Guide to Whole House Ventilation

Choosing a ventilation system for new homes

Improved air tightness means that choosing the correct ventilation system is becoming an increasingly important consideration in residential new-builds.  Many new concepts and products have emerged over the last few years in this field.  However there is still some confusion with regard to why and how these technologies should be deployed, not just to satisfy the requirements of the latest building regulations, but to deliver acceptable levels of indoor air quality to protect both occupants and the building fabric.

Why ventilate??

Fresh, clean air is one of the most essential requirements for a comfortable and healthy living environment.  Apart from the obvious need for fresh air to breathe, ventilation is required to dilute and remove pollutants, odours and moisture that are generated through everyday living.

How to ventilate

Current UK and Northern Ireland building regulations specifically recognise 4 distinct ventilation strategies

  1.  Trickle vents in all rooms, with intermittent extract fans in bathroom and kitchens.  This is the traditional configuration found in most homes built in the last 20 years.
  2. Passive Stack Ventilation (PSV) extract from kitchens and bathrooms, with trickle supply vents in remaining rooms.  This option is relatively uncommon inNorthern Ireland.
  3. Continuous mechanical extract (MEV) from kitchens and bathrooms, with trickle supply vents in remaining rooms. 
  4. Continuous Mechanical Ventilation with Heat Recovery (MVHR) ducted to all rooms.

In this guide, we will consider MEV and MVHR, plus another system, PIV, which although not specifically listed in the regulations is being used increasingly in new-build homes.  For information on conventional extract fans, see our guide to Bathroom Extract

Continuous Mechanical Extract (MEV)

This option replaces the conventional intermittent extract fans with a system that continually extracts air from the bathrooms and kitchens at a much lower rate.  This can be either centralised, in the form of a central extract unit ducted to each “wet” room, or de-centralised, in the form of smaller localised continuously running "dMEV" fans like the Greenwood Unity CV2 fan. The key advantage of this type of system is a more consistent and controllable level of background ventilation, giving improved indoor air quality.  The downside is that all the heat energy in the extracted air is lost, and since the MEV system only extracts air, trickle vents must still be included in the non-extract rooms to allow fresh unheated air into the building. 

Whole House Mechanical Ventilation with Heat Recovery (MVHR)

A ducted MVHR unit removes stale and moist air from all bathrooms and kitchens, and simultaneously provides other rooms with a supply of filtered fresh air that has been pre-warmed by a counter-flow heat exchanger. Because the MVHR system provides both supply & extract, those pesky trickle vents are no longer required, and the continuous extract from wet rooms removes the need for separate bathroom extract fans.    MVHR is the most sophisticated and energy efficient solution, but it is also the most expensive to install.   It delivers the most significant benefits in well insulated houses built to a high level of airtightness (<3 m3/m2h).  Although it is the most complicated type of whole house ventilation, installing an MVHR system is not beyond the abilities of a keen housebuilder or plumber.  Vent Store offers a wide range of MVHR units from Silavent, Greenwood and S&P, along with all the ductwork required for a complete installation, and we can help you select the correct sized unit for your house.  

Positive Input Ventilation (PIV)

PIV consists of a central, usually loft mounted fan unit, which continually pushes filtered fresh air into the house via a single supply terminal. Because the supply air is taken from the loft space rather than directly from outside, some of the heat lost through the ceiling is theoretically recovered, however PIV units generally require an integral electric heater to temper the supply air and avoid draughts.  The Greenwood Centair range offers a complete range of PIV units with some clever energy saving features.  Since PIV does not provide any extraction, conventional intermittent extract fans are still required in kitchens and bathrooms – see guide.  In addition to this, for buildings built to moderately high design airtightness levels ( <5 m3/m2h), trickle vents should still be installed in all rooms to allow the supply air to permeate the entire house. Larger houses may also require two or more units to achieve the minimum whole house ventilation rates.  One advantage of PIV over MEV is that the supply air is filtered, helping to reduce airborne contamination.