Following a number of CO incidents relating to condensing boilers with air/gas ratio control valves Gas Safe Register in conjunction with the HHIC have issued a new guidance note to clarify the action levels at measured CO/CO₂ ratios. All installers should read Technical Bulletin 126 on the Gas Safe Register website.

The crux of the Guidance Note states the following:

Boilers fitted with an air/gas ratio mixing valve where the CO/CO₂ ratio exceeds 0.008 are classified as "Immediately Dangerous"

Boilers fitted with an air/gas ratio mixing valve where the CO/CO₂ ratio exceeds 0.004 but does not exceed 0.008 are classified as "At Risk". (Unless the manufacturer has specified a ratio lower than 0.004)

It is also proposed that future revisions to BS7967 and GIUSP will also adopt 0.004 as the action level for all boilers fitted with a gas/air ratio valve.

Some boiler manufacturers already define safe combustion levels for their boilers and recommend that a combustion check is performed when commissioning.

The contact details for all boiler manufactured who are members of the HHIC are given in the appendix of Technical Bulletin 126.

Nov 2009.