ok and still pressing both switches (checked this as had problem previously). The pump sounds like it is still working and is getting hot. The green light is constant on the front and doesn't show any flashing errors.

Checked to see if the fan is still working, it doesnt sound like it is coming on when pressing the CH switch but when I took the sealed cover it seem to be spinning. Could it be the air pressure switch or PCB issue?

I'm kinda out of ideas/knowledge here so any help would be appreciated before I have to call someone out.

 

[transam]

Biasi riva ? Prisma combi ?????

If your h.w is working u can discount the following pump , fan , ignition pcb , aps , gas valve , assuming it is not an external fault , R.Stat any time clock !!!

if the boiler has an inbuilt time clock , could be that , central heating thermistor (sensor) , or control board ???? you say it is not a divertor valve !!! pressing both switche's ???? meaning
In C.H mode the switch on the end of divertor valve (H.W switch) should not be activated ????

 

[Agile]

Is the sliding switch under the timeclock correctly moving?

Has it ever been removed?

Are you waiting a good five minutes before assuming the CH is not working?

Is selector switch in snowflake position?

Tony

 

[Gregsta]

I'm not sure it is either a Riva or Prisma, it just says Biasi 24s.....

The sliding switch appears to be working correctly, when switched off the green light goes off and in constant or timed mode it activates the diverter valve correctly for CH. This has not been moved or changed recently. The time clock is functioning in itself and when the CH is set to come on it activates the diverter valve and depresses the CH switch accordingly. (I can't see very easily whether the DHW switch is being depressed at the same time however - is this incorrect if the case??? Would this being depressed all the time not fire the bolier constantly???)

Yes I have waited a good while to see if the CH is not working, more than an hour with no ignition. The selector switch is in the snowflake position, I have tried the summer/sun position with no joy either way.

Transam - you mention the central heating thermistor, is there any way to test/isolate this as the cause? Typical it was pretty cold again last night!!

 

[Agile]

A failed o/c sensor will not stop this boiler from working. A too low resistance CH sensor would stop it working on DHW so its not that. Any they rarely fail low resistance unles they are wet.

There is little to stop it working on CH apart from the external stat connections.

It would be very rare but the PCB might have a fault but most unlikely.

A simple test is turn off power for an hour and then reconnect and see if the boiler fires up for about 25 seconds. If so then the diverter valve might be physicaly jammed in the DHW mode.

But if that was the case it would still fire up every 15 min to keep the boiler hot but you dont report that.

Tony

 

[transam]

When u say the pump is working ???

 

[transam]

When u say the pump is working ??? would I be correct to say , thay u are saying the pump is working when the boiler is in the C.H mode ???
but not firing ??? if so ??? primary flow switch ???? may not be making ???
not unheard of for it to make in H'W mode but not in C.H mode ????

? doubled up on posting sorry ignore 1st one )

 

[Gregsta]

Yes when in CH mode or th CH being activated the pump start running and gets warm, but no ignition.

Not sure if you are referring to the primary flow being the micro switch attached to the diverter valve. If so this is being connected properly as I have manualy removed and pressed, the pump activates but no ignition.

I have tried turning off for an hour and back on but no joy, any other suggestions would be appreciated.

 

[transam]

The primary flow switch is activated by the pump , irrespective of wether H'W or C.H is in use ,this flow switch need's to make in order to activate the ignition sequence , you say that it does in the c'h mode but nothing further occur's ???e,g the micro switch on primary flow switch is depressed ?????

Incidentally agile is an approved biasi service agent .

 

[transam]

Incidentally O'P , if you go into a website , The Joy Of Being A Heating Engineer . Biasi prisma down to detail page 22 will give u the exact sequence of operation ??? ,

Are you sure the primary flow switch is makeing on C.H ??? that would be the one facing you directly not the one behind or partially obsured by the gas valve ??? see u later I am off to bed !!

 

[Homesafe]

CHeating sensor!

 

[Gregsta]

Right, was confusing the primary flow switch for a min, I'm not sure if this is working then without taking another look later today. I have a friend coming round who knows a bit about boilers and reckons he might be able to help me. Will check the sensor as well.

Cheers guys

 

[transam]

If you have H.W the primary flow switch is working in the H>W mode !!!

BUT is it working in the C>H mode ???? does the microswitch on the primary flow switch make ??? when the pump run's the pin in the flow switch must come out & hit microswitch , to start ign sequence ???? does it ???????????

 

[Gregsta]

Right - fixed, It was the heat sensor (thermistor) £15 to replace from my local plumbers merchants. Replaced that and it fired right up.

Thanks for the suggestions and help!