Open-flued appliances per BS 5440-2 rules of thumb. Room-sealed appliances need no vents. Guidance only — always verify against manufacturer instructions and current Gas Safe / IGEM standards.
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This free ventilation calculator gives you the free vent area required for an open-flued gas appliance, based on its net heat input and where it is installed. It is aimed at UK Gas Safe registered engineers checking existing ventilation on service visits or specifying vents for a new open-flued installation.
Enter the appliance's net input in kW and choose the location: in a room, in a compartment ventilated to a room, or in a compartment ventilated directly to outside. The calculator applies the BS 5440-2 rules of thumb — 5 cm² per kW over 7 kW for a room, 10 cm² per kW at both high and low level for a compartment vented to a room, and 5 cm² per kW at both levels for a compartment vented to outside.
Room: (net kW − 7) × 5 cm²; Compartment to room: net kW × 10 cm² high and low; Compartment to outside: net kW × 5 cm² high and low
Net kW is the appliance's net heat input from the data badge. For an appliance in a room, the first 7 kW needs no purpose-provided ventilation, and each kW above that needs 5 cm² of free area. A compartment ventilated to an internal room needs 10 cm² per kW at both high and low level; a compartment ventilated directly to outside needs 5 cm² per kW at both levels. All figures are free area — the actual open area of the vent, not its overall face size.
Enter the appliance's net input in kW from the data badge or manufacturer's instructions. If only gross input is stated, divide by 1.11 for natural gas or 1.09 for LPG to get net.
Choose whether the appliance is in a room, in a compartment ventilated to a room, or in a compartment ventilated to outside. Compartments need vents at both high and low level.
The calculator shows the free vent area in cm² — a single figure for a room, or separate high-level and low-level figures for a compartment. Compare this against the free area marked on the vents fitted, not their overall dimensions.
These figures are the BS 5440-2 rules of thumb for open-flued appliances. Open-flued (flueless-air) appliances draw their combustion air from the room or compartment they sit in, so inadequate ventilation directly affects safe combustion. Room-sealed appliances — which include almost all modern condensing boilers — take their air from outside through the flue and need no purpose-provided vents under these rules.
The 7 kW allowance for appliances in a room reflects adventitious ventilation in a typical dwelling. Multiple appliances sharing a space, flueless appliances, unusually airtight rooms and non-domestic settings all fall outside this simple check and must be assessed against the full standard.
Vent free area is the clear open area of the grille, which is often much smaller than its face size — check the manufacturer's marked free area. Guidance only — always verify against manufacturer instructions and current Gas Safe / IGEM standards before signing off ventilation.
Open-flued appliances per BS 5440-2 rules of thumb. Room-sealed appliances need no vents. Guidance only — always verify against manufacturer instructions and current Gas Safe / IGEM standards.
As a rule of thumb from BS 5440-2, an open-flued appliance in a room needs 5 cm² of free vent area per kW of net input above 7 kW — so an 18 kW net appliance needs (18 − 7) × 5 = 55 cm². Compartments need vents at both high and low level: 10 cm² per kW if vented to a room, or 5 cm² per kW if vented to outside.
No. Room-sealed appliances draw combustion air from outside through the flue, so they need no purpose-provided ventilation under these rules. Almost all modern condensing boilers are room-sealed. Compartment installations should still follow the manufacturer's instructions on clearances and any cooling requirements they specify.
Use net input. This calculator's formulas are based on the appliance's net heat input in kW. If the data badge only gives gross input, convert it: divide by 1.11 for natural gas or 1.09 for LPG. Using gross figures would oversize the vents slightly, but always work from the correct net figure.
Free area is the actual clear open area air can pass through, not the overall size of the grille. A 100 cm² grille may only have 40–50 cm² of free area once louvres and mesh are accounted for. Good-quality vents have their free area marked by the manufacturer — that is the figure to compare against the calculated requirement.
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