Open-Flued Ventilation Calculator

How it works

This exercise uses the room-vent method in Approved Document J for England for specified flued gas appliances. It excludes decorative fuel effect fires, flueless appliances and compartment openings. The 7 kW allowance is conditional: the guidance links it to older dwellings with air permeability above 5 m³/(h·m²). It is not a universal exemption for small appliances.

Calculated area (cm²) = max(0, total rated net input kW − permitted allowance kW) × 5. The calculator starts with no allowance. Select 7 kW only when the exercise or applicable design conditions establish that it is permitted.

Check the appliance instructions, air route and applicable requirements separately. Room-sealed combustion air and compartment cooling are different checks. This room calculation does not specify all ventilation needed for every installation.

Worked example

At 20 kW net, no allowance gives 20 × 5 = 100 cm². Where the 7 kW allowance is permitted, (20 − 7) × 5 = 65 cm².

Convert vent free area from cm² to mm²

There are 100 mm² in 1 cm², because both dimensions are multiplied by ten: 10 mm × 10 mm = 100 mm². Multiply cm² by 100 to get mm², or divide mm² by 100 to get cm². The 65 cm² result above is therefore 6500 mm², not 650 mm².

This also helps when reading vent specifications. A stated free area of 6000 mm² is 60 cm², so it is smaller than a calculated requirement of 65 cm². Compare the free-area figures in the same units before choosing a vent.

Grille dimensions are not the free area

The outside dimensions include solid material. A grille measuring 150 mm × 100 mm covers 15,000 mm², or 150 cm², but that is not necessarily its open area. In a hypothetical exercise where 40% is open, free area would be 150 × 0.40 = 60 cm²: still short of 65 cm².

Use the manufacturer’s declared equivalent area for the complete vent assembly where available. Do not assume a percentage from its appearance, treat the nominal duct diameter as its capacity, or add unsuitable flyscreen to a gas-appliance air vent.

A room calculation does not size compartment vents

Identify the appliance and where its air supply comes from before applying a rule. This result does not specify the high- and low-level openings for a cupboard or compartment. Dividing a room result in half does not establish the size of those openings; their purpose, position and route to air must follow the applicable procedure.

The flue flow and spillage guide explains the separate checks on the combustion-product route. Use it alongside the ventilation calculation, and the gas-fire revision notes when studying heater-specific scenarios.

Method: Approved Document J, England — stated room-vent conditions

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