Which size ventilation
system do I need?

The Danish building regulation BR18 determines that a home's basic air change must be provided by a ventilation system, with supply air in the living spaces and exhaust in the bathroom, toilet, kitchen, utility room and basement.

Requirements for outdoor air supply

The basic requirement is the addition of a basic air change of 0.3 litres of air per second per square metre of heated floor space (overall dimensions). For a home with a ceiling height of 2.5 m, this is the equivalent of the air in the room being changed once every two hours. Converted to m3 of air, 1.08 m3 an hour per square metre is required.

For a house of 125 m2, the minimum ventilation requirement will therefore be 125 x 1.08 = 135 m3/h.

Exhaust requirements

In addition to the requirements for outdoor air supply, the ventilation in a home must fulfil minimum exhaust requirements from the kitchen, bathroom with/without toilet, utility room and basement respectively.

Minimum exhaust requirement in:

  • Kitchen 15 l/s = 54 m3/h*
  • Bathroom 15 l/s = 54 m3/h
  • Toilet without bath, utility room 10 l/s = 36 m3/h
  • Basement (single family dwelling) 10 l/s = 36 m3/h


Get our full 32-page guide for selection and installation of residential ventilation right here: Selection Guide for residential ventilation

How the cooker hood is installed is critical

Calculating the exhaust requirement in a kitchen depends on whether an cooker hood has been installed which is connected to the ventilation system or a separate cooker hood. In multi-occupancy buildings, the cooker hood is often connected to a decentralised ventilation system, whereby the basic ventilation such as forced ventilation (in connection with cooking) takes place through the cooker hood. In these cases it should be factored, in terms of air volume, that the cooker hood has an extraction capacity of at least 75%. This is not the case with stand-alone extractors. This has considerable bearing on the dimensions of the ventilation system, as shown in these two examples:

Calculation example 1:

For a house of 125 m2, the basic ventilation requirement is 1.08 x 125 = 135 m3/h. With a kitchen with an extractor hood connected to the ventilation system, two toilet-bathrooms and a utility room, the exhaust requirement excluding the kitchen is: 54 + 54 + 36 = 144 m3/h. The extractor has an extraction capacity of 75% at
140 m3/h. The total air volume will thus be 144 + 140 = 288 m3/h.

Here, the ventilation system must be able to supply 288 m3/h at the specific back pressure in the duct system and at a max. power consumption of 1,000 J/m3 in accordance with the Danish BR18.

Calculation example 2:

For a house of 125 m2, the basic ventilation requirement is 1.08 x 125 = 135 m3/h.
With a kitchen with a separate cooker hood, two toilet-bathrooms and a utility room, the exhaust requirement excluding the kitchen is: 54 + 54 + 36 = 144 m3/h. Add to that Dantherm's recommended 54 m3/h for the statutory exhaust via control valve. The requirement of an extraction capacity of at least 75% still applies, but does not need to be factored in. The total air volume will thus be 144 + 54 = 198 m3/h.

Here, the ventilation system must be able to supply 198 m3/h at the specific back pressure in the duct system and at a max. power consumption of 1,000 J/m3 in accordance with the Danish BR18.

Additionally, replacement air must always be ensured when the cooker hood is in operation. Manual opening of windows does not qualify as a way of meeting the requirements.

The highest requirement applies

In general, the supply of air and exhaust needs to be balanced. Therefore, the highest figure determines the required ventilation system capacity.

For more information, please download Dantherm’s full Selection Guide for residential ventilation.