MVHR Systems: Mechanical Ventilation with Heat Recovery — Design, Installation, and Part F Compliance

What Is MVHR and Why Does It Matter?

Mechanical Ventilation with Heat Recovery (MVHR) extracts stale, moisture-laden air from wet rooms and kitchens while simultaneously supplying fresh filtered air to living spaces. A heat exchanger core transfers typically 85–95% of the thermal energy from the outgoing air to the incoming stream, dramatically reducing the ventilation heat loss that accounts for 20–40% of a well-insulated home's total energy demand.

Since the 2022 amendments to Approved Document F, MVHR has moved from a niche specification into mainstream residential and commercial new-build work. Part F 2022 raised minimum whole-dwelling ventilation rates and tightened testing requirements, making MVHR the most straightforward route to compliance in airtight dwellings where background ventilators cannot provide sufficient air changes without unacceptable draughts.

When to Specify MVHR

MVHR is most cost-effective in dwellings with an air permeability below approximately 5 m³/(h·m²) at 50 Pa. In very airtight properties (≤3 m³/(h·m²)), natural and intermittent mechanical extract ventilation strategies struggle to meet Part F minimum rates without causing discomfort, making whole-dwelling MVHR the default choice.

Key indicators that MVHR should be specified:

• New-build dwellings targeting Passivhaus or near-zero energy performance
• Deep retrofit projects where walls, roof, and floor have been insulated and air-sealed
• Dwellings where occupants suffer from allergies — MVHR incoming filters (typically G4/F7) remove pollen and particulates
• Buildings in high-pollution urban areas where opening windows is undesirable

System Components

MVHR Unit

The central unit contains two counter-flow or cross-flow air streams separated by a heat exchanger core. Better units use enthalpy (hygroscopic) cores that also transfer moisture, reducing internal humidity swings. Key specifications to check:

• Specific Fan Power (SFP): Part F 2022 requires SFP ≤0.45 W/(l/s) at design flow rate for residential systems. This is the power consumed per litre per second of airflow.
• Heat recovery efficiency: Minimum 73% dry heat recovery efficiency under EN 13141-7 test conditions for Passivhaus compliance; most quality units exceed 85%.
• Noise: Units installed inside the thermal envelope should not exceed 25–30 dBLw at design flow. External loft-mounted units are noisier but thermally decoupled.
• Filter class: G4 on extract side, F7 (ISO ePM1 ≥50%) on supply side is the current Passivhaus recommendation.

Ductwork

Semi-rigid insulated duct (typically 75 mm or 90 mm spigots expanding to 125 mm or 160 mm main runs) is the preferred system for residential work. Rigid galvanised or PVC circular duct suits commercial applications. Key rules:

• Duct velocity: 2–3 m/s in distribution branches, ≤4 m/s in main spines to control noise and resistance
• Maximum duct temperature on supply side in winter: keep duct runs inside the thermal envelope wherever possible to prevent condensation and recover heat
• Bends and fittings add equivalent length: a 90° elbow = approximately 1.5–2 m straight duct for resistance purposes

Detailed duct sizing guidance is in our separate guide to Ventilation Ducting.

Extract Valves

Circular or square extract valves are fitted in kitchen (above hob area), bathroom, en-suite, utility room, and WC ceilings. For bathrooms and shower rooms, fit humidity-sensitive valves that automatically boost extract rate when relative humidity exceeds 70–75%. See our guide to Bathroom Extractor Fans for valve types.

Supply Diffusers

Supply diffusers in bedrooms and living spaces should be adjustable, low-velocity, and positioned to avoid draughts on occupants. Ceiling-mounted diffusers with 360° throw pattern are preferred; avoid pointing supply jets directly at seating areas.

Outdoor Air and Exhaust Terminals

Both fresh air intake and exhaust discharge penetrate the external envelope. Separation of at least 1 metre (ideally 2 metres) between intake and exhaust is required to prevent short-circuit recirculation. Fit bird-proof grilles with insect mesh. On extract terminals, a weather cap with backflow preventer stops wind-driven reverse flow.

Part F 2022 Design Requirements

Approved Document F Volume 1 (dwellings) sets whole-dwelling ventilation rates as a minimum. For a four-bedroom detached house, the design flow rate is calculated from:

• Total extract from wet rooms: kitchen 13 l/s continuous, each bathroom/shower room 8 l/s, WC 6 l/s, utility 8 l/s
• Supply to habitable rooms: calculated from floor area — at least 0.3 l/s/m² of habitable room area
• The higher of the two calculations sets the system design flow rate

Part F also mandates commissioning to SAP Q airflow testing protocol with results submitted to Building Control. From 2022, commissioning certificates from an appropriately qualified person (CIBSE Commissioning Code A or equivalent) are required for all new-build MVHR installations.

Electrical Connection

MVHR units are low-wattage appliances (typically 50–200 W) but must be on a switched, fused spur or dedicated circuit:

• Single-phase 230 V supply via 6 A MCB or fused connection unit (FCU) — see our guide to FCUs and Switched Fused Spurs
• Run from a circuit that is not on a time switch — MVHR must operate continuously or at minimum ventilation rate at all times
• Cable route must not breach the airtightness layer — use appropriate airtight grommet or sleeve where cables pass through membranes
• Many units include a summer bypass mode (automatically bypasses heat exchanger in hot weather for free-cooling) — no additional wiring required, thermostatically controlled internally

For wiring and cable selection, refer to our UK Electrical Cable Types guide and our guide to consumer unit MCB/RCBO selection.

Installation Best Practice

Location of the MVHR Unit

The unit should be inside the thermal and airtightness envelope to prevent frost damage to the heat exchanger and duct condensation on supply runs. Insulated loft plant rooms, utility cupboards, and airing cupboards are common locations. Avoid uninsulated cold lofts unless the unit has frost protection rated to −15°C.

Condensate Drainage

Heat exchangers produce condensate from the warm extract stream, especially in winter. Every MVHR unit requires a 22 mm or 32 mm condensate drain, trapped and connected to the foul drainage system or run to an external gully. A dry trap risks drain gas entering the unit.

Airtightness

MVHR only performs as designed in a reasonably airtight building. All duct penetrations through external walls or the airtight membrane must be sealed with airtight tape, foam, or proprietary collars. Sealing failure allows cold air infiltration that defeats the purpose of heat recovery.

Filter Maintenance Access

Filters require inspection every 3–6 months and replacement typically annually. Ensure the unit is accessible: minimum 600 mm clear in front of the filter access panel. Communicate maintenance requirements to the end user or building manager at handover.

Commissioning and Testing

Post-installation commissioning must include:

1. Airflow measurement at every supply diffuser and extract valve using an anemometer or balancing hood — record against design rates
2. Balancing — adjust valve settings until each terminal is within ±10% of design flow and total system flow matches MVHR unit setting
3. Electrical checks — voltage, earth continuity, insulation resistance on supply cable
4. Noise check at occupied positions — should not exceed 25 dBLw(A) in bedrooms with unit at design speed
5. SAP Q documentation — complete commissioning certificate with measured flows, unit model, and SFP calculation

Common Faults and Remedies

Summary: Key Specification Points

MVHR is now standard practice on airtight new-build and deep retrofit projects. Specify units with SFP ≤0.45 W/(l/s), heat recovery efficiency ≥85%, and F7 supply filters. Size ductwork for 2–3 m/s velocity, keep runs inside the thermal envelope, and provide condensate drainage from day one. Commission to Part F 2022 SAP Q protocol and hand over maintenance documentation to the client. A correctly designed and commissioned MVHR system delivers comfortable, low-energy ventilation for the life of the building with minimal running cost.