Open-flued appliance air supply (diagram)

The diagram has two panels. In the first, an open-flued appliance stands in a room and draws its combustion air through a permanent louvred wall vent. In the second, it sits in a cupboard or compartment served by vents at high and low level. The green dashed arrows trace the air path through the vents, and the amber arrows show the products of combustion rising up the flue to outside.

Air in

• Combustion needs oxygen. Too little air means incomplete burning, which can produce carbon monoxide.
• The appliance takes its air from the space it stands in, so that space must replace the air as fast as it is used. More heat input needs more air.
• Purpose-provided vents have fixed, non-adjustable openings; check that each stays clear.

Up the flue

• The vent supplies air and the flue removes the products. Both paths have to work.
• At the draught diverter, room air enters the flue as dilution air and joins the products going outside. That is normal.
• A powerful extractor fan can lower the room pressure enough to pull products back down the flue and spill them into the room.

The compartment

Cool air enters through the low vent, is warmed by the appliance, rises and leaves through the high vent. That circulation feeds combustion and carries heat away; one opening, or two at the same height, would not drive it. Approved Document J gives, per kW of net input, 5 cm² high and 10 cm² low for vents direct to outside, or 10 cm² high and 20 cm² low into a room, which then needs its own 5 cm² per kW vent to outside.

If the low vent is painted over or blocked, the appliance is starved of combustion air and the compartment overheats, risking incomplete combustion and carbon monoxide.

Compare room-sealed

A room-sealed appliance with a balanced flue draws its air from outside and sends products outside. Its combustion is sealed from the room (the room itself need not be airtight), so it does not rely on room air or a room vent.