What an action level is, how to compare a ratio with the appliance's limit, and what to do when a combustion check fails.
What an action level is
An action level is a reading at which you must take a defined action instead of carrying on as normal. It is not a target to adjust the appliance to. Check action levels against the current procedure: limits and the conditions that apply them get revised, and a remembered figure can be out of date.
Where the manufacturer gives no figure of its own, BS 7967 sets the fallback level to use: 0.0080 for a central heating boiler without an air/gas ratio valve, 0.0040 for one fitted with an air/gas ratio valve, and 0.0200 for a water heater, flued or flueless. Always check the appliance's own instructions first; these fallback figures only apply when none are given.
Comparing the reading
Use the limit in the model and fuel instructions. A generic ratio chart, the analyser's own pass/fail indicator or a flame-colour guide cannot overrule it. Read the exact wording: where the instructions require a ratio below 0.0040, a stable reading of 0.0040 does not meet it.
Within the limit
Record the results. The appliance may stay in service only if the other checks also pass. A good ratio does not cancel:
- a CO concentration above the appliance's CO limit
- heat staining around the flue connection, which can indicate spillage or a failing joint
- the flue check: clean burning does not prove the products are being removed, so still do the spillage test on an open-flued appliance or the specified flue-integrity check on a room-sealed one
Above the limit
Keep the appliance out of normal use while an above-limit result is investigated. Check the analyser, sampling point, casing and firing rate against the instructions before attributing the result to an appliance fault. Follow any instrument warning or required safety action immediately; do not wait through a hazardous condition for a more favourable number. Repeat the measurement only under safe, prescribed test conditions. A confirmed excessive CO/CO2 ratio means unsatisfactory combustion even when the boiler still lights and heats water. Check the CO concentration separately: either failed limit needs attention.
- Validate the reading and compare it with the limit for this model, fuel and test condition.
- If the failure is confirmed, do not commission or leave the appliance in normal use. Follow the manufacturer's procedure to find and correct the cause within its permitted settings.
- Repeat the analysis under the specified conditions and record the before-and-after readings and work done.
- If satisfactory combustion cannot be achieved, follow the current unsafe-situations procedure. For a flued boiler, a confirmed combustion-reading failure is At Risk; separate evidence of products entering the room requires its own assessment and may make the situation Immediately Dangerous.