An ordered check of an open-flued appliance's combustion-air vent: type, permanence, free area, route to outside, obstructions and extract fans.
Check the air supply of an open-flued appliance on every visit, as carefully as the flue. Work through the vent in this order.
- Identify the appliance type. An open-flued appliance takes its air from the room or compartment, so it depends on the vent. A room-sealed appliance needs no room vent for combustion; a flueless one relies on the room's ventilation.
- Find the vent that serves it. Confirm it supplies this appliance and is not just background ventilation.
- Check it is permanent. No flap, shutter, cover or anything else that lets it be closed.
- Check the area. Use the ventilator's declared equivalent area, not its outside size: the grille and louvres take up part of the opening. Compare it with what the appliance needs for its heat input. If a higher-output appliance has replaced the old one, the old vent may now be too small.
- Follow the route to outside. Air taken through an adjoining room is acceptable only if that room has its own vent to outside; a door undercut or a transfer fan does not do it. A compartment needs both its high and low vents clear.
- Look for obstructions. Furniture, boxed-in pipework, paint or wallpaper over a grille, or an air brick plastered over during decorating or blocked by render, debris or an extension.
- Check it has not been defeated. Customers block vents to stop draughts. A blocked required vent starves the appliance of air and is a recognised defect.
- Check extract fans. A fan can depressurise the room and pull products back down an open flue. In a kitchen with an open-flued gas appliance, Approved Document J limits the extract fan to 20 litres per second, and the appliance must be shown not to spill with the fan running.
If the vent fails
- Treat the appliance as unsafe until the correct permanent vent is restored, and follow the unsafe-situations procedure.
- Never accept an openable window or adventitious air in place of a required vent.
- Headaches that improve away from the property, especially with appliance sooting, suggest possible carbon monoxide exposure. Move occupants into fresh air and seek urgent medical and gas-safety assistance before continuing fault diagnosis.
Grilles, mesh and site-built vents
- A grille or mesh that keeps out birds and animals must have openings of at least 5 mm. Fine insect mesh is smaller than that, so it does not belong on a combustion-air vent.
- A vent built on site from several parts, such as an air brick, a duct and an inside grille, counts as the equivalent area of its most restrictive part. The parts' areas never add together.