Two worked room-vent sizes for an open-flued boiler, with and without the 7 kW allowance, and how to choose and keep the vent.
Ventilation is calculated, never guessed: an open-flued appliance short of air burns incompletely and can spill carbon monoxide into the room. Size the vent from the net heat input, using the method that applies to this appliance and this building, not from room size or flue diameter.
Approved Document J gives 500 mm² (5 cm²) of permanent vent per kW of net input for an open-flued appliance in a room. Older, leakier dwellings with air permeability above 5 m³/(h·m²) may deduct the first 7 kW.
Example 1: older house, allowance applies
A 16 kW net open-flued boiler in a room.
- Chargeable input: 16 kW − 7 kW = 9 kW.
- Free area: 9 kW × 5 cm²/kW = 45 cm².
- In square millimetres: 45 × 100 = 4,500 mm².
Example 2: new house, no allowance
Take a new dwelling with an 18 kW net open-flued boiler. Its design permeability is 3.0 m³/(h·m²), below the 5.0 air-permeability threshold. Count every kW; do not deduct the older-dwelling 7 kW allowance.
- Free area: 18 kW × 500 mm²/kW = 9,000 mm².
- In square centimetres: 9,000 ÷ 100 = 90 cm².
Choosing and keeping the vent
- Select the ventilator by the declared equivalent area of the complete assembly. Suppose a calculated requirement is 5,000 mm². A grille with a 100 mm × 100 mm outside frame appears to cover 10,000 mm², but if its label declares 4,500 mm² equivalent area, it is 500 mm² short. Choose an assembly rated for at least 5,000 mm² in this example; the inlet and outlet of one through-wall vent are not two areas to add together.
- The air path must reach outside. A grille into a sealed cupboard with no outside connection supplies nothing.
- The vent must be permanent. A user-operated cover does not meet the requirement.
- A vent blocked later changes the air supply the appliance was commissioned with, which is why ventilation is rechecked at every visit.
- A compartment, or a route through another room, uses different figures. Never split a room-vent answer between cupboard openings.
Free area and equivalent area
Ventilation tables give the free area a vent must provide, or the equivalent area for ventilators of more complex design.
- Free area is the open area left for air once blades, louvres and mesh are allowed for, so it is always less than the vent's outside size.
- Equivalent area comes from an airflow test of the complete ventilator. Baffles and other resistance can make it smaller than its measured free area; it is not an arbitrary safety margin added by the maker.
- Use the figure marked on the ventilator or given in its literature whenever it is available. For a simple vent without internal baffles and without a declared equivalent area, Approved Document J allows the minimum unobstructed cross-sectional area to be used. Do not infer a complex vent’s rating from its outside frame. The declared figure assumes a clean vent: fluff, dust or paint can reduce the air that actually passes.