A worked free-area calculation for an open-flued appliance, framed so you verify the rate in the standard.
Open-flued appliances draw their combustion air from the room, so above a certain input they need a permanent air vent that cannot be blocked. This worked example shows the shape of the calculation.
Method - many candidates learn a rule of the form: provide a fixed effective free area for each kW of input above a threshold. For this worked example, use 5 square centimetres (cm2) of effective free area for every kW above a 7 kW threshold. Treat that rate and threshold as the example input and confirm the current values in the applicable standard for the appliance and room type.
Step 1 - find the chargeable input. For an open-flued appliance of 16 kW net input: 16 - 7 = 9 kW above the threshold.
Step 2 - apply the rate. Free area required = 9 kW x 5 cm2 per kW = 45 cm2 of effective free area.
Step 3 - choose the vent. A grille has an overall size and a smaller effective free area once the bars and mesh are accounted for, so size from the effective free area quoted by the maker, not the outside dimensions. The vent must be permanent and not blockable.
Step 4 - consider the path. Air taken directly from outside, drawn from an adjoining room, or supplied to an appliance in a compartment can be treated differently, and a compartment may need both high and low vents. Always check the route the air actually takes and the worst case for the room.