Sizing high and low vents for an appliance in a compartment

First identify the appliance and air route. An open-flued appliance takes combustion air from the compartment; a room-sealed one takes it through its outside air route. Either arrangement may have compartment cooling requirements. Follow the appliance instructions and applicable design method.

Why position matters

Where high and low openings are required, cooler air can enter below and warmed air leave above. Adding their areas together does not establish that each opening is adequate. Nor should equal areas be assumed for every appliance type.

Two calculation exercises

Consider a 24 kW net appliance. Suppose the applicable compartment-to-room method specifies 20 cm²/kW low and 10 cm²/kW high for the open-flued arrangement:

• Low opening: 24 × 20 = 480 cm².
• High opening: 24 × 10 = 240 cm².

For a different, room-sealed arrangement whose specified method requires 10 cm²/kW at each level, each opening would be 240 cm². These examples distinguish the methods; they do not prescribe vents for every boiler. Some manufacturer-approved arrangements do not require cooling vents when the stated clearances and conditions are met.

A route directly outside may use different rates from a route into a room. Check the complete path, including any required room-to-outside opening. Do not deduct a room's 7 kW allowance automatically from the compartment openings.

Choose suitable ventilators from their declared equivalent areas. The frame size is not the available air area, and added unsuitable screening can make an otherwise adequate opening unacceptable.