Ventilation of your boiler room

Enter the boilers and the room and see how large the low-level vent (air supply) and the high-level vent (air exhaust) must be, as a grille and as a round, square or rectangular duct. Calculated to NBN/DTD B 61-001, NBN/DTD B 61-002 and NBN D 51-003.

1. Appliances in the room

Total nominal output0 kW

Use the manufacturer’s nominal output (for a boiler, the output at 80/60 °C). Add up all appliances in the same room.

Liquefied gas (propane or butane)

2. Building

Effective chimney height (m)

Air supply

Grilles and 90° bends in a row in the supply

The air exhaust runs as a duct inside the chimney stack (ends 0.5 to 1.5 m below the chimney top)

Number Diameter (mm) Cross-section

Needed for the air exhaust in the chimney stack and with room-sealed appliances only.

Where does the combustion air come from?

Through another room (transfer opening) is only allowed in an existing building and up to 30 kW.

Cooker hood or tumble dryer exhausting outside in the same room (m³/h, 0 = none)

How is the room ventilated now?

3. Room, grilles and ducts

Length (m) Width (m) Height (m)

Free area of the grille (%)

The free or net area is on the data sheet of the grille. With a fine mesh against vermin it is often between 30 and 50 %.

Fixed side of a rectangular duct (mm, empty = 2 to 1)

Only have a certain height or width free? Enter it and we work out the other side.

Section through the room

Sketch, not to scale. The values at the openings are the minimum free area you need.

How we calculate

This calculation

Which standard

From 70 kW in one room, NBN/DTD B 61-001:2021 (boiler rooms and chimneys) applies. Below 70 kW, NBN/DTD B 61-002:2021 applies to central heating boilers and NBN D 51-003 to other gas appliances, such as a water heater or gas fire. Chapter 6 of NBN D 51-003 applies to every natural gas appliance.

The 2019 standards NBN B 61-001 and B 61-002 were withdrawn on 8 April 2021. Since then the 2021 technical documents serve as code of good practice: B 61-001 takes over the 1986 text, B 61-002 the 2006 text. We therefore calculate with the minimum cross-sections from those texts and not with the 2019 airflow method.

Good to know

Free area is the net opening, without louvres and mesh. A 300 x 200 mm grille with 50 % free area gives 300 cm².
A duct has no louvres: its inner cross-section is the free area. Round, square or rectangular is fine, as long as the cross-section is large enough.
You may split the free area over more than one opening, as long as together they are large enough.
A window, a grille with a slide or a damper does not count: the openings must always stay open.
A long duct with many bends needs a larger cross-section.

This calculation helps you with the design. It does not replace the text of the standard, the appliance manuals or the inspection. Ventichim only supplies the materials. No rights can be derived from the result.