Where the analyser probe goes on a room-sealed boiler, what the air-intake point is for, and why test points are resealed.
Two points, two jobs
A room-sealed condensing boiler may have two test points on its flue turret or adapter: one for the flue products and one for the incoming combustion air. Identify them from the appliance instructions, because their purposes are different.
The combustion sample
Insert the probe into the designated flue-gas sampling point. The specified sampling arrangement provides the gas sample needed for the appliance’s combustion check. Follow the specified probe depth, sampling-point seal, casing condition and firing rates, and let the readings stabilise before recording.
On an open-flued appliance, air admitted at a draught diverter can dilute gas concentrations. Use the appliance instructions or the recognised procedure for that appliance type: some methods place the probe in the secondary flue through the diverter. Being above it is not an automatic reason to reject the sample. Position, stable readings and the required interpretation all matter.
The air-intake check
Use the air-intake point only when the manufacturer requires an air-supply or flue-integrity check. It does not give the combustion sample. Do not assume it will read clean: flue products entering the air supply can change the readings, so it does not guarantee 20.9% oxygen or zero CO. Judge the result against the model's specified method and limits.
Points that give the wrong answer
Sampling at any of these does not give a true picture of what the burner is producing:
- a gas valve test point
- the combustion air inlet
- the flue terminal outside
- an improvised sampling hole
A perfectly calibrated analyser in the wrong place still gives the wrong answer. If the designated points or the method are unclear, obtain the correct appliance instructions before testing.
After testing
Refit the sampling-point plugs and check their seals as instructed. Leaving a point open can compromise the combustion circuit.
Exam cues
- Products of combustion on a room-sealed boiler: the dedicated flue-gas sampling point on the turret or flue adapter.
- The air-intake point: an air-supply or flue-integrity check where the manufacturer requires one, never the combustion sample.