Mechanical ventilation with heat recovery MVHR

Ventilation is necessary in buildings to remove ‘stale’ air and replace it with ‘fresh’ air:

• Helping to moderate internal temperatures.
• Replenishing oxygen.
• Reducing the accumulation of moisture, odours, bacteria, dust, carbon dioxide, smoke and other contaminants that can build up during occupied periods.
• Creating air movement which improves the comfort of occupants.

Very broadly, ventilation in buildings can be classified as ‘natural’ or ‘mechanical’.

• Mechanical (or ‘forced’) ventilation tends to be driven by fans.
• Natural ventilation is driven by ‘natural’ pressure differences from one part of the building to another.

Traditionally, many buildings were ventilated largely by natural ventilation or 'infiltration', with fresh air entering spaces when doors or windows were opened, or through gaps in the fabric of the building. However, as the building regulations have become increasingly strict, and build quality has improved, buildings have become more air-tight and ventilation is increasingly provided mechanically.

To reduce the heat lost to the outside as a result of this mechanical ventilation, mechanical ventilation with heat recovery (MVHR) can be used.

Approved document F: Volume 1: Dwellings, 2021 edition, defines mechanical ventilation with heat recovery as: 'A mechanically driven ventilation system that both continuously supplies outdoor air to the inside of the dwelling and continuously extracts indoor air and discharges it to the outside. For the purposes of this approved document, the guidance for mechanical ventilation with heat recovery applies to centralised or decentralised supply and extract systems, with or without heat recovery.’

Whole-house, or multi-room MVHR involves installing ductwork to each room, typically in ceiling voids, and running these ducts to a heat recovery unit, often located in the loft space. Air is continually extracted from rooms that might be warm and humid, such as bathrooms and kitchens. This is passed through a heat exchanger in the heat recovery unit where ‘waste’ heat is recovered from the air before it is rejected to the outside. The heat exchanger uses the recovered heat to ‘temper’ incoming fresh air that is used to supply the non-humid ‘habitable’ rooms such as bedrooms and living rooms. Tempering the incoming air reduces the need for heating, and so reduces energy use.

The standard, background level of ventilation can be boosted manually or automatically when bathrooms or kitchens are in use. The heat exchanger can be by-passed in warmer conditions so that the fresh supply air remains cool. Typically filters will be included in the system to remove potential pollutants such as pollen and dust form the supply air.

Care must be taken when installing MHVR to ensure that ducts do not provide a flanking route for noise to transmit between spaces. It is also important to properly size and locate the heat recovery unit and ductwork so that any sound generated by the system itself is minimal.

MVHR has become popular in new homes, but installation in existing properties can be disruptive due to the routing of ducts throughout the property.

Regular maintenance is required to ensure that fans are operating correctly and filters remain clean.

[edit] Optimised ductwork design

MVHR ductwork should be arranged so the connection to the world is as short as possible. Insulate both ducts that connect the MVHR to the world. This is intended to prevent condensation on the outside of the incoming pipe and the inside of the outgoing pipe. Have all the duct runs between rooms and MVHR within the thermal envelope of the house. This means they need no insulating. The air that they're carrying is as warm as the house air, essentially so if they run through a cold space they will cool. This is either your warmed fresh air that is cooling, or it's your heat laden stale air. Loss of heat from either represents a loss of heat from the warm envelope.

NB Domestic Ventilation Compliance Guide, 2010 Edition (with 2011 amendments), published by the Department for Communities and Local Government in July 2011, defines continuous mechanical balanced ventilation with heat recovery (MVHR) as: ‘… a ventilation system that comprises central ducted supply and extract fans, air being supplier into the habitable rooms via a heat recovery unit.’

[edit] Related articles on Designing Buildings

• Air conditioning.
• Air infiltration testing.
• Building services.
• Condensation.
• Cross ventilation.
• Displacement ventilation.
• Domestic ventilation systems performance.
• Exhaust air heat pump.
• Flanking sound.
• Heat recovery.
• Humidity.
• HVAC.
• Interstitial condensation.
• Mechanical ventilation.
• Natural ventilation.
• Reasons for using MVHR systems.
• Stack effect.
• Thermal comfort.
• Ventilation.