MVHR for London Homes: Mechanical Ventilation with Heat Recovery Explained

An MVHR system has four components: the central heat recovery unit (the heart of the system, containing the heat exchanger, two fans — one extract, one supply — and the filter set); the duct network (supply ducts running to habitable rooms, extract ducts from wet rooms); the external connections (fresh air intake grille and exhaust grille — these are typically on an external wall or roof, positioned to avoid short-circuiting where exhaust air is immediately drawn back into the intake); and the room terminals (supply diffusers in bedrooms and living rooms; extract grilles in kitchens, bathrooms, and utility rooms). In operation: Room air is continuously extracted from the kitchen, bathrooms, and utility rooms via the extract duct network. This warm, humid, stale air passes through the heat exchanger in the central unit. Fresh outside air is simultaneously drawn through the supply side of the heat exchanger. In the heat exchanger, heat transfers from the warm outgoing air to the cool incoming air — the two airstreams do not mix (the air crosses but doesn't share a common path). The pre-warmed fresh air is then supplied via the supply duct network to the bedrooms and living rooms. The cooled exhaust air is discharged to the outside. The heat recovery efficiency of a good MVHR unit is 80-90% — meaning that 80-90% of the heat that would otherwise be lost with the extracted air is recovered and transferred back into the supply air. Modern MVHR units also include bypass mode: in summer, when the outside air is warmer than the extract air, the heat exchanger is bypassed and the incoming air is not pre-warmed (which would overheat the building).

MVHR is most cost-effective and most beneficial in dwellings that meet certain conditions. Highly airtight buildings: For MVHR to recover significant heat, the building must be substantially airtight — otherwise, uncontrolled air infiltration through gaps in the building fabric provides more ventilation than the MVHR ducts, and the heat recovery benefit is diluted. An airtightness test result (air permeability) of ≤3 m³/(h.m²) at 50Pa is typically considered the threshold below which MVHR provides a clear benefit over simpler ventilation strategies. New extensions can be built to this standard, but existing Victorian London terraces typically have air permeability of 10-20 m³/(h.m²) at 50Pa without significant remedial airtightness work. Highly insulated buildings: Where a building has been upgraded to very low heat loss (triple-glazed windows, external wall insulation, high-performance roof insulation), the ventilation heat loss becomes a higher proportion of the total heat demand — recovering ventilation heat becomes more valuable. Renovation projects involving extensive airtightness improvement: Where a deep retrofit is being planned (upgrading insulation, replacing windows, installing airtight membranes), adding MVHR at the same time is much more cost-effective than retrofitting it later. Buildings with poor natural ventilation: Where the house layout or window positions do not allow effective cross-ventilation (common in deep narrow London terraces), MVHR provides more reliable ventilation than relying on openable windows. Buildings with air quality concerns: MVHR with HEPA filters removes a significant proportion of pollen, particulates, and urban pollutants from the supply air — a benefit in London's air quality environment, particularly for residents with asthma or allergies.

Building Regulations Part F (Ventilation) requires that dwellings have adequate means of ventilation to maintain indoor air quality. For new dwellings and extensions, the specific requirements depend on the airtightness of the building: Where the dwelling has an air permeability greater than 5 m³/(h.m²) at 50Pa (the typical situation for an existing Victorian house before airtightness improvement work), natural ventilation (background trickle ventilators in windows, intermittent extract fans in wet rooms) is generally compliant. Where the air permeability is ≤5 m³/(h.m²) at 50Pa (achievable in a well-built new extension or airtightly renovated property), natural ventilation is likely to be insufficient and mechanical ventilation will be required by the regulations. In practice for extensions: a standard single-storey rear extension to an existing Victorian house is unlikely to require MVHR — the overall dwelling permeability remains relatively high due to the existing house. A whole-house deep retrofit that achieves very low air permeability throughout will require MVHR to comply with Part F. Airtightness testing: Where Part L (Energy Efficiency) or Part F compliance depends on the air permeability of the dwelling, an airtightness test (blower door test) is required to demonstrate compliance. The test is carried out by an accredited testing organisation and is typically required at completion of new dwellings (SAP calculation), and for new extensions where the airtightness target is relevant to the energy performance calculation.

Installing MVHR in a new extension is much simpler than retrofitting it into an existing house, because the duct runs can be designed into the construction programme rather than being threaded through completed ceilings and walls. In a new single-storey extension, the MVHR unit is typically located in the eaves, loft, or utility space, with supply ducts feeding the main living space and extract ducts from the kitchen and any adjacent bathroom. Typical installation costs for a whole-house MVHR system in a London home: 2-bedroom flat or small house (simple layout, 60-80m² floor area): £3,500-£6,000 all-in for supply, installation, and commissioning. 3-4 bedroom terraced house (more complex duct runs, existing ceilings and floors): £6,000-£12,000. Larger properties or complex retrofits: £10,000-£20,000+. Commissioning (balancing the airflow rates to the designed values in every room) is an essential final step and should be included in any MVHR installation contract. The commissioning engineer measures the actual airflow at every terminal and adjusts the system until all flows match the design. Without commissioning, rooms may be significantly over- or under-ventilated. Running costs: MVHR fans run continuously at low speed (typically 30-70 watts total fan power for a whole-house system). Annual electricity cost at continuous operation: approximately £50-£120/year depending on the unit and tariff. Filter replacement: F7 HEPA filters typically cost £40-£80 per set and should be replaced every 6-12 months.