What Approved Document J covers, where flue products must discharge, and how flue-flow and spillage tests prove an open flue works.
Approved Document J is the Building Regulations guidance on combustion appliances. It sets out what a flue must achieve, and the flue-flow and spillage tests are how you prove on site that an open flue achieves it.
What Approved Document J covers
- Combustion appliances, their flues and chimneys, hearths and fuel storage.
- Air supply for combustion, safe discharge of the products of combustion, and carbon monoxide warning.
- A carbon monoxide alarm is generally expected when a fixed combustion appliance is installed.
- Solid fuel and oil as well as gas. The hazards are the same whatever is burned, so a wood burner fitted beside a gas appliance is judged against the same document.
- Not drainage (Part H), general ventilation (Part F) or electrical safety (Part P), which have their own Approved Documents.
It works alongside the gas safety regulations, not instead of them. The building guidance deals with air supply, flueing and protection against carbon monoxide; the gas safety regulations govern the safe installation, maintenance and use of the gas system. Gas work has to satisfy both.
Where the products come out
Products of combustion must discharge into free outside air and must not find their way back in.
- Keep terminals clear of windows, doors and air vents. A terminal too close to an opening lets products drift straight back inside, and height alone does not remove that risk, so distances to openings are controlled too.
- Position air inlets so combustion and ventilation air can be drawn reliably.
- Take exact clearances from BS 5440-1 and the manufacturer's instructions.
Proving an open flue
Two tests, in this order, answer different questions.
- Flue flow, before the appliance is operated. Smoke in the flue should be drawn up and out of the correct terminal, with none appearing in rooms or elsewhere along the route. A pass supports flue continuity and draw; it says nothing about the appliance firing.
- Spillage, with the appliance running at the setting and after the warm-up its instructions specify. Use smoke at the position specified for the appliance, such as its draught diverter, canopy or fire opening. It should be drawn towards the flue rather than returning into the room. Close windows, external doors and adjustable outside vents. Run each fan being checked at its maximum usable setting and vary internal doors to find the most adverse pressure at the appliance; include appliance-room fans with that room's door shut.
Acting on the result
- Smoke that still returns into the room after the prescribed warm-up and any permitted repeat is a failed test. Keep the appliance out of use, follow the unsafe-situations procedure and correct the cause.
- A pass with a fan off does not cancel a fail with it on. Opening a window to get a pass does not prove the permanent air supply is adequate.
- Repeat a failed test only where the instructions allow it. Never keep re-running a spilling appliance until it passes.
- Record the conditions, result, defect and action.