Installing a single-flow MVHR vent in the bathroom
Your bathroom only has a simple air grille, but the condensation persists on the mirror and the walls. A passive grille creates no draught: the damp air only leaves through natural air currents, often insufficient. Replacing this grille with a self-regulating single-flow mechanical ventilation vent provides continuous, silent extraction, regulated according to needs. This guide is aimed at DIYers who already have an extraction duct towards the outside and want to equip it with a motorised vent.
Choosing a self-regulating single-flow mechanical ventilation vent
A self-regulating single-flow mechanical ventilation vent maintains a constant air flow whatever the wind or the network pressure. For a bathroom, a flow rate of 30 m³/h is recommended to effectively extract the humidity from showers; the regulations require a minimum of 15 m³/h continuously. The vents exist in 100 mm or 125 mm diameter: measure your duct before buying. Prefer a silent model, stated under 30 dB(A), with a non-return flap that blocks the cold air when the ventilation is off.
- Switch off the power at the consumer unit and check the absence of voltage with a tester before any electrical work.
- Unscrew the old grille, dust the duct with a vacuum cleaner fitted with a flexible hose attachment and check its diameter (100 or 125 mm).
- Mount the vent's base on the wall centring it on the duct, then screw it firmly checking the horizontality with a spirit level.
- Connect the vent to the network: insert the connection duct into the existing duct and seal the joints with aluminium tape.
- Connect the power supply and the dedicated switch, restore the power and test the flow rate by placing a sheet of paper under the vent.
Connecting the vent to the extraction network
The vent must be connected to the extraction network that carries the air towards the outside, generally a 80 or 100 mm PVC duct rising to the roof. If your grille opens onto a simple duct, check that it is not blocked by leaves or a nest: a blocked passage makes the ventilation useless and can be dangerous if the warm air meets an obstacle. In a house fitted with a unit, the vent can be connected to the MVHR riser provided it is compatible with the type of unit (self-regulating or humidity-sensitive).
Wiring the switch respecting the NF C 15-100 standard
The mechanical ventilation vent must be controlled by a dedicated wall switch, placed at the entrance to the room, or connected to the lighting circuit so it runs as soon as the light comes on. Use a 2 or 3 conductor cable of 1.5 mm², protected by a 10 A circuit breaker in a damp room. Strictly respect the safety zones: no switch, socket or junction box must be located in volumes 0, 1 and 2 of the bathroom.
Precautions and mistakes to avoid
- Choosing a vent with a diameter larger than the duct and forcing it in: you risk cracking the duct and reducing the flow rate.
- Forgetting the non-return flap: the cold air from the duct will enter the room when the ventilation is off.
- Connecting the ventilation to the lighting circuit without suitable protection: the cumulative load can trip the consumer unit.
- Wiring the switch in the bathroom safety zones, forbidden by the NF C 15-100 standard.
- Ignoring a blocked duct: the ventilation forces, heats up and no longer renews the room's air.
In half a day, you replace a passive grille with real active ventilation that freshens your bathroom on a lasting basis. The condensation disappears, mould takes hold less quickly and the room breathes, for a modest investment of around a hundred euros. If your electrical installation is old or doubtful, entrust the connection to an electrician: the rest of the job remains perfectly within your reach.