As London homes become better insulated, ventilation becomes more critical. Here is what mechanical ventilation with heat recovery involves and when to specify it.
One of the unintended consequences of improving the thermal performance of a period London property is that it becomes less able to breathe. Old draughty houses had poor insulation but excellent accidental ventilation — air infiltration through gaps around windows, under doors, and through floor voids continuously renewed the internal air. Insulate and draught-proof that house, and the ventilation fails.
Mechanical Ventilation with Heat Recovery (MVHR) is the correct whole-house ventilation solution for a well-insulated London renovation. Here is what it involves.
Why ventilation matters
Air quality in a poorly ventilated house degrades in ways that are directly harmful. Carbon dioxide from occupants' breath accumulates. Volatile organic compounds (VOCs) from materials, cleaning products, and furniture off-gas into the air. Moisture from cooking, showering, and respiration generates humidity that, without adequate ventilation, produces condensation on cold surfaces and black mould growth.
The UK Building Regulations (Part F) establish minimum ventilation requirements. The requirements are designed to ensure adequate fresh air and to remove moisture before it causes problems. In a standard house, these requirements are met by background ventilation (trickle vents in windows, airbricks) plus extract fans in bathrooms and kitchens. In a well-insulated, well-draught-proofed house, this approach is often inadequate.
What MVHR is
An MVHR system extracts stale air from wet rooms (bathrooms, kitchens, utility rooms) and supplies fresh air to habitable rooms (bedrooms, living rooms). At the core of the system is a heat exchanger: outgoing warm stale air passes through one side of the exchanger; incoming cold fresh air passes through the other. The two airstreams do not mix, but the outgoing air transfers its heat to the incoming air. Efficiency ratings of 85–95% are achievable with modern units — meaning 85–95% of the heat in the outgoing air is recovered.
The result is a house that is continuously ventilated with fresh, pre-warmed air, without the cold draughts and heat loss associated with opening windows or relying on extract fans.
When MVHR is the right choice
MVHR is most appropriate when:
- —The house is being significantly insulated and draught-proofed as part of the renovation
- —The house is new or will be taken to near-Passivhaus airtightness levels
- —The client has respiratory sensitivities or air quality concerns
- —The house is in an urban environment (London) where window ventilation brings in pollution and noise
MVHR is less appropriate in a period property that is not being significantly insulated — the system performs best in an airtight building and is less efficient if air leaks around the unit. In a draughty Victorian terrace being refurbished cosmetically, individual extract fans in wet rooms and trickle vents in windows may be more practical.
System components
Central unit: The MVHR unit itself — the heat exchanger plus fans. Typically located in a loft, utility room, or plant room. The unit must be accessible for filter maintenance (filters should be cleaned or replaced every 3–6 months). Size ranges from domestic units serving 2–3 bedrooms (approximately 200–300 m³/h) to larger units for substantial houses.
Ductwork: The most demanding element of the installation. Fresh air supply ducts run from the unit to each habitable room; extract ducts run from wet rooms back to the unit. In a renovation, ductwork must be concealed — typically in ceiling voids, running within floor constructions, or in a dedicated service void. The duct layout must be designed before any ceilings are fixed.
Duct sizing affects performance — undersized ducts increase resistance and reduce airflow. Rigid circular ducts have lower resistance than rectangular or flexible ducts. Flexible ductwork is easier to route in retrofits but has significantly higher resistance and should be minimised.
External connections: The unit requires an external air intake (fresh air in) and an external air exhaust (stale air out). These must be positioned to avoid recirculation — the intake and exhaust should be on different elevations or separated by sufficient distance. In a period London terrace, finding appropriate positions for external grilles without affecting the character of the building requires thought.
Commissioning: The system must be balanced — airflow at each supply and extract grille adjusted to the correct rate for the room served. An unbalanced system produces uneven ventilation and noise from high-velocity air at undersized grilles. Commissioning should be carried out by a qualified engineer and documented.
Heat recovery efficiency: what the numbers mean
An efficiency rating of 90% means that 90% of the heat in the outgoing air is transferred to the incoming air. On a winter day where the incoming air is 5°C and the internal temperature is 20°C, the temperature difference is 15°C. At 90% recovery, the incoming air enters the house at 5 + (0.9 × 15) = 18.5°C — almost at room temperature, rather than 5°C.
This prevents cold draughts and reduces heating demand. The energy saving depends on the volume of air being ventilated and the hours of operation, but for a well-insulated house, MVHR typically represents a 5–10% reduction in heating energy consumption.
Noise considerations
A poorly designed or poorly installed MVHR system is audible. Sources of noise:
- —The unit itself — vibration transmitted through inadequate anti-vibration mounts
- —Ductwork transmission — low-frequency noise from the fan transmitted along rigid ducts
- —Grille noise — air velocity too high at the grille, causing aerodynamic noise
The correct approach: the unit should be on anti-vibration mounts; attenuators should be installed in the duct runs close to the unit; grilles should be sized for the target airflow velocity (below 2 m/s is typically inaudible). In a bedroom or home cinema, MVHR grilles should be particularly carefully specified.
Integration with heating systems
MVHR can be combined with heating. Some units include a small electric heating element that supplements heat recovery on very cold days. More sophisticated systems integrate with a heat pump, using the ventilation airstream to distribute heat around the house.
For a high-specification renovation with a heat pump and MVHR, the combination of good insulation, heat recovery ventilation, and efficient heat distribution creates a building with very low running costs and excellent air quality — the performance standard that is increasingly expected in new-build but rarely achieved in period renovation.
Cost
A whole-house MVHR installation for a London townhouse (4–5 bedrooms):
| Scope | Installed cost |
|---|---|
| Basic system, standard ductwork, new build | £6,000–10,000 |
| Full retrofit in existing building, concealed ductwork | £12,000–20,000 |
| High-specification system with attenuators, balanced commissioning | £18,000–30,000 |
These costs include equipment, installation, and commissioning. They do not include making good to ceilings and walls after duct installation in a retrofit.
ASAAN's approach
ASAAN specifies and manages MVHR installations as part of renovation projects where thermal improvement is being undertaken. We coordinate ductwork routing with the structural and joinery programme and require commissioning documentation before sign-off.
If you are planning a renovation that includes significant insulation and draught-proofing, contact us to discuss ventilation strategy.
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