What is the purpose of a mechanical ventilation system in a home, and how does it work?

How a mechanical ventilation system works in the home

Controlled mechanical ventilation automatically renews the indoor air. It removes stale air and replaces it with fresh air from outside. The entire system is based on the principle of directed traffic flow from one room to another.

Air sweeping

Fresh air enters the main rooms first: the living room and bedrooms. It then travels through the apartment to the kitchen, bathroom, and toilet, where it absorbs moisture before being vacuumed up. This continuous circulation ensures that the air in every room is refreshed, leaving no area untouched.

In a home equipped with a single-flow mechanical ventilation system, the air follows a specific path: outside, air intake, main room, under the door, utility room, vent, duct, unit, and then out to the outside. This journey is described in detail in the encyclopedia article on theControlled mechanical ventilation and its systems.

System components

A mechanical ventilation system has four main components. The fan in the enclosure creates a vacuum that draws in polluted air through the vents and expels it from the home. Each of the other elements has a specific function along this route:

• Air inlets: they allow outside air to enter the main rooms.
• Exhaust vents: they draw in contaminated air from utility rooms.
• Ducts: they carry the air drawn in through the vents to the motorized unit.

In practice, exhaust vents are installed in every utility room with a water outlet: kitchen, bathroom, toilet, and laundry room. Make sure none of these parts are omitted when mapping out the network.

Traffic rules

The decree of march 24, 1982 (a french regulatory text establishing ventilation requirements for residential units) mandates general and continuous ventilation of residential units. It requires air inlets in the main rooms, outlets in the utility rooms, and free airflow between the two.

This free movement takes place through the interior doors. ADEME (the french agency for ecological transition, a public agency responsible for energy and the environment) generally recommends a gap of approximately 2 cm under doors. For its part, the FFB (french building federation, the sector’s leading professional organization in France) uses standard values of 1 cm or 2 cm, depending on the door and its configuration.

Contrary to popular belief, closing the doors does not improve ventilation if it blocks the airflow. A closed door remains compatible with scanning as long as there is a gap or a transfer grid.

Self-regulating or humidity-controlled airflow rates

Among the types of mechanical ventilation systems, single-flow ventilation is divided into two categories. The difference lies in how the airflow is controlled: fixed in one case, and adjusted based on humidity in the other.

Self-regulating single-flow mechanical ventilation system

A self-regulating single-flow mechanical ventilation system operates with a single duct network dedicated to exhaust. Its operating characteristics can be summarized in a few points:

• Unheated fresh air: the incoming air comes directly from outside.
• Flow rate set during installation: the installer sets it based on the specific dwelling.
• Constant airflow: it does not vary with humidity, weather, or the number of occupants.
• Optional high speed: some models offer this feature on a case-by-case basis.

This high-speed setting is used during activities that produce more moisture, such as extended cooking. DomeaShop includes these devices in its line ofSelf-regulating constant-flow mechanical ventilation systems.

Single-flow, humidity-controlled mechanical ventilation system

A humidity-controlled mechanical ventilation system adjusts the airflow based on the humidity of the air. While taking a shower, your mouth opens wider. It then gradually closes as the air is purified and the stale air is vented out.

In the Hygro A system, only the exhaust vents are humidity-controlled; the air inlets remain self-regulating and pressure-sensitive. In the Hygro B system, the air inlets also adjust, which limits the amount of fresh air drawn in and reduces heat loss when demand is low. Both of these versions are included in the lineup ofType A and B humidity-controlled mechanical ventilation systems.

Installation suitable for residential use

Single-flow, humidity-controlled mechanical ventilation is well-suited for renovation projects and older homes. Their air leaks do not ensure controlled moisture removal. Installation follows a set procedure: choosing the location for the unit—often in the attic or a closet—mapping out the ductwork, installing vents in wet areas, installing air intakes in the main rooms, making the electrical connections, and then commissioning the system.

For new residential buildings subject to RE2020 (french environmental regulations applicable to new construction) as of january 1, 2022, the installation must be inspected upon completion of construction by a qualified professional, in accordance with the decree of august 4, 2021 (french regulatory text). In addition, NF DTU 68.3 (a french standard that establishes technical rules for construction site implementation) governs the design and installation of this ventilation system.

Dual-flow mechanical ventilation with heat recovery

Dual-flow ventilation does more than just extract air—it also brings in fresh air. This ventilation system recovers heat from the outgoing air to warm the incoming air.

Two air networks

A dual-flow mechanical ventilation system has two separate networks, each with its own role in the airflow path:

• Exhaust system: it extracts stale air from damp rooms.
• Air distribution system: it distributes fresh air throughout the living room and bedrooms.
• Heat exchanger: both fluid streams pass through it before reaching their destination.

Outdoor air is first filtered, passes through the heat exchanger, and then flows into the living areas. The exhaust air also passes through the heat exchanger before being discharged outside.

A dual-flow system therefore does not require any air inlets above the windows or on the exterior walls of the bedrooms and living room. This lack of openings can improve sound insulation against street noise.

Heat exchanger and air quality

In the heat exchanger, heat from the exhaust air is transferred to the incoming air without the two air streams mixing. This way, the fresh air enters the rooms already warmed up in the winter.

According to ADEME, heat recovery reaches up to 70% for certain systems. It reaches as high as 90% for the highest-performing models.

The fresh air filters limit the entry of dust, particles, and pollen, thereby improving indoor air quality. Depending on the equipment, the supply air is filtered through an M5 or F7 filter, and the exhaust air is filtered through a G4 filter.

Choose based on the house

Some units have a bypass feature that bypasses the heat exchanger on hot summer nights to draw in outside air directly. An antifreeze system also protects the heat exchanger when temperatures drop well below freezing.

A dual-flow system remains more expensive than a single-flow system, especially in renovation projects: two duct networks, their insulation, a condensate drain, and a more complex installation process. This is the preferred option when the house is highly energy-efficient and airtight. For a passive house, the airtightness standard is 0.6 Vol/h at 50 Pa.

Using and maintaining the ventilation system

Once installed, the effectiveness of a mechanical ventilation system depends on how it is used and maintained. A few simple, regular steps will ensure proper air circulation and extend the life of the unit.

Continuous operation

A mechanical ventilation system is designed to run day and night. Its operating costs remain modest, and certain technologies are already optimizing its performance:

• Annual energy consumption: a 20-watt mechanical ventilation system running 24 hours a day consumes approximately 175 kWh per year.
• Calculation: 0.020 kW multiplied by 8,760 hours; the actual cost depends on the price of electricity.
• EC motors: energy-efficient motors reduce power draw.
• Sensors and control: CO2 and humidity sensors adjust the ventilation system to the home's needs.

So don't turn off the appliance every day to save money: turning it off daily promotes moisture, condensation, and the buildup of pollutants.

Check for anomalies

A simple test is to hold a sheet of paper up to your mouth: if it stays stuck to it, the suction is working. This method is not a substitute for a professional flow measurement. At the same time, keep an eye out for these common signs and faults:

• Warning signs: condensation on windows, lingering odors, an unusually loud motor, or very dusty air intakes.
• Removed air vents: a common oversight after replacing windows.
• Damaged ductwork: crushed ducts in the attic, improperly placed vents, or a duct box that has become inaccessible.

Depending on the layout of the unit, an inaccessible cabinet prevents any maintenance. Make sure there is unobstructed access to the attic or the closet where it is stored.

Maintaining equipment

Clean the intake and exhaust vents once a quarter, taking care not to get the humidity sensor on a humidity-controlled vent wet. The air intakes of a single-flow system should be cleaned of dust every month, and its fan once a year. DomeaShop recommends professional maintenance every 3 to 5 years, for an average lifespan of 10 to 20 years. In a dual-flow system, the filters should be cleaned every 6 months and replaced at least once a year. The motor and fans are inspected annually, and a thorough inspection of the ducts, air ducts, and heat exchanger is performed every 5 to 10 years.