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What is ventilation - System ABCD

"Ventilation noun [U] (providing air): the movement of fresh air around a closed space, or the system that does this." (Cambridge dictionary)

Ventilation in the form as we know it can best be described as:
"The more or less continuous replacement of the polluted (indoor) air by clean(er) (outdoor) air".

The main reason for ventilation is the positive influence on air quality. Everyone has ventilated at some time or has come into indirect contact with it. Some examples of this are: Opening a window (blowing out air), ventilation via window grilles or ventilators.

Ventilation and health are really connected

Air quality indoors is influenced by five factors:

      1. Moisture - per person this is 300 to 700 grams of water vapour per hour
      2. CO2 - We produce 20 litres of CO2 per hour
      3. Fine dust from cooking and gas- and wood-burning stoves
      4. Volatile organic compounds/concentrations from building materials
      5. Radioactive gas Radon/Thoron from the ground/soil

The first three factors are produced by human beings and have an impact on living comfort when ventilation is poor/less good.
Research also shows that poor air quality has a major impact on:

      • Concentration
      • Fatigue
      • Headaches
      • Poor sleep
      • Reduced zest for work

Improving indoor air quality increases pleasure and quality of life.

Ventilation in your home, what are the options?

The importance of ventilation has become increasingly high on the agenda of governments, housing corporations and homeowners in recent years. Partly due to the measures that government/municipal institutions prescribe, but also due to subsidies and awareness of the effects of "poor ventilation".

A home can be ventilated via four different basic principles, also called "(ventilation) system ABCD”. The four different ventilation methods are described as follows:

Ventilation system A

"Natural inlet with Natural outlet"

System A, the natural supply and extraction of air. This method of ventilation is entirely dependent on the wind force (pressure) on the facade and positions of the wall, window or door grilles. In practice, especially in the (cold) winter period, we see that the majority of people with a System A partially or even completely close the grilles. In this way, there is hardly any or no ventilation anymore. The positive consequences are that no heat is lost, but will not outweigh the negative consequences such as breathing in polluted air, concentration problems, poor sleep, headaches, dry to itching eyes etcetera.

Ventilation of air (opening the window) is also covered by System A.

System A
Advantages Disadvantages
  • Grilles can be placed directly between glass and frame, so not expensive
  • No electricity costs
  • Easily achievable during renovation
  • Since 2006, this form of ventilation is no longer permitted in newly built houses
  • Air circulation in the room
  • Wind dependent; with no wind little or no ventilation
  • Not adjustable on the basis of volume, moisture, CO2. Often only by a manually adjustable slide in the grille

Ventilation system B

"Mechanical supply with natural outlet"

System B, a ventilation method in which air is pushed out through overpressure. The supplied air is blown into the house by means of a (mechanical) fan. The often colder outside air is dispersed through the duct system and ends up in rooms such as a bedroom, living room, kitchen, bathroom, toilet and so on. By creating a higher overpressure than outside, the air will be forced outside through window grilles or wall grilles.

The disadvantage of this system is that in wet rooms the air is not discharged properly.

System B
Advantages Disadvantages
  • Constant flow of fresh air
  • Cold downdraught
  • Heat loss
  • High overpressure
  • Odours spread easily through the house

Ventilation system C

"Natural supply with Mechanical extraction"

System C, a ventilation method in which air enters the house via underpressure through window grilles. The negative pressure is caused by the extraction of the mechanical device. The extraction points are installed as standard in wet rooms, such as toilets, bathrooms, storerooms and kitchens. The window or wall grilles for natural supply can often be found in living areas, such as bedrooms, kitchen and living room. The air will flow through overflow* from the living space to a wet room.

*overflow = the movement of air by means of underpressure or overpressure from one room to another by means of a gap under the door.

System C
Advantages Disadvantages
  • Adjustable (settings, CO2, humidity, temperature)
  • Easy to install
  • Heat loss
  • Ventilation on the basis of underpressure
  • Light draught and feeling cold air
  • Air enters unfiltered

Mechanical ventilation systems can be operated wirelessly and via sensors (moisture, CO2) but also via a fixed three-position switch. View the possibilities and the complete range of System C fans here.

Ventilation system D

"Balanced ventilation with heat recovery based on Mechanical supply and exhaust"

System D, a ventilation method in which heat is recovered from the air. The principle is as follows: the air is supplied into the so-called "living areas" and extracted from "wet areas". This air passes through the heat exchanger and is re-used. The recovered heat is then mixed with the fresh air from outside. The heated and filtered air is then blown back into the living spaces via the supply valves. The D system can be identified by four connections. Two connections lead to the outside (intake and outlet) and two to the inside (supply and return) of air.

System D
Advantages Disadvantages
  • Pleasant climate
  • Little heat loss
  • High efficiency
  • Adjustable (settings, CO2, humidity, temperature)
  • Filtration of air (G3, G4, F7 filters)
  • Separate supply and return
  • Initial purchase
  • Major intervention in case of later application/renovation
  • Replace filters (1x every 6 to 12 months depending on location and use of the house)

The thermal efficiency of this process depends on the ventilation system used (MVHR) and is between 90% and over 98%. Most D systems can be controlled wirelessly and via sensors, take a look at the different System D fans here.


The four different images of the systems (ABCD) don’t explain very well the differences in, for example, construction, approach, comfort, budget and so on. To make the difference even better and more visual, we have tried to compare the two most common systems 'system CD'. These are compared to the most common decisions when choosing or determining a ventilation system indoors.

   System C  System D
 Power consumption + + + + + + +
 Sound + + + + + +
 Efficiency - + + + + +
 Maintenance + + + + + +
 Maintenance costs + + + + +
 Comfort + + + + + + + +
 Purchase + + + + + +
 Air quality + + + + + + + +

Advice on ventilation

Our ventilation consultants speak with hundreds of people every day. We are proud to be called The United Kingdom's largest ventilation specialists. With our free ventilation advice, we help home-owners, businesses and municipalities take a step further in improving and making air quality more sustainable, resulting in a healthier UK.

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Take a look at our contact page and get to know us.

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