Flue flow test: proving a flue pulls before the spillage test

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

1. Inspect the flue. Prepare it as the procedure requires, including any warming it specifies.
2. Release smoke at the prescribed position.
3. Watch where it goes: it should be drawn up and out of the correct terminal.
4. 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. When an appliance on a lined chimney is replaced, the old liner is normally replaced too, unless it is suitable for the new appliance's whole lifespan.

A pass shows the flue is continuous and drawing under the test conditions. 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. Where the flue runs through a neighbour's property and access is refused, take reasonable steps to check it is sound, such as writing to the neighbour and asking to see records of examinations.
• Roof height. Wind around roofs and nearby structures creates pressure differences and turbulence. Follow the required terminal position and height, look at local obstructions and check how the flue performs: a tall flue can still draw badly.
• 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 is one possible explanation of the symptom; inspect for other defects and never accept unresolved spillage.
• 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. Keep the specified clearance from combustible material, the required fire separation and access for inspection. Fitting a sleeve does not mean these are met: check each one.

Construction details to check before testing

For a factory-made metal chimney passing through a wall, the sleeve allows the heated metal to expand without damaging the chimney or surrounding construction. It does not allow joints hidden in the wall, and it does not replace the manufacturer's supports. Plan the appliance connection so the appliance can be withdrawn while the chimney remains assembled.

Read the chimney designation with its instructions. G50, for example, identifies soot-fire resistance with 50 mm separation from combustible material. The figure is not the flue diameter, metal thickness or operating temperature; other parts of the designation describe other performance properties.

Wet operation needs a liner suitable for condensate: W identifies wet-condition resistance, whereas D is for dry operation. Suitability also depends on corrosion resistance, the appliance and fuel, and provision for draining condensate without pockets where it can collect.

In a rising connecting flue with spigot-and-socket joints, put the sockets uppermost so moisture running down the inner wall stays inside. The joints must also be gas-tight. A purpose-made inspection or cleaning cover in the flue needs the same gas-tightness and thermal insulation as the system it closes; a gap intended to drain moisture would also release flue gases. Use compatible components and allow access for cleaning.

When checking a flue's area through an offset, measure across the gas passage at 90° to the local flow. An angled cut can appear larger than the true cross-section. The change of direction must not reduce the passage below the required area.