Basic system: single flow by extraction

Less sensitive to climatic variations than natural ventilation, a CMV system provides general and continuous replacement of air.

The principle consists of extracting stale air from so-called service rooms, such as kitchens and bathrooms, through fan-driven extraction outlets.

Since the pressure in the building is therefore lowered, new air is drawn in through air inlets installed in the main rooms. This, in turn, produces a transfer of air from the main rooms to the service rooms, providing full ventilation of the home. Gaps left in the lower parts of interior doors achieve this.

Exterior air inlets are fitted into the woodwork of picture windows, in window casings, or directly in the masonry. Extraction outlets, which can have a double flow rate in kitchens, are linked to the extraction fan by flexible ducts for the individual dwellings, and through a shared network of galvanized steel ducts for collective dwellings and apartment buildings.

The airtightness of the ducts must meet European requirements (Eurovent classification A, B, and C, the most efficient). The French company ABB Vim offers connection joints prefitted with an airtight seal in this field.

Martine Bianchina, marketing manager at ABB Vim, explains that “The presence of this seal makes assembly easier, reduces installation time, and offers very good airtightness that makes it possible to obtain a class C rating [leakage rate less than 1%], use a smaller fan, and therefore consume less energy.”

As variations in the flow rate are often large for an entire building, which in turn generates significant pressure losses in the network, the company also sells a pressure regulator (“Reglavent”) which is placed near the fan.

“This system controls the flow rates and makes up for pressure differences in order to obtain a constant,” says Bianchina.

Humidity control

The air inlets and extraction outlets of a single-flow CMV system can be specified to be self-regulating, which helps to ensure a constant flow rate whatever the climatic conditions or variations in pressure affecting the network.

The outlets adjust even more precisely to the required flow rates when they are controlled by the humidity present in the rooms, which also prevents condensation and helps save energy. Air humidity is measured with hygrometers, which control the operation of the outlets.

This type of humidity-controlled ventilation system has been launched in France by Aldes, one of the country’s main manufacturers of ventilation systems. Bernard Moysan, partnership relations manager at the company, explains that “Installing a humidity-controlled CMV system makes it possible to regulate the rate of air replacement, depending on the occupancy of the building and the staleness of the air.

“Occupants are not always present, and when they are, they occupy different rooms at different times, producing greater or lesser amounts of contamination in the air. With humidity-regulated air inlets and outlets, flow rates are higher when occupants are present and almost negligible when they are not, an adaptation to actual requirements that can lead to energy savings of up to 50%.”

While it is possible to have humidity-controlled CMV systems where only the extraction outlets are humidity controlled, these result in lower energy savings.

Double-flow system for recovering heat

A double-flow mechanical ventilation system is based on a central unit that takes in fresh air and exhausts stale air. The air inlets in the main rooms or living areas and the air outlets in the service rooms are connected using ducting to the central unit, which has an air intake from the outside.

This type of system has the advantage of lower noise levels, because there are no air inlets at the front of the building.

This design and mode of operation offer the possibility in winter of recovering heat from the extracted air to heat the incoming air. The installation then includes a heat exchanger (mostly a plate-type exchanger) through which flows the stale air. This is extracted on average at 68°F, and heats the fresh air entering from the outside. The air drawn in can reach 57° if the outside temperature is at 32°, and the interior temperature is at 68°. This is because the exchanger recovers in general 60% to 65% of the heat losses, which results in savings of 15% to 20% in heating costs when compared to a single-flow system.

In an apartment block, the inlet and extraction fans and vertical ducts are shared, but there is a heat exchanger in each apartment.

Ventilation controlled by CO₂, occupancy

In some buildings, the ventilation system can be controlled by interior concentrations of CO₂, carbon dioxide being recognized as an indicator of interior air quality.

The concentration of CO₂ depends on the number of people, but also on particular activities (smoking areas) as well as the CO₂ already present in the incoming air, which varies significantly depending on whether the building is in a rural, urban, or industrial area. This principle makes it possible to obtain constant air quality, regardless of the level of occupancy.

For example, after a meeting room has emptied, ventilation can continue to operate until the concentration of CO₂ in the air stabilizes below a threshold set at 0.1%, or 0.3% in nonsmoking areas. A CMV system controlled by CO₂ levels can be for a single area; the regulation of flow rate is then ensured by the fan by means of a frequency controller. For multiple areas, the flow rate can be adjusted in each room through use of motorized registers.

With other systems, the quantity of air exhausted or drawn in depends on the number of people present. The outlets are then linked to an access barrier that incorporates a detection sensor and counter, or they stop operating automatically when occupants are not detected.

The flow rate is set according to the usual occupation of the rooms, but can be modified in case of change. These automatic control systems produce savings in energy and maintenance.

Discretion assured

CMV systems in France have improved over the years, particularly in their performance and reliability, but they also have become as discreet as possible.

Ventilation often seemed to be a source of drafts and noise. Ventilation outlets in particular have much improved, as testified by Laurent Barbarin, managing director of Anjos, which manufactures all types of fresh air inlets and extraction outlets.

“These inlets and outlets have continued to advance over the last few years in engineering, acoustics and appearance, with prices remaining stable in order to meet the requirements of the market.”

The latest air inlets can achieve a sound insulation of 30, 35, 40, or 45 dBA when fronting onto a road, depending on the level of traffic noise.

Where required, these units can be fitted with silencers or acoustic hoods. As for the sound pressure level (level of noise produced in the room) emitted by the fan and the extraction outlets, this can vary between 30 and 35 dBA at minimum flow rates for the main rooms, and between 35 and 50 dBA at maximum flow rates for service rooms.

To increase sound insulation, the extraction units can be positioned on anti-vibration blocks or pedestals fitted using an elastomer; similarly, the ventilation ducts can be insulated with glass fiber. Most outlets display the optimum volume of air that they can move quietly and efficiently.

As well as being quieter, the new generation of air inlets being used in French buildings are also distinctly better looking. Small in size, they are available in various colors that can be neutral (white, gray, black etc.) or bright (red, blue, green, yellow, etc.) to blend in with the decor.