Last edited 05 Nov 2020

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, of the building regulations ‘Means of ventilation’ describes mechanical ventilation with heat recovery as, ‘continuous mechanical supply and extract with heat recovery’. It is commonly used in housing to extract stale air and supply fresh air whilst recovering up to 95% of heat that would otherwise have been lost from the extract air and using it to pre-heat the supply air.

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.