Mechanical ventilation (VMC): how it works in French properties

How a VMC works

A typical VMC simple flux (single-flow) system works like this: a central extraction unit, usually in the roof space or a cupboard, pulls stale, humid air out of the property through grilles positioned in the rooms where moisture and odours are generated (bathrooms, WC, kitchen). Fresh air enters the property through small trickle vents fitted to windows, or through dedicated supply grilles in living areas and bedrooms.

The fan runs continuously at a low speed, with automatic or manual boost in the kitchen and bathrooms when needed. Because air is constantly being replaced, humidity doesn't build up. This matters more than it sounds: humidity is the primary cause of mould growth in French properties, and mould is a common complaint in older apartments that lack adequate ventilation.

VMC simple flux vs VMC double flux

Simple flux extracts stale air but doesn't recover the heat it carries out of the building. In winter, you're expelling warm air and drawing in cold air, an unavoidable energy loss.

VMC double flux (dual-flow or heat-recovery ventilation) addresses this. The unit contains a heat exchanger: outgoing stale air and incoming fresh air pass through adjacent channels without mixing, and heat transfers from the warm exhaust air to the cold incoming air. In practice, a good double-flux unit recovers 75-90% of the heat that would otherwise be lost. That makes a measurable difference to heating costs and to the DPE energy rating.

Double flux units are larger, more expensive, and require proper ducting to each room. They make most sense in well-insulated properties where the heat loss through ventilation is proportionally significant. They are also eligible for MaPrimeRenov' grants, provided a certified contractor carries out the installation.

Why VMC matters for property condition

Without a working VMC, or any ventilation at all, moisture from cooking, bathing and breathing has nowhere to go. It condenses on cold surfaces: external walls, windows, corners. Over time this leads to mould growth and, in severe cases, structural damp. In the south of France, where properties are often shut up for part of the year, this is a particular problem.

A non-functional or absent VMC will be noted in a property survey and will show up in the DPE calculation. Buyers and tenants have become more aware of this. If you're purchasing an older property, check whether a VMC is installed and, if so, when it was last serviced. VMC units require periodic cleaning of filters and grilles, and the fan unit itself has a service life of roughly 10-15 years.

VMC and French building regulations

For any property built or substantially renovated after 1982, VMC is required under French building code. Older properties are not legally required to retrofit one, but the absence affects DPE ratings and can create issues for rental properties, since properties with poor energy performance face tightening restrictions on letting from 2025 onwards.

Related terms

• DPE: the energy performance certificate that is affected by ventilation quality
• MaPrimeRenov': VMC double flux installations can qualify for this grant
• RGE certification: required for contractors installing VMC under a grant scheme

Further reading

• Cooling and ventilation systems on the Cote d'Azur