Our homes need a constant supply of fresh air to keep us and the buildings healthy. Ventilation is key to providing this.

We create pollution with our daily activities, like bathing, laundry and cooking and things in our home create pollution like furniture gives off formaldehyde and carpets and furnishings are sometimes made with man made fibres which release chemicals into the air.

Humans have a range of humidity thats is comfortable to live in, its between 45% and 65%, if humidity gets too high or too low we find it uncomfortable to live in. Humidity outside of this range also encourages things like mould, bacteria and viruses to grow.


Our homes used to be built with lots of ventilation in mind. We needed good air flow to keep fires in hearths burning and to remove pollutants from the air.

Cracks in walls and gaps in floors allow air to travel through the house. And with lights in ceilings and unsealed loft hatches the air can zip through a home, especially when it’s windy outside.

Controlled ventilation

In more recent times we are moving towards controlling the air that comes in and goes out, and this is what we mean when we say ventilation.

Buildings do need ventilation. Floor and roof timbers need to be well ventilated to ensure that they do not suffer from condensation. Damp timbers can lead to mould and rot.

We also have appliances in the home which require fresh air, like gas boilers, fires and log burners.

Purge ventilation

The most basic type of purge ventilation is to open a window or door. This allows a large volume of air to enter or leave the property, but depends on the homeowner to do it. It’s a lovely thing to do in spring, summer or autumn to open a window to allow all the fresh air in. It’s not so nice to have to do it in mid winter when its freezing cold.

Some ventilation systems have a purge function, similar to that of a kitchen extractor hood.

Background or Trickle Ventilation

Background or trickle ventilation is exactly that. Ventilation that comes in constantly and in a trickle or low flow. It can be through vents in windows or walls. it can be natural or mechanical

Passive Stack Ventilation (PSV)

Passive stack ventilation has vents in “wet” rooms like kitchens. bathrooms and toilets connected by near vertical ducts to ridge or roof terminals. The warm moist air in the wet rooms is drawn up by the wind effect and the stack effect.

Fresh clean air is brought into the home by background ventilators or trickle vents in habitable rooms like bedrooms. Internal doors should have a gap at the bottom to allow free passage of air around the house.

Most standard trickle ventilators or uncontrolled and allow air in all the time, unless manually closed. However, a recent development is to have humidity controlled ventilators which only allow air in when the humidity in a room is too high.

One of the down sides to PSV is that it is reliant on wind and weather conditions, it won’t work on hot still summer days, when it is most needed.

Intermittent extract fans

Intermittent extract ventilation is the most common type of ventilation in people homes. extractor fans are located in wet rooms and remove moisture and other pollutants quickly. Typically they are intermittent and only operate when required either by a connection to a light switch or via a humidity sensor. the fans can be mounted in walls, ceilings or in windows. replacement air enters the property either by uncontrolled infiltration or back ground ventilators like trickle vents in windows.

Most intermittent fans are connected directly to outside by short ducts, however some inline fans in loft spaces are connected to outside by longer ducts.

Mechanical Extract Ventilation

This type of ventilation continuously extracts stale air from wet rooms, fresh air is introduced by background ventilators in habitable rooms.

Often a central ventilation unit is positioned in a cupboard or loft space a ducts connect it to all the wet rooms. The system is dual speed giving a constant tackle ventilation or high speed boost flow. The boost function is usually controlled by light switches, manually or by humidity.

Single room heat recovery ventilation

This type of ventilation is similar to extractor fans, but they also bring fresh air into the wet rooms and warm it up from the warm stale air going out.

Often they provide constant trickle ventilation and provide for a boost function, which is either controlled by a light switch or by humidity sensor.

As they are normally situated in wet rooms there needs to be other background ventilators for habitable rooms.

One other issue is that the incoming and outgoing air streams can mix because the external grilles are so close.

Mechanical Ventilation with Heat Recovery (MVHR)

Whole house mechanical ventilation with heat recovery is perhaps the most complex system to use. It is difficult to retrofit into homes, but fairly easy to plan and install if the building id a new build.

It consists of a central unit which has both inlet ventilators and extract ventilators. These are balanced so that incoming air is close in flow to outgoing air. The central unit has a heat exchanger which heats up the incoming air using heat from the outgoing air.

The extractor vents are situated in the wet rooms and the inlet vents are in the habitable rooms.

This means that clean fresh air comes in warm and the stale moisture and pollutant laden air is cooled before leaving the building.

This type of system works best if infiltration is reduced to a minimum, properties with air change rates of less than 5m3/hr/m2 @ 50Pa.

Houses that aren’t this air tight, will still benefit from MVHR, but the system wont be as energy efficient.

MVHR has the added benefits of reducing condensation, and can be supplied with air filters, although the most effective filters will cause an increase of energy use, as the fans need to overcome the resistance of filters.

Although the systems are balanced, some are provided with a boost function to help remove moisture laden air from wet rooms like bathrooms and kitchens.

Positive Input Ventilation (PIV)

Positive Input Ventilation is a relatively new development to the ventilation market. The fan is typically sited in the loft space and a duct connects to an inlet ventilator in the landing ceiling.

Air is taken from the loft and forced into the home through the ductwork. This creates a slight positive pressure in the home. Stale, moisture laden air is pushed out through outlet ventilators, or more commonly the building fabric. These systems are often recommended for problems with radon or moisture and condensation.

Positive Input Ventilation ca nee an efficient way of ventilating air tight homes. The down side is that in homes that aren’t air tight, the staled moisture laden air is pushed through the building fabric. This can leads to problems within walls and floors as the warm air cools and the water vapour in the air condenses.