Mechanical ventilation with heat recovery

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8.3.5Design considerations

MVHR systems shall ensure compatibility and satisfactory performance. Issues to be taken into account include:

  1. performance
  2. systemised approach
  3. type and position of air valves and terminals
  4. control of condensation
  5. protection from cold.


The MVHR system should be designed to provide satisfactory performance and be installed according to the design and manufacturer’s recommendations. Variations from the design should maintain the satisfactory performance of the system and be approved by the designer.

Issues that should be taken into account include:

  • ventilation rates as set out in appropriate building regulations and standards
  • fan capacity, accounting for airflow resistance of the system
  • ensuring the even distribution of airflow, taking into account airflow resistance, including from bends and fittings

Airflow resistance should be calculated using figures for air valves and terminals determined in accordance with BS EN 13141-2 and data supplied by the duct manufacturer. Ductwork should be as direct as possible to reduce the number of bends.

Allowance should be made for air transfer within the home. Where gaps between the underside of internal doors and the floor finish are used for air transfer, the guidance in Chapter 9.1 ‘A consistent approach to finishes’ should be considered.

Systemised approach

The MVHR system should be designed as a complete package, taking into account the performance of all components and materials, to ensure compatibility and the performance requirements of the system.

Particular consideration should be given where components from different manufacturers are specified on the same system.

Type and position of air valves and terminals

Air valves should be selected according to location and function, ensuring appropriate specification for:

  • wall or ceiling location
  • supply or extract function
  • the velocity of the system.

To create cross-ventilation within a room and to ensure satisfactory operation, air valves on low velocity systems should be

  • positioned on the opposite side of the room from internal door openings
  • a minimum of 200mm from walls, where located on a ceiling
  • a maximum of 400mm from the ceiling, where located on a wall
  • a minimum of 600mm (on plan) from hobs in kitchens
  • positioned to account for the likely location of tall furniture and to avoid draughts over beds and seating areas
  • lockable, where adjustable.

To prevent cross-contamination, intake terminals should generally be separated from exhaust terminals and other potential sources of pollution by a minimum of 1m measured on plan. Increased separation distances may be required between the intake and any:

  • soil and vent pipe terminal
  • boiler flue outlet
  • biomass or solid fuel chimney terminal.

Terminals should prevent the entry of birds and animals.

Control of condensation

Ductwork should be insulated to prevent condensation formation where:

  • it passes through spaces outside the insulated parts of the home, such as a roof void
  • carrying cold air through spaces that are within the insulated parts of the home.

This can be achieved by using suitable pre-insulated ductwork, or a proprietary insulation system with a thermal resistance equivalent to a minimum of 25mm of insulating material, with a thermal conductivity of 0.04W/Mk.

Ductwork insulation, including that used for proprietary duct insulation systems and pre-insulated ducts should be:

  • inert, durable and suitable for use with the ductwork system
  • continuous and vapour resistant
  • not adversely affected by moisture vapour
  • installed in a neat and workmanlike manner to ensure that there are no gaps
  • installed in accordance with the manufacturer’s recommendations.

Where a vapour control layer is incorporated, the joints should be sealed using appropriate tapes or sealants as recommended by the manufacturer.

Table 1: Ductwork insulation

Type of ductDuctwork continuously insulated: ductwork located inside the insulated part of the homeDuctwork continuously insulated: ductwork located outside the insulated part of the home
Service (supply and extract)NoYes⁽¹⁾


1 Additional insulation should be provided to protect the system from the cold.

Any condensate that forms within the fan unit or ductwork should be able to drain to a suitable outfall. Fan units should be located to enable connection of the condensate drain to the soil and waste system via a dry trap.

Protection from cold

MVHR systems should be protected from the effects of cold. Issues to be taken into account include:

  • performance in relation to indoor air quality
  • the manufacturer’s recommendations where any parts are located outside the insulated part of the home
  • insulation of ductwork and other system components.

To prevent damage to the components and ensure satisfactory operation, MVHR systems should be fitted with automatic frost protection.

Horizontal sections of service ductwork, outside the insulated parts of the home, should be insulated to achieve a thermal resistance equivalent to at least 150mm of insulating material with a thermal conductivity of 0.04W/Mk. This may be achieved by installing the ductwork between the layers of horizontal insulation.

Condensate drains located outside the insulated part of the home should be insulated to prevent freezing.