Open-flued spillage test: an ordered method

A spillage test checks that a running open-flued appliance sends its products of combustion up the flue instead of into the room. Spillage means products escaping into the room; it is not a gas leak.

Find the smoke observation position in the appliance instructions. On a model with a draught diverter, that opening lies between the primary flue (appliance outlet to diverter) and the secondary flue (diverter to terminal). It shields the burner from changes in the flue draught and lets in dilution air, which goes outside with the products. A fire may instead specify its opening or canopy for the smoke check. A diverter does not make spillage harmless.

1. Look for warnings of possible spillage: heat-damaged paint near the diverter, soot marks, a CO alarm that has sounded, or persistent yellow tipping on a burner designed for a blue flame. None of these tells you the cause. Investigate and carry out the required checks before the appliance is used again.
2. Confirm the flue-flow test has passed and the air supply is suitable.
3. Close windows, external doors and adjustable vents to outside; keep permanent air vents clear. Try the fans and internal doors in different combinations to find the one that lowers the pressure at the appliance most. Include a check with the appliance room's door closed and every fan in that room running at its highest usable setting.
4. Run the appliance at the setting and for the warm-up period its instructions specify.
5. Hold the smoke source at the position specified for this appliance. Watch whether the smoke is drawn into the flue or escapes into the room.
6. If the first check fails, allow only the extra warm-up and repeat that the procedure allows: the draught can change as the flue heats.
7. Stop if a permitted repeat still shows spillage. Never keep re-running a spilling appliance until it seems to pass.
8. Record the conditions and the result.

If it fails

1. If smoke still comes back into the room at the observation point after the required warm-up and any permitted repeat, the test has failed: the appliance is unsafe until the cause is put right.
2. Spillage with a fan running can mean its pull has upset the flue draught, even if the appliance passed with that fan off. Opening a window to get a pass does not make the installation safe: it must pass under the required test conditions.
3. Make the appliance safe under the unsafe-situations procedure and do not leave it in use.
4. If occupants report headaches that ease away from home, treat it as a carbon monoxide risk: advise them about the symptoms, fresh air and getting medical advice.
5. Investigate and correct the cause, then repeat the required checks.

Finding the worst case

Which fans and door positions pull hardest on the appliance depends on the building's layout, so a single test can miss the worst case.

• Close any adjustable vents to outside as well as the windows and external doors.
• Check relevant extract fans, including cooker hoods and tumble dryers, at their highest usable setting when running. Include the effect of ceiling fans. A ceiling fan matters even though it blows no air outdoors: its air currents can change the pressure at the appliance and disturb the flue draught.
• Vary the internal doors and operating fans as the layout requires. Include the appliance-room door closed with all fans in that room running; a fan elsewhere may matter with a connecting door open.

Yellow flames

On a burner designed to burn blue, persistent yellow tipping or a lazy yellow flame can warn of incomplete combustion, which may produce carbon monoxide. A common cause is too little primary air for the gas being burned, such as a primary air opening clogged with dust or lint, or an oversized injector. Closing down the primary air makes it worse.

The colour does not tell you how much carbon monoxide there is or which fault you have. Compare the flame with the manufacturer's description, then check the burner, the air supply and the flue before the appliance is used again. Appliances designed to show a yellow flame, such as fuel-effect fires, are the exception.