Understanding what supply-air ventilation (VMI) actually does
Principle: fresh air supply, slight positive pressure, and the path of air through the dwelling
A supply-air ventilation system injects fresh air into a buffer area (often a hallway or attic) through a fan and a short duct run. The goal is a slight positive pressure that drives air from dry rooms toward wet rooms. On site, you verify the supply point, the transfer under doors, and you avoid short-circuiting (supply too close to an exhaust point).
Filtration, preheating and airflow rates: what changes from one unit to another and according to the settings
Depending on the model, filtration may target mainly dust, or also pollen. Preheating is sometimes built in, or available as an option, which affects energy use and the risk of nuisance tripping. As for settings, you size the useful airflow and anticipate pressure losses related to the ductwork, clogged filters and silencers.
Stale-air removal: the role of existing vents, air inlets and building leakage
VMI does not extract air. Stale air leaves through existing exhaust grilles (if an extraction system is in place), high-level grilles, or through the building’s air leakage path. You check that the airflow path is coherent, that there is no spillage from combustion appliances, and that the system works effectively in wet rooms (humidity and odours). To set the correct sizing of air inlets and exhaust grilles, also check compatibility with the existing ventilation layout.
Positive pressure: expected benefits and key site risks
Why positive pressure can help with certain odours and unwanted air infiltration
A supply-air ventilation system puts the volume under slight positive pressure. Filtered air is introduced into dry areas and exits through planned leaks and openings. Expected result on site: less parasite air entering from the crawlspace, garage or attic. Fewer odours following air infiltration. Before commissioning, check the air paths between rooms, and identify uncontrolled openings (smoke test).
Humidity: what supply-air ventilation can improve (and what it cannot solve) in a damp dwelling
Positive pressure can limit the draw of humid air from cold spaces and improve dilution if extraction from wet rooms follows. However, it does not dehumidify on its own. It does not address water leaks, rising damp, or thermal bridges. Keep extraction operational, and check that the flow rates remain consistent with occupancy and internal gains.
Risks if the system is poorly sized: condensation, flue draft issues, slamming doors, acoustic discomfort
In a leaky envelope, exfiltration can carry vapour into cold zones and trigger condensation. With a non-sealed combustion appliance, positive pressure can disturb draft and create reverse draft. Add slamming doors, whistling at grilles and fan noise. Measure pressure and sound level, then adjust the controls, silencers and air passages. To anticipate these issues, use dew point calculation to prevent condensation on site.
When supply-air ventilation is appropriate, and when it is not
Suitable cases: light renovation, house that is difficult to duct, need for simple and fast air renewal
A supply-air ventilation system is appropriate when you are aiming for controlled positive pressure without building a full extract network. In light renovation or in a house that is difficult to duct, you install a supply unit, with upstream filtration and sometimes slight preheating. It is effective on a short-duration job.
Unsuitable cases: very airtight dwelling, severe moisture issues, presence of combustion, damp cellars/crawlspaces
Avoid it in a very airtight dwelling or after extensive air-tightness work. Positive pressure increases exfiltration through leaks and can move water vapour into cold zones. Be cautious where there is a natural-draft appliance or an open fireplace. In damp cellars and crawlspaces, moisture-laden air can rise through airtightness defects.
Conditions of use to set with the client: habits, heating, occasional airing, humidity monitoring
Set the operating conditions in the quote. Airflow adjusted and maintained, transfer openings identified (grilles, transfers, door undercuts), and filters maintained. Check consistency with the heating system, especially if the air is reheated. Organise humidity monitoring over a few weeks to confirm the settings, then formalise the occasional airing instructions.
VMI vs heat recovery VMC: technical opinion on performance and efficiency
Heat recovery: why supply-air ventilation does not replace a heat recovery unit for energy savings
| Criterion | VMI | Heat recovery VMC |
|---|---|---|
| Principle | Supply air, positive pressure | Balanced supply and extract |
| Heat from extracted air | Not recovered | Recovered via heat exchanger |
| Heating impact | Reduces unwanted air ingress but does not exchange energy | Limits losses linked to air renewal |
In practical terms, a VMI can stabilise flows and improve indoor air quality, but thermally it remains a “fresh-air only” ventilation system. Of the two, only the heat recovery unit aims for an overall efficiency gain through heat recovery.
Indoor air quality: supply-side filtration vs airflow balancing in heat recovery systems
A supply-air ventilation system filters incoming air at the unit. It does not finely control exhaust rates room by room. A heat recovery system, with dedicated ducts, keeps airflow balancing. You can set operating modes and limit the transfer of odours and humidity between zones.
Comfort and maintenance: noise, filter changes, access, electricity consumption
On site, comfort depends mainly on sizing and pressure losses. VMI has a concentrated noise point at the supply outlet. Heat recovery systems can be noisy at the terminals if the ductwork is poorly balanced. In both cases, plan a filter maintenance schedule. Heat recovery also requires access to the exchanger and duct airtightness. Electricity consumption follows airflow and pressure.
Diagnostic and implementation method for a defensible opinion in 2026
Pre-design: envelope condition, simple measurements (CO₂, humidity), identification of air inlets and outlets
Carry out a technical visit focused on airflow. Record visible airtightness defects, volumes, rooms with existing extraction, and keep dated readings (CO₂, relative humidity) for at least two representative rooms.
| Measurement | Reference range | Site interpretation |
|---|---|---|
| CO₂ | < 1,000 | Air renewal generally sufficient |
| Relative humidity | 40 to 60 | Hygrothermal comfort zone. Above this, risk of condensation |
Map the air inlets, outlets, unused ducts and air paths (door undercuts, grilles, gaps). Without a transfer path, positive pressure is lost.
Sizing and settings: airflow rates, supply location, preheating management and filters
On a supply-air ventilation system, place the supply point in the heart of the heated volume, with easy filter access. Set the airflow based on measured values, not by feel, and record the settings (airflow, noise, pressure losses). Preheating is chosen when the incoming air would otherwise create supply temperatures too low for the terminals and ductwork to operate properly.
Post-installation checks: smoke tests/tracer tests, balancing air transfers, maintenance and monitoring protocol
Verify the airflow direction by smoke tracing and correct any short-circuit paths. Adjust the air transfers between rooms, then formalise a maintenance protocol (filters, air intake, fan) and follow-up with repeated CO₂ and humidity measurements.
