How to calculate the required permanent ventilation free area for domestic open-flued appliances.
Open-flued gas appliances draw air for combustion from the room in which they are installed. To ensure safe, clean combustion and prevent carbon monoxide release, a permanent ventilation opening to the outside air must be provided under BS 5440-2:
Sizing Rules for Appliances in Rooms
For open-flued appliances (such as boilers or water heaters) installed in a room (not a compartment):
- First 7 kW of Net Heat Input: Requires no ventilation (0 cm² free area).
- Each kW above 7 kW: Requires 5 cm² of free area per kW.
Sizing Formula
$$\text{Required Ventilation Free Area} = (\text{Net Input in kW} - 7) \times 5 \text{ cm}^2$$
Example Calculation
A boiler has a net heat input of 18 kW installed in a domestic utility room:
- Deduct the first 7 kW: $18\text{ kW} - 7\text{ kW} = 11\text{ kW}$.
- Multiply the remaining input by 5: $11 \times 5\text{ cm}^2 = 55\text{ cm}^2$.
- The required permanent opening to outside air is 55 cm² of free area.
Key Installation Guidelines
- Permanent Openings: Vents must not be closable or adjustable by the user.
- Grille Material: Grilles must have a mesh size of at least 6 mm (or flyscreen removed) to prevent dust/insects blocking the air flow.
- Vent Position: Vents should be positioned where they cannot be easily blocked or cause cold draughts that tempt users to seal them.
*Original revision guidance; confirm standard calculations against BS 5440-2.*