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 reduces the effect of changing flue draught on the burner and admits dilution air that joins products travelling outside. 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 proves the cause on its own. Investigate and carry out the required checks before further use.
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 relevant fan and internal-door combinations to find which most reduces pressure at the appliance. Include a check with the appliance-room door closed and every fan in that room running at its maximum 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 moves towards the flue rather than escaping into the room.
6. If the first observation is unsatisfactory, follow only the further warm-up and repeat that the procedure allows; the draught may 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. Smoke still returning into the room from the specified observation area after the required warm-up and any permitted repeat is a failed test: the appliance is unsafe until the cause is corrected.
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 for a pass does not show that the installation is safe 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

The fan combination and internal-door positions that put the greatest suction on the appliance vary with the building layout. One pass may therefore miss the adverse 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.
• 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 means combustion is incomplete, and incomplete combustion can 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.