VentAssist

Using the result

Areas shown are free areas, not the outside dimensions of a grille. Use the vent manufacturer’s certified free area, and check location, blockage, other appliances and the flue’s performance on site.

7 kW adventitious air allowance: Diagram 32 note 5 applies to the outside air supply for an open flued appliance in a room, or via an adjoining room, only when the older dwelling is assessed above 5.0 m³/(h·m²) at 50 Pa. It does not reduce the high and low compartment vents. DFE fires have a separate exception in paragraph 3.12.

For a flueless appliance, Diagram 33 also calls for controllable ventilation: an openable window, door or similar opening directly outside, or, in a kitchen, mechanical extract. A suitable outside door can meet this provision without a separate window. Check the applicable Approved Document F edition and the manufacturer’s instructions.

Scope and sources

Type A (flueless), Type B (open flued) and Type C (room sealed) follow BS EN 1749:2020. Ventilation figures follow Approved Document J, paragraphs 3.10–3.16 and Diagrams 32–33 for England, gas appliances rated up to 70 kW net. Also see HSE guidance on ventilation and flues.

DFE fires without a measured flue area, multiple appliances, extract fans, LPG cases and unusual layouts require the applicable standards and manufacturer’s instructions. BS 5440-2:2023 must be consulted where relevant; the calculator does not reproduce the full standard. The app is an aid for competent engineers; it does not determine that an installation is safe.