MVHR installation
Plain English. Practical detail. UK context.
MVHR means mechanical ventilation with heat recovery. It extracts stale air and supplies filtered outdoor air through a heat exchanger that recovers some heat from the outgoing airstream. It is a ventilation system, not an air conditioner, and it should not be confused with a simple bathroom extractor fan.
Last reviewed: (editorial review against the public sources listed below).
Where it works best
A designer should consider the building’s airtightness, room layout, occupancy, moisture production and available duct routes. MVHR can be particularly useful in appropriately airtight buildings, but a unit cannot compensate for every uncontrolled air leak. It needs supply and extract paths, transfer routes between rooms, correctly positioned external terminals and access for maintenance. The best location is not simply the smallest unused cupboard. Noise, vibration, condensate drainage and protection of ducts in cold spaces affect successful operation.
What installation includes
Duct size, bends, fittings and resistance influence airflow and fan energy. Crushing flexible duct into a tight void can undermine the design even when the unit is capable. Penetrations through fire-resisting elements need suitable detailing. Commissioning involves measuring and adjusting airflow at terminals, not just switching the unit on. Ask for the design, commissioning results and an explanation of normal, boost and summer-bypass operation. Room labels and accessible controls help occupants use it properly.
Common mistakes
Turning the system off for long periods because it is noisy can allow moisture and air quality problems to develop. The cause may be installation, commissioning or maintenance rather than an unavoidable feature. Filters need inspection and replacement as specified; blocked filters change performance. MVHR does not mean windows can never be opened, but its normal operation should not depend on ad hoc window opening to correct a deficient installation. Cooker extraction requires a considered arrangement, not an assumed connection into the heat recovery unit.
Alternatives and applicable guidance
Mechanical extract ventilation, decentralised ventilation or other compliant strategies may suit particular refurbishments. England’s Approved Document F provides ventilation guidance, with different editions and transitional arrangements depending on the work; other UK nations use their own requirements. Agree which standards apply to the project. Handover should include filter access, cleaning, maintenance, control settings and what to do during an alarm or fault. Keep terminal settings recorded so later redecorating or cleaning does not erase the commissioned arrangement.
Frequently asked questions
Does MVHR cool the house like air conditioning?
Not normally. Summer bypass avoids some unwanted heat recovery, but is not equivalent to active cooling.
Can I fit a unit without commissioning?
Measured balancing and commissioning are essential to establish the intended airflows.
Should it run continuously?
Follow the system design and user instructions. Most whole-house systems are intended for ongoing operation with suitable boost control.
Sources and further reading
Public sources and further reading. Sources checked: 8 November 2026. Consult the original for details and later changes.