Glossary
MVHR: heat recovery ventilation for airtight London homes
A whole-house ventilation system recovering heat from extracted air. Common in low-energy designs.
A whole-house ventilation system recovering heat from extracted air. Common in low-energy designs.
MVHR is a whole-house ventilation system that continuously extracts stale air from wet rooms, supplies fresh air to habitable rooms, and passes one stream through a heat exchanger to recover typically 85 to 92% of the heat from the other. It is System 4 in Approved Document F.
Below about 3 m³/(h·m²) air permeability, natural background ventilation can no longer be relied on and continuous mechanical ventilation becomes the sensible system. Above roughly 5 m³/(h·m²), MVHR recovers heat from air that is leaking out elsewhere, and the payback collapses.
In a retrofit the practical obstacle is ductwork. A full MVHR system needs rigid or semi-rigid ducts to every room, which is straightforward in a new-build or a deep retrofit and difficult in a finished Victorian terrace.
System 4 must be commissioned and the flow rates measured and recorded. Whole-dwelling ventilation rates are set by occupancy and floor area, with minimum extract of 13 l/s from a bathroom, 8 l/s from a WC and 13 l/s from a kitchen under continuous operation, with boost rates above that.
Ducts must be insulated where they pass through unheated space — otherwise condensation forms inside the duct in the loft, which is the most common MVHR defect found in practice.
System 1 is background ventilators plus intermittent extract, which is what most extensions use. System 3 is continuous mechanical extract, cheaper to install than MVHR and often the right compromise in a retrofit where supply ducting is impossible.
A worked Bromley example of mechanical ventilation with heat recovery (mvhr): with 48 conservation areas — the Bromley Town Conservation Area and the Chislehurst Conservation Area among the best known — London Borough of Bromley (Bromley Council) sees mechanical ventilation with heat recovery (mvhr) constantly, and there is no borough-wide Article 4 direction, so permitted development generally survives outside conservation areas. Where mechanical ventilation with heat recovery (mvhr) pushes a Bromley scheme out of permitted development, the route becomes a £548 application decided in 8 weeks rather than a £249 prior approval in 6 weeks.
| Heat recovery efficiency | 85–92% typical |
|---|---|
| Approved Document F system | System 4 |
| Sensible below | 3 m³/(h·m²) air permeability |
| Kitchen continuous extract | 13 l/s |
| Bathroom continuous extract | 8 l/s |
| Commissioning | Mandatory, with recorded flow rates |
Only after airtightness work. In a leaky house continuous extract usually makes more sense.
It satisfies the regulations, but purge ventilation via openable windows is still required.
Usually the loft or a utility cupboard, with insulated ducts through unheated space.
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