When a manufacturer has not specified the maximum amount of CO in the flue of a condensing boiler, the HHIC states that the maximum amount of CO must not be more than 350 PPM (Parts Per Million) and the Combustion Ratio is less than or equal to 0.0040.
We would be most interested to receive comments about this (do email office@co-gassafety.co.uk).. CO-Gas Safety considers that this is a retrograde step and that a maximum of 200 PPM of CO is achievable by most manufacturers immediately and a lower amount should be possible soon.