Choose the correct point for combustion sampling or an air-intake check, and reseal the test points afterwards.
A room-sealed condensing boiler may have separate sampling points for flue products and incoming combustion air. Identify them from the appliance instructions: their purposes are different.
- Combustion sample: Use the designated flue-gas point to sample the products leaving the appliance. Follow the specified probe depth, sampling-point seal, casing condition and firing rates, and allow readings to stabilise.
- Air-intake check: Use the air-intake point when the manufacturer requires a flue-integrity or combustion-air check. Flue products entering the air supply can change these readings, so this point does not guarantee 20.9% oxygen or zero CO. Judge the result against the model’s specified method and limits. It is not a substitute for sampling the combustion products.
- After testing: Refit the sampling-point plugs and check their seals as instructed. Leaving a point open can compromise the combustion circuit.
Do not improvise a sampling hole or assume sampling at the external terminal is an acceptable alternative. If the designated points or method are unclear, obtain the correct appliance instructions before testing.
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