Flues, Chimneys and Combustion Product Discharge
What the exam tests
Flues, chimneys and how combustion products leave the building. You must recognise flue types, check terminal positions, verify flue integrity and understand spillage and flue-flow testing.
Core rules
- Flue types include open flue, room-sealed balanced flue and fan-assisted flue. The appliance instructions determine the permitted type, length, bends and terminal position.
- Terminal clearances protect openings, vents, windows, doors, boundaries and adjacent buildings. A terminal under an opening window or beside an air brick is a defect even if the flue draws on the day.
- Open-flued appliances need a flue-flow test and a spillage test after installation and service. A smoke pellet or match test must show satisfactory clearance and no spillage into the room.
- Flue integrity includes joints, supports, slope, access for inspection and the condition of the chimney or liner.
- A failed spillage test means the appliance must not be left in use.
Worked example
A wall-mounted open-flued water heater is serviced. The flue-flow test is satisfactory, but the spillage test shows products curling back into the room after several minutes. The installer checks the ventilation and finds the permanent vent has been blocked by insulation. He clears the vent, repeats the spillage test and only then passes the appliance. If spillage persists, the appliance is classified unsafe. The reasoning: test under real operating conditions, fix the cause, then retest.
Marks-losing traps
Measuring terminal clearance to a fixed window when the window opens; accepting a flue that passes a smoke test but spills under sustained operation; ignoring a blocked ventilation grille; failing to check a shared chimney.