Ventilation System B: mechanical supply, natural exhaust

Ventilation system B supplies fresh air to your home in a controlled way. Unlike completely natural ventilation (system A), the supply openings are actively supported by a fan. The exhaust of used, humid air takes place naturally through grilles in wet rooms. This makes system B an interesting choice for those who want more control over air quality, without installing a fully mechanical exhaust system.

How exactly does ventilation system B work?

With ventilation system B, a fan draws in fresh outside air, filters it and blows it through ducts to the dry rooms: living room and bedrooms. The influx of purified air creates a slight overpressure. This overpressure pushes the used air naturally towards the wet rooms – bathroom, kitchen and toilet – where it exits via exhaust grilles. So the system is a combination of mechanical supply and natural exhaust.

Advantages and disadvantages of ventilation system B

Advantages:

• Filtering: the supplied air passes through a filter, keeping dust and pollen out. Ideal for people with allergies.
• Constant flow rate: you can precisely adjust the fan, ensuring sufficient fresh air always enters, regardless of wind or temperature.
• Easy to integrate: because the exhaust is natural, you need less ductwork than with system D. This makes it suitable for renovation projects.

Disadvantages:

• No heat recovery: the heat from the exhaust air is lost, unlike with ventilation system D. This can increase the energy bill.
• Noise production: the fan produces a slight noise. Proper placement and sound dampening are important.
• Maintenance: filters need to be replaced regularly and the fan periodically cleaned.

Difference with other ventilation systems

The Flemish building code distinguishes four basic types, each with their own characteristics:

• System A: completely natural, supply and exhaust via grilles, without fans.
• System B: mechanical supply, natural exhaust.
• System C: natural supply, mechanical exhaust via a central extraction unit, so the reverse.
• System D: fully mechanical with heat recovery, the most energy-efficient option.

For a thorough comparison, it's best to read our separate page about the different ventilation systems.

What does a ventilation system B cost?

The total price depends on factors such as the size of the home, the number of rooms to ventilate, the degree of insulation and the accessibility of the installation site. On average, you should budget between €1,500 and €3,000 for a basic installation, excluding any built-in zones and finishing. For an exact tailored price, it's best to request a non-binding quote from a certified installer.

Installation: new build or renovation

System B can be installed in both new-build homes and existing homes. Because the exhaust is natural, you don't need an extensive duct network. However, space must be provided for the supply unit and the duct system to the dry rooms. In renovation situations, it is sometimes necessary to lower ceilings or walls to conceal the ducts, but that is less invasive than with system D.

Maintenance of ventilation system B

For a consistently healthy indoor climate, regular maintenance is essential. Replace the filters at least twice a year and vacuum the supply grilles in the living areas. Have the fan and the ductwork professionally cleaned every two to three years. This prevents bacterial growth and keeps the system running efficiently.

Regulations in Flanders

In Flanders, ventilation is legally required in new homes and in major energy renovations. System B meets the EPB regulations, provided it is correctly dimensioned. For specific requirements per municipality, it's best to consult the Local Energy and Climate Pact or contact a ventilation expert in your region.