Air inlets and extract vents: a properly sized MVHR system for effective ventilation

Understanding the role of air inlets and extract vents in ventilation

Balancing fresh and extracted air: the principle behind a working MVHR system

Effective ventilation is a circuit. Fresh air comes in through the air inlets, stale air goes out through the extract vents. If either is missing, the MVHR system strains, makes noise, and pulls air from wherever it can. The goal is a continuous air path between rooms, with undercuts beneath doors.

Knowing where to place air inlets (on windows) and vents (in wet rooms)

Air inlets go on the windows of living rooms, such as the lounge and bedrooms. Vents go in the wet rooms, kitchen, bathroom, WC, utility room, generally high on the wall. On site, also check that nothing blocks the grilles, paint, cladding, furniture.

Spotting the signs of poor ventilation on site (condensation, odours, mould)

Warning signs come quickly. Frequent condensation on glazing, odours that linger after a shower or cooking, mould in corners or behind furniture. At this point, check airflow rates, duct condition, and whether the air inlets are actually open. These are your early warning signs before real damage sets in. To go further on technical points that often get overlooked, see also duct condition.

Sizing: how to calculate MVHR airflow rates without getting it wrong

Determining regulatory airflow rates based on the dwelling and its rooms

Start with the minimums from the decree of 24 March 1982. They depend on the type of dwelling and the number of main rooms. You add up the extract rates of the service rooms. In practice, the kitchen often carries the peak rate. A solid baseline avoids ventilation that is either too weak or needlessly noisy.

Accounting for pressure drop: ductwork, lengths, elbows and fittings

Next, translate the plan into metres of ducting and equivalent lengths for every elbow, tee, vent, damper, silencer and filter. The sum gives the pressure to overcome (Pa). Aim for a short network with few changes of direction. Keep a simple margin for fouling.

Choosing the right MVHR unit: single-flow, humidity-controlled, heat-recovery

Select the unit based on its performance curve. It must deliver the total airflow at the calculated pressure level. In single-flow, the humidity-controlled version modulates but must still respect the minimums. In heat-recovery systems, also size the supply air and allow for losses through the heat exchanger and filters to keep ventilation stable (see also single-flow, the humidity-controlled option).

Air inlets: choosing and installing them correctly to avoid client complaints

Selecting the right type of air inlet: self-regulating, humidity-controlled, acoustic

An air inlet feeds the ventilation system with fresh air. Choose it based on the system. The self-regulating type keeps a near-constant flow rate. The humidity-controlled type modulates according to humidity, useful in a dwelling with variable occupancy. The acoustic type adds attenuation where the dwelling faces a noisy street.

Limiting noise, draughts and fouling: good installation practice

Follow the manufacturer's instructions and check MVHR compatibility. Install it high on the window, with a continuous seal and fixings that don't twist the frame. Take care with airtightness on the interior side, and keep the exterior passage clear. Avoid paint and site dust getting into the damper. Warn the client. Regular dusting limits whistling and blockages.

Handling special cases: external/internal wall insulation, new windows, sensitive rooms

With external or internal wall insulation, use a suitable sleeve and maintain insulation continuity to avoid thermal bridges. With new windows, plan for the recesses and compatible accessories ahead of time. In bedrooms or sensitive rooms, favour acoustic models and avoid blowing air directly onto a bed or sofa.

Extract vents: placement, settings and airflow checks

Placing vents correctly in the kitchen, bathroom, WC and utility room

Good ventilation starts with logical placement. In the kitchen, position the vent high, ideally on the ceiling or high on the wall, near the cooking area without putting it directly in the path of steam or grease. In the bathroom, aim for the wet zone, up high, while keeping the vent accessible for cleaning. In the WC, favour a high position, on the toilet side. In the utility room, place it near the sources of humidity (tumble dryer, drying rack).

Making the adjustments: calibration, balancing, maintenance markers

Set each vent to its target airflow (calibration), then check that the whole network stays consistent (balancing). Work within the manufacturer's settings, without improvising. Note the setting positions and add a simple marker (label, photo, date) to make maintenance easier and prevent drift after cleaning.

Checking on site: airflow measurements, smoke tests, simple traceable checks

Check on site with a flow-measuring cone or a suitable anemometer. Complement this with a smoke pencil or paper test to confirm flow direction and spot a fouled vent or an airtightness defect. Keep a record. A dated and signed table of per-room measurements secures the handover and future inspections. To go further, see airflow measurements and the right tools for the job.

In 2026, secure your ventilation jobs with a clear handover method

Documenting sizing and installation: plans, photos, product datasheets

For ventilation that stands up to time and inspections, keep a record of the sizing. Airflow calculation notes, a layout plan of the ductwork, and the location of vents and air inlets. Add the product datasheets (unit, vents, ducts, fittings) and the setting instructions. A few dated photos make a difference, taken before cladding is closed up and in the loft.

Coordinating ventilation with airtightness and insulation: avoiding classic mistakes

Ventilation cannot be handled in isolation. Plan ahead for penetrations through membranes and vapour barriers. Take care with sleeves and adhesive tapes, or you create leaks and unwanted return airflow. Also think about insulating ducts in unheated spaces. Skip that, and you're inviting condensation and losses. Aim for clean, accessible junctions.

Setting up a handover checklist and a maintenance guide for the client

At handover, validate everything in writing. Airflow rates measured at the vents, settings, noise, direction of operation, filter access, condensate drainage if applicable. Hand over a signed checklist, the installation diagram, and a simple maintenance guide. For example, cleaning the vents and replacing filters according to the manual. The client breathes easier, and so do you.