As homes become more airtight through better insulation and draught-proofing, ventilation becomes more important, not less. A very airtight building needs a controlled ventilation strategy, or moisture, CO2, and pollutants accumulate. Mechanical Ventilation with Heat Recovery (MVHR) is the most thermally efficient way to ventilate an airtight home: it continuously extracts stale air from wet rooms and supplies fresh air to living spaces, and recovers 80-95% of the heat from the outgoing air before it's expelled.
MVHR is most appropriate for new builds and major renovations where high levels of airtightness can be achieved. Installing it in a leaky old house makes little sense: the heat recovery benefit is negated by uncontrolled air infiltration through the building fabric.
How MVHR Works
An MVHR unit contains two airstreams: an extract duct network pulling warm, stale, moist air out of kitchens, bathrooms, and utility rooms; and a supply duct network pushing fresh, filtered air into bedrooms, living rooms, and other habitable spaces. The two airstreams pass through a heat exchanger inside the unit, where the outgoing warm air transfers its heat to the incoming cold fresh air without the airstreams mixing.
The result: the incoming air arrives at near-room temperature (typically within 1-3°C of indoor temperature), significantly reducing the heating energy needed to maintain comfort. On a cold winter day, instead of introducing fresh air at 2°C into a room at 20°C (an 18°C temperature difference to heat), MVHR introduces it at perhaps 17°C (only a 3°C difference).
Additional benefits include: continuously filtered incoming air (reducing allergens and particulates), consistent internal humidity control (reducing condensation and mould risk), and the ability to design out traditional bathroom extract fans and trickle vents in windows.
When MVHR Makes Sense
MVHR is most appropriate for:
- New builds designed to Passivhaus or near-Passivhaus airtightness standards
- Major extensions or renovations where the building envelope is being significantly improved and tested
- Loft conversions where the new space is insulated and airtight to modern standards
- Buildings with occupants who have respiratory conditions or allergies, where air quality is a priority
MVHR makes less sense for standard existing buildings with typical levels of air infiltration, where the unit would be running ducts through a building that also has uncontrolled air movement through gaps and cracks. In these cases, demand-controlled local extract fans in wet rooms (dMEV) or centralised extract systems (MEV) are simpler and cheaper solutions.
Design Requirements
MVHR only performs well if it's properly designed and installed. Poorly designed MVHR systems are common and frustrating: they're noisy, they don't achieve their claimed efficiency, and the ductwork routing is a permanent reminder of inadequate planning.
Key design requirements:
Ductwork layout. Ducts should be as short and straight as possible, with smooth bends rather than sharp right angles. Long, complex duct runs with multiple bends reduce airflow and efficiency. Rigid ducting is preferable to flexible duct, which collapses, kinks, and has higher resistance. The unit location (typically in a loft, utility room, or plant room) determines the duct run lengths.
Balanced flow rates. Supply and extract flows must be balanced. If extract exceeds supply, the building goes slightly negative in pressure, pulling in cold air through any available gap. If supply exceeds extract, moisture doesn't get removed effectively. Commissioning balances the flows after installation.
Airtightness first. MVHR is not a substitute for airtightness: it requires it. An MVHR installation should be accompanied by a target airtightness of no worse than 3m3/hr/m2 at 50Pa, and ideally better. Air pressure testing (blower door test) at completion verifies this.
Filter maintenance. MVHR units filter incoming air. Filters need replacing every 6-12 months depending on outdoor air quality and usage. Neglected filters reduce efficiency significantly and can become a source of microbial growth. Factor maintenance costs and access into the installation design.
Commission properly and verify performance. A commissioned MVHR system with measured flow rates and a tested airtightness result gives you confidence it's working as designed. A system that's never been balanced and never had an airtightness test is unlikely to be delivering its claimed efficiency.
Costs
MVHR system costs depend significantly on house size and ductwork complexity:
| House type | Typical installed cost (2025) |
|---|---|
| 2-3 bedroom new build | £3,500 - £6,000 |
| 4-5 bedroom new build | £5,500 - £9,000 |
| Retrofit to existing house (complex) | £8,000 - £15,000+ |
Running costs are low: MVHR units typically consume 50-100W, costing £80-£160 per year in electricity at current rates. Against the heating energy saved (which varies significantly by build quality and climate), the payback on MVHR in a new build is typically 5-10 years if properly designed.
Unit Selection
Reputable MVHR brands available in the UK include Zehnder, Genvex, Paul Novus, Vaillant recoVAIR, and STIEBEL ELTRON. The efficiency claims of MVHR units are tested to a European standard (Eurovent certification), so efficiency comparisons between manufacturers are meaningful. Look for units with heat recovery efficiency above 85%, low specific fan power (SFP below 0.45 Wh/m3), and a track record of filter availability in the UK market (filter maintenance is a long-term commitment).