CCN1Revision card

Open-flued appliance air supply (diagram)

Note 17 of 26 · free to read · All Core Domestic Gas Safety notes

Why an open-flued appliance needs permanent purpose-provided ventilation, and how combustion air and products move.

Schematic of an open-flued gas appliance inside a room or compartment. Permanent purpose-provided air vents at high and low level let outside air into the room; blue arrows trace combustion air from the vents to the appliance; the products of combustion rise up the flue and discharge to outside above the appliance. Notes explain that the vents must be permanent and that blocked vents risk incomplete combustion and spillage. — Open-flued appliance air supply

The schematic shows an open-flued appliance inside a room or compartment. Unlike a room-sealed appliance, it draws the air it burns from that space.

Outside air enters through permanent purpose-provided air vents, shown on the left wall. These vents are sized for the appliance and must not be closable.

For an appliance in a compartment, ventilation is usually provided at both high and low level so air can circulate around the appliance.

The blue arrows trace combustion air from outside, through the vents, into the room and to the appliance.

The products of combustion are carried up the flue and discharge to outside, above the appliance.

If the vents are blocked or undersized, the appliance is starved of air. That can lead to incomplete combustion and to products spilling back into the room, which is a carbon monoxide risk.

This card explains the ventilation principle only. The actual free areas, vent positions and flue requirements come from the current Approved Document J, BS 5440 and the manufacturer instructions.