MVHR stands for Mechanical Ventilation with Heat Recovery. It is a whole-house ventilation system that uses a central unit containing a heat exchanger to recover heat from stale outgoing air before it is expelled from the building, and uses that recovered heat to warm fresh incoming air. The system continuously extracts from wet rooms (kitchen, bathrooms, en-suites, utility room) and continuously supplies fresh air to habitable rooms (bedrooms, sitting room, dining room). A bypass mode allows the system to vent heat in summer without recovery. Heat recovery efficiency is typically 70-95%, meaning that in winter almost all the heat in the exhaust air is recovered rather than lost.

When is MVHR required?

MVHR is not mandatory under the Building Regulations for all new dwellings, but it is the natural ventilation strategy for highly airtight buildings (air permeability below around 3 m3/h.m2 @ 50 Pa) because at that level of airtightness, background (trickle) ventilation through window frames and air bricks becomes insufficient to meet the ventilation rates required by Approved Document F. It is always specified in Passivhaus projects and is increasingly common in high-performance new builds designed to Part L 2021 standards. The SAP calculation takes MVHR into account - a system with a high heat recovery efficiency and low specific fan power (SFP) improves the SAP score.

What is specific fan power (SFP) for MVHR?

Specific fan power (SFP) is the electrical power consumed by the fans divided by the volume flow rate of air moved, expressed in W/(l/s) or W/(m3/h). It is a measure of how efficiently the fans convert electricity into air movement. Lower SFP values mean more efficient fans. Approved Document F requires an SFP not exceeding 2.0 W/(l/s) for continuous mechanical supply and extract systems in new dwellings. Good MVHR systems achieve SFP of 0.5-1.0 W/(l/s). High SFP is often caused by undersized ductwork creating excessive resistance - careful duct sizing and layout is critical to achieving a low SFP.

What maintenance does an MVHR system require?

MVHR systems require regular filter changes (typically every 6-12 months depending on filter grade and local air quality), annual cleaning of the heat exchanger core, and periodic checking and balancing of the air flow rates through each supply and extract valve. Filters are typically G4 or F7 grade and must be changed to maintain air quality and prevent the unit from working harder than necessary (increased SFP). If filters are neglected, the heat exchanger can become contaminated with dust and grease (especially the kitchen extract path), reducing efficiency and potentially becoming a hygiene issue. The summer bypass function should be checked annually.

Thermal & Energy noun / abbreviation

MVHR

/ ɛm viː eɪtʃ ɑː /

Also known as: mechanical ventilation with heat recovery, heat recovery ventilation, HRV, whole-house ventilation

Definition

MVHR (Mechanical Ventilation with Heat Recovery) is a whole-house ventilation system that continuously extracts stale air from wet rooms and supplies fresh air to habitable rooms via a central unit containing a heat exchanger. The exchanger transfers 70-95% of the heat from outgoing stale air to incoming fresh air, dramatically reducing ventilation heat loss. MVHR is essential in highly airtight buildings (below ~3 m3/h.m2 @ 50 Pa) where background ventilation through trickle vents is insufficient. Governed by Approved Document F (ventilation) and assessed in the SAP energy calculation under Part L.

In practice

An MVHR system consists of the central unit (containing the heat exchanger, fans, and filters), a network of supply ducts to habitable rooms and extract ducts from wet rooms, and terminal valves (supply diffusers and extract grilles) in each room. Duct layout is critical - ducts should be as short and straight as possible, with gentle bends, to minimise resistance and achieve a low specific fan power (SFP). Rigid circular ducts (typically 75mm or 125mm diameter) are preferred over flexible ducting; flexible ducting can be compressed at bends, dramatically increasing resistance.

Commissioning is essential - all supply and extract valves must be balanced to achieve the design airflow rates (typically 25-35 l/s total for a 4-bedroom house). An unbalanced system either over-ventilates (wasting energy) or under-ventilates (causing humidity and air quality problems). The system must be commissioned by a competent person and results recorded. Filters must be labelled with the change date and accessible to the occupant. A common installation error is running the kitchen extract through the MVHR unit without a grease filter or bypass - kitchen grease contaminates the heat exchanger and voids the warranty.

Building Regulations

Ventilation standardApproved Document F - continuous whole-house ventilation (System 4)

Max SFP2.0 W/(l/s) for continuous supply and extract systems

CommissioningMust be commissioned and results submitted to Building Control

SAP inputHeat recovery efficiency and SFP feed into the Part L SAP calculation

Approved Document F (Ventilation) classifies MVHR as a System 4 (continuous mechanical supply and extract with heat recovery) ventilation strategy. It requires minimum flow rates to each room type, maximum SFP of 2.0 W/(l/s), commissioning by a competent person, and provision of user instructions. The heat recovery efficiency and SFP of the specific MVHR unit must be entered into the SAP calculation under Approved Document L - a high-efficiency unit with low SFP significantly improves the SAP score. The commissioning certificate and air permeability test result must both be submitted to Building Control before a completion certificate is issued.

Full Building Regulations guidance

MVHR

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