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Ventilation of Dwellings - Selection and Sizing

When installing an MVHR system, it is important to choose a system which is SAP Appendix Q listed. This means it has been tested by an independent organisation and has official efficiency figures which can be used as part of a BER assessment.

MVHR System Operation

MVHR systems are designed to run continuously. As a result the design of the unit is very important.

Some units use backward-curve fan technology. This offers slightly lower power consumption but means that they cannot offer constant extract pressure. Other units use forward-curve fan technology. These units can offer constant extract pressure and a more comfortable experience for the end user.

The controls supplied with the unit must be suitable for continuous operation and should provide indication to the occupant that the system is operating correctly. Controls should also indicate if a fault has occurred on system or if maintenance or filter cleaning is needed.

Sizing of MVHR systems

An MVHR system should be capable of providing adequate general ventilation at all times and of meeting requirements for extract ventilation that may need to be met from time to time. The sizing is a three step process:

1. Calculate the general ventilation rate

Part F of the Building Regulations requires that the general ventilation rate should be the greater of:

a) 5 l/s plus 4 l/s per person, e.g. 25 l/s for a five person dwelling


b) 0.3 l/s per m2 internal floor area, e.g. 30 l/s for a 100 m2 dwelling.

It is likely that the requirement will be determined by method (b) in most cases.

2. Apply the adjustment for air infiltration

The adjustment for air infiltration must be deducted from the general ventilation rate. This deduction is calculated as follows:

0.04 x V (l/s), for dwellings of more than 1 storey


0.06 x V (l/s), for single storey dwellings

V is the gross internal volume of the dwelling in m3.

This figure is subtracted from the calculated general ventilation rate to give the minimum capacity of the MVHR system.

3. Check the extract rate

In order to meet extract requirements, the system may require a higher extract or boost capacity depending on the number of wet rooms (kitchens, bathrooms, etc.). The extract rate to be provided for each wet room is specified in the table below.

MVHR Systems: Minimum extract rates for Wet Rooms
Wet Room Minimum extract rate (l/s)
Kitchen 13
Utility Room 8
Bathroom 8
Toilet (no bath or shower) 6

The required overall extract rate is calculated by adding together the relevant individual extract rates specified in the Table. If the result proves higher than the calculated minimum capacity, the system should be capable of meeting the higher extract rate or boost capacity.

Air Distribution

An air supply should be provided to each habitable room.

Extract will be required from all wet rooms. The system should be capable of an extract rate from each wet room at least equal to that specified in the Table above.

Transfer of air and cross ventilation between rooms should be possible. This will normally be provided by a 10mm gap under doors

Purge Ventilation

A facility for purge ventilation should be provided in all wet rooms.

This can be provided in one of two ways:

  1. Including an opening window or external door. The area of the opening part should be at least 5% of the floor area of the room.
  2. Providing purge ventilation using the heat recovery ventilation. In the case of a MVHR system this can be achieved using a smart extract grille fitted with either an occupancy sensor and timer or a humidity controlled sensor.

Where possible option 2 with an automatic grille is better as this requires no input from the end user.