A practical checklist for judging whether an existing combustion-air vent is doing its job.
When you inspect an appliance, the combustion-air vent deserves as much attention as the flue. Use this as an ordered check, then confirm the figures against the current standard.
- Find the vent and confirm it actually serves the appliance, rather than being a general background vent that has nothing to do with combustion air.
- Check it is permanent and fixed open. A vent fitted with a closable flap, or anything that lets it be shut, is not reliable purpose-provided ventilation.
- Estimate the effective free area. The hole in the wall is not the free area; the grille, mesh and louvres reduce it, so judge the area the air can actually pass through against what the appliance needs.
- Confirm the path the air takes. A vent direct to outside, a vent from an adjoining room, and the high and low vents serving an appliance compartment are each treated differently, so check the route is suitable.
- Look for obstructions. Furniture, boxed-in pipework, decorating that has painted or papered over a grille, or external air bricks blocked by render, debris or a later extension all reduce the air below what was designed.
- Check it has not been defeated. Vents are sometimes deliberately blocked to stop draughts, so treat a blocked or removed required vent as a recognised defect.
A vent that is undersized, blocked or closable cannot be relied on to keep combustion safe, so where you find one act under the unsafe situations procedure and confirm the correct size and type against the current standard.