A step-by-step way to prove an open-flued appliance clears its combustion products to outside.
A spillage test checks that an open-flued appliance is pulling its combustion products into the flue and out of the building rather than into the room. Work through it methodically:
- Prepare the room as found and close windows and external doors so you test realistic conditions.
- Run the appliance at its full heat input and allow the flue to warm so a normal draught can establish, typically a few minutes.
- Hold a smoke source near the edge of the draught diverter or canopy and watch whether the smoke is drawn cleanly into the flue.
- Repeat the worst case: switch on any extract fans in the property and, where relevant, test with internal doors both open and closed to find the condition that most reduces the air available to the appliance.
- If smoke spills back into the room at any stage, allow time and repeat to confirm, because a cold flue can take a moment to start.
- Record the result. Persistent spillage means the appliance is not safely clearing combustion products and must be handled under the unsafe situations procedure.
Spillage points to causes such as a blocked or undersized flue, insufficient air supply, or a fan in the property overpowering the natural draught. Find and fix the cause; do not leave an appliance in use that fails this test.