Ferroli boiler blows out steam

[vader]

I have a Ferroli Falcon combi boiler that is again causing a problem. It is heating up ok and the water is heating constantly now. However the water is very hot when initially is turned on (with the heating on) then goes cold before the boiler ignites and heats the water ok. When the water is switched off it takes about a minute before the heating fires up again and there is a large cloud of what looks like steam out of the flue. After another minute there is a noise similar to air trapped and this cycles the heating off/on with the same happening. Pressure is about 1 bar going to nearly 2 bar when boiler is on. Can anyone suggest what it might be?

 

[bengasman]

Sounds like sludge clogging up the water heat exchanger. Was the system flushed and treated upon installation? Regular maintenance?

 

[vader]

Thanks for the reply. The boiler is fairly old and the system was recently drained and refilled due to radiator valve replacement it was flushed using just the mains water. There was no black sludge in the water and it was clean initially. Is there a good product to flush out the boiler?

 

[gas4you]

All combis, when the heating is on, will produce extra hot water for a few secs then go cool then come back up to set dhw temp.

I don't know this particular combi, but I would be looking at the control devices that are switching over from ch to dhw.

Can't see how sludge will just make it take longer to fire up for dhw

X800 is a good powerful cleansing chemical