Single-flow mechanical ventilation

Single-flow mechanical ventilation renews the air by extracting stale air (from kitchens and bathrooms) and drawing in fresh air through air inlets located in the walls or window and door frames of living areas. There is no heat exchanger: the heat from the extracted air is not recovered. It is a robust, economical, and easy-to-operate solution, widely used in homes and some commercial building technical rooms. However, it can generate heat loss in winter, create negative pressure if poorly adjusted, and provides no filtration of the incoming air (except with additional local grilles or filters).

Functioning and common variations

Principle

- An extraction unit draws air from the vents (kitchen, bathroom, toilet).

- Fresh air enters naturally via self-regulating air inlets (constant flow rate) or humidity-controlled air inlets (variable flow rate depending on humidity).

- The transit takes place from dry room to wet room, under the doors (undercutting).

Variants

- Self-regulating: fixed flow rates (outlets and inlets), stable but not very adaptable to occupancy.

- Humidity-controlled A: vents vary according to RH, air inlets are fixed.

- Humidity-controlled B: vents and inlets vary according to RH (more economical in mid-season).

Key elements

- Sealed and insulated networks in attics/cellars to limit losses and condensation.

- Acoustic attenuators, access hatches, well-positioned connections to avoid noise and air short circuits.

- Simple power supply (often one speed + high speed for cooking).

Advantages, limitations and points to consider of a single-flow mechanical ventilation system

Strengths

• Simplicity/cost: readily available materials, installation and maintenance.
• Robustness: less risk of deviations than a complex system.
• Hygro-adjustable: adapts flow rates to the actual humidity → savings compared to self-regulating.

Boundaries

• Heat loss in winter (no recovery); risk of discomfort near air inlets.
• Limited IAQ: fresh air is not filtered (outdoor pollen/PM), except by local filters.
• Overpressure possible (air intake noises, combustion appliance draft to be secured).
• Noise at the vents if flow rates/pressures are incorrectly adjusted.

Points to consider

• Sizing: regulatory flow rates per room, consistent with actual use.
• Hygro A/B choice according to climate/occupancy; avoid self-regulating in dwellings with very variable conditions.
• Air inlets: place away from sources of pollution (traffic, nearby hoods); provide cleanable grilles.
• Networks: sealing, slopes for condensate drainage, thermal insulation in cold areas.
• Maintenance: regular cleaning of vents/grilles, replacement of motor/wheel if clogged; check ΔP.
• Safety: compatibility with gas appliances (air supply, detectors); prevents backflow.

Anecdote — “Peaceful bathrooms in Brest”

In Brest, a student residence equipped with a single-flow, self-regulating ventilation system was complaining of persistent odors and condensation. An audit revealed clogged vents, air inlets blocked by residents, and an undersized unit. The system was upgraded to a humidity-controlled B system, the vents were cleaned and replaced, new air inlets were installed with basic pollen screens, and the ductwork was recalibrated (airtightness and attenuation). The result: stabilized humidity levels, eliminated odors, and improved thermal comfort despite the lack of heat recovery—proofing that a well-adjusted single-flow system can do the job very well.