How the flue-flow test proves the route before the spillage test, and what makes a natural-draught flue pull or fail.
A flue-flow test checks that the flue gives an effective route to the correct terminal, with no leakage into the building or next door. Do it before the appliance is operated for the spillage test.
The test
- Inspect the flue. Prepare it as the procedure requires, including any warming it specifies.
- Release smoke at the prescribed position.
- Watch where it goes: it should be drawn up and out of the correct terminal.
- Check along the route for smoke in rooms, roofspaces, voids or adjoining property.
Smoke at the wrong terminal, a down-draught, no flow, or leakage along the route is a failed test. Stop, find and fix the cause, then repeat the checks before operating the appliance. A new liner or a cleared blockage still has to be proved.
A pass supports flue continuity and draw. It does not show that the appliance clears its products when firing; the spillage test does that.
What makes a natural-draught flue pull
- Route. As vertical as practicable, with few bends. Long horizontal runs and bends reduce the draught.
- Integrity. Check the joints and casing for the whole run. Damage can let combustion products, including carbon monoxide, escape into the building instead of reaching the terminal.
- Roof height. Wind over a roof creates turbulent, higher-pressure air near the surface, ridges and nearby walls. A terminal lifted clear of it draws reliably.
- Wind. A taller building upwind can cause a down-draught that reverses the flow, so the appliance spills in strong wind but not in calm weather.
- Temperature. A cold flue in an unheated loft draws weakly until it warms and may spill briefly at start-up. That explains the symptom; it never makes unresolved spillage acceptable.
- Chimneys. An oversized or unsound chimney needs a suitable liner to give a sound, correctly sized flue route. The chimney is still examined, cleaned and repaired, and the finished flue is inspected and tested.
- Voids. Where a flue passes through a floor, ceiling or roof void, keep the manufacturer's clearance from combustible materials, usually with a sleeve or an air gap.