Commissioning CO levels and an out-of-tolerance CO2 reading

Whose limits apply

Use the boiler manufacturer's commissioning figures first. Where no commissioning CO level is supplied for a condensing boiler with an air/gas ratio valve, the TB 143 flowchart checks for CO below 350 ppm and a CO/CO2 ratio below 0.0040. Readings should be well inside the applicable limits. Check maximum rate and, where possible and specified, minimum rate. BS 7967 gives the same 0.0040 ratio for this type of boiler. Acceptable CO and ratio readings are part of commissioning; they do not replace the gas, flue and other installation checks.

When CO or the ratio fails at commissioning

• Check that no seals, such as those on flue extensions, were displaced during installation. Put right what you find and check again.
• Leave the air/gas ratio valve alone. It is factory set and is not adjusted during commissioning unless the manufacturer advises it.
• If the readings still fail, record CO and the ratio at both maximum and minimum rate, turn the boiler off and call the manufacturer.
• The boiler is not commissioned until the problem is found and put right. If commissioning cannot be completed, it is disconnected from the gas supply.

CO2 out of tolerance at a service

Some manufacturers also give a CO2 range at maximum and minimum rate and ask for it to be checked at each service, alongside CO and the ratio. A CO2 figure outside that range needs investigating even when CO and the ratio pass. The Worcester Greenstar 8000 Life instructions list these checks:

• the gas inlet pressure and the gas rate
• the fan pressure
• the flue and air intake, including any blockage in the condensate disposal
• the condition of the burner
• leaks or obstructions in the exhaust path

Condensate drains from the part of the boiler the flue gases pass through, which is why its disposal is on the list. Water-side performance can need its own investigation, but it does not replace these combustion checks at the prescribed firing rate. For this model, the gas valve and venturi are factory set: if CO2 remains outside tolerance after the other causes have been checked, contact Worcester Bosch before adjustment. Follow the instructions for the actual boiler and fuel.