Air inlet readings, visual flue checks, the 20.6% oxygen limit, the TB 143 commissioning order and the meaning of net flue temperature.
Why the check exists
Since 1 April 2014, commissioning a new condensing boiler has included measuring CO and the CO/CO2 ratio and recording them on the Benchmark commissioning checklist. Gas Safe Register's Technical Bulletin 143, developed with the Heating and Hotwater Industry Council, sets out the generic procedure for boilers with an air/gas ratio valve, and manufacturers base their own instructions on it. It does not cover servicing, where the manufacturer's instructions and BS 7967 apply.
The order of checks
- Zero the analyser in fresh air outside, as its instructions say.
- Set the boiler to maximum rate by the method in its instructions and let combustion stabilise. Keep the probe out during that stabilisation period as the procedure directs, avoiding excessive startup gas levels at the sensor.
- Check flue integrity at the air inlet test point.
- Still at maximum rate, check CO and the ratio at the flue gas sampling point, letting the readings stabilise before recording.
- Where possible and specified, set minimum rate, let it stabilise and check CO and the ratio again. If the instructions do not say how to set minimum rate, ask the manufacturer.
- Cap the test points, refit the case correctly and record the readings.
The flue integrity check
On a concentric flue, the air inlet test point samples the combustion air on its way in, so it should read almost like fresh air. With the case fitted and the boiler at maximum rate, TB 143 gives oxygen of at least 20.6%, or carbon dioxide of no more than 0.2% on an analyser that measures carbon dioxide. A reading of exactly 20.6% oxygen meets that minimum. Follow any stricter manufacturer limit; for example, some instructions require CO2 to be below 0.2%.
A passing reading does not check every joint, support or installation detail. Complete the visual checks of the flue's assembly, fixing, support and permitted length as well.
A valid failed reading indicates possible mixing of combustion products with the incoming air. Investigate the flue: its components assembled, fixed and supported as the instructions require, and the flue and terminal clear of obstruction. Some manufacturers also list the seals around the burner, the internal flue and the case. Then repeat the check. If it still fails, turn the boiler off and call the manufacturer. The boiler is not commissioned until the problem is found and put right, and if commissioning cannot be completed it is disconnected from the gas supply.
Where there is no air inlet test point, the analyser cannot check integrity. Confirm instead that the flue has been checked visually: the whole system correctly assembled, fixed and supported, within the maximum flue length, and following the guidance for flues in voids where it applies.
Net flue temperature
Net temperature is the flue-gas temperature minus the combustion-air reference temperature. It contributes to the loss and efficiency calculations; the temperature difference alone is not a quantity of heat. Use the specified air sensor and location. For example, the KANE458s uses its internal temperature when no T2 probe is connected, so its position matters if that temperature differs from the incoming air.