Biasi Boiler Prisima 24se Hot water problem

[tonyramponi]

Hi,

My Biasi boiler (24se model dated 1997) has developed a problem whereby it keeps switching on/off in 10 second cycles while in DHW mode. CH is fine. It only appears to be the flame/gas which is being switched off, the fan keeps running and the voltage stays on to the pump which leads me to believe the dhw pressure switch is working and the diverter is correctly switching over. Water comes out hot from the tap in line with when the boiler is actually on.

The temperature of the boiler never gets above about 65-70 but it will cut in and out on it's way up to this maximum. I can hear a relay chattering as it tries to make its mind up if is going on to stay on or go off.

I suspect the secondary dhw exchanger may be blocked but is this likely if the boiler cuts out before reaching temperature?

Further weirdness is that the boiler will occasionally switch on for a few seconds after you turn the tap off when there is CH demand present. It will then switch off for around 30 seconds before switching back on.

Temperature probes seem to be measuring OK.

Anybody any Ideas where else I could look?

My current plan is to take out both the pump and dhw exchanger for a quick inspection, or am I barking up the wrong tree?

 

[transam]

75 degs c is the normal ( limited ) temp of the primary circuit when in the HW mode !
This would or should (I think ?) give a max hw temp , dependant on flow rate of 55 degs c ( I think ?)

30 sec switch on & off when ch 1st comes on , this is normal for this boiler , its a quirk if u like of the anti short cycling device built into the electronics , it can be overidden on the main board via a dip switch !

sounds like a plate exc prob ??

 

[tonyramponi]

Thanks, I'll look at that first.

What causes the boiler to cut out when theres a plate exchanger problem?

Is it something to do with the temperature differential between the two temp sensors?

 

[Agile]

I think this is likely to be a gas/combustion related fault which will need a gas registered engineer to sort out for you.

I dont expect any parts will be needed.

Tony

 

[tonyramponi]

A cryptic response from a man in the know!

If no parts are required then I guess something either needs cleaning or has gone out of adjustment. I would guess something to do with the gas valve, it looks like it could be a new one as it has the part number written on it in permanent marker - perhaps it was never set up properly? I did try to measure the modulation voltage and it seemed to be a bit off what was in the service manual.

Whats bothering me is I can't quite figure out what would cause it to shut off in Hot water mode but not CH. There seems to be only one sensor and a water route difference between the two operations.

I've currently got the plate Heat exchanger out and it was full of black crud. If cleaning it make no difference I guess I'll need to call an expert in.