high CO

[paul551uk]

Been out to service a boiler today old unidare with a selectos burner. Had a 0.85 60 nozzle so changed for a new one old pressure was at 7.5 so adjusted to 8 bar could't get the smoke level to 0-1 it was about 5 i added more air up until the flame blew out then had to adjust back down again still about 4-5. took a change with my analyser but co went throw the roof and had to pull out quickly. changed for another new nozzle this time a 0.75 60 and made no difference. flue ways are clear and no sign of any build up through access hatch it has about 600mm of enamal before entering a concrete liner. i fired the boiler with the access hatch open briefly blaming a blockage in the flue but it made no difference.
Any ideas guys ?

 

[Best]

Been there before with Selectos burners on any boiler.
Is that a downdraft boiler? Boiler shell, looking on the side view shaped like an 'L' and very similar copy of the old Thorn ODY boilers?
Sounds very likely the nozzle/diffuser assembly is not set up right and you were then over airing the burner trying to reduce the smoke number. Are you familiar with burners with that adjustment? Someone might have set it up wrong previously.
If the final diffuser is a separate adjustable part, then it might need loosened and moved closer or further from nozzle. The entire nozzle assembly might also need adjusted back or forward. The two variations make it tricky. You can try just altering slightly it all and retesting with analyser several times until you start to get CO2 over 10% with normal CO levels if you have no idea where to begin.
I prefer 80 degrees nozzle and normally a solid nozzle for those burners
If you have the burner type code number, maybe basic instructions available.

 

[WHPES]

I second the diffuser as being the potential issue