How Do You Ventilate Your Home—With System A, B, C or D?

How Do You Ventilate Your Home—with System A, B, C or D?

Home ventilation is a very important aspect of creating a pleasant and healthy living environment for the occupants. The outer shells of new homes are generally made increasingly airtight. Without ventilation, the air inside the home would quickly become stale due to excessive humidity, odours, CO2 and other impurities. The occupants might develop health problems. Home ventilation is also essential to avoid fungal growth and prevent house mite infestation. This article discusses four different ventilation systems.

System A

This is the simplest system, but not the most efficient. In the dry rooms (living room, office and bedrooms) vents are placed above the windows to supply fresh air from outside. In the damp rooms (bathroom, kitchen and toilet) the contaminated air is removed by natural circulation through vertical exhaust ducts with adjustable vents. Vents in the interior walls or doors, or gaps beneath the doors, enable air flow from the dry rooms to the damp rooms. With this system the degree of ventilation is strongly dependent upon the temperature and pressure differences between the outdoor and indoor climates.

System B

With system B, fresh air is supplied by fans in the dry rooms. Air flow to the damp rooms and contaminated air exhaust are the same as in system A. However compared to the previous system, the indoor climate is less weather dependent. Electricity consumption is invariably somewhat higher as a result of running the fan.

System C

System C is actually the inverse of system B. It uses natural air supply through air vents in the windows of the dry rooms, while contaminated air is removed from the damp rooms mechanically by fans. This system can ensure good air quality under various weather conditions thanks to the installation of a limited network of exhaust ducts

There is also a variant called system C+. It is largely the same as system C, but with smart control of mechanical exhaust. This can be achieved by using motion sensors, among other options. When someone enters a damp room, the ventilation air flow is automatically increased. In addition, CO2 sensors can also be triggered to increase the ventilation air flow if the CO2 level is too high.

System D

System D is essentially a combination of systems B and C, with fans for mechanical air supply as well as mechanical exhaust. The system controller keeps the air supply and exhaust constantly in balance. That avoids positive or negative pressure inside the home and makes the system less dependent on changeable weather conditions

Example of system D
Example of a type D ventilation system with heat recovery. (Courtesy of

System D can be complemented with heat recovery. The incoming, cold outdoor air is routed to a heat exchanger where it can be preheated by heat extracted from the contaminated exhaust air, which is already at room temperature. And this without mixing the two air streams. Such heat recovery reduces energy consumption. In the summer, the process is reversed: warm outdoor air is partially cooled in the heat exchanger by the cooler indoor air. In this way, the need for air conditioning is reduced, which lowers the energy bill.