Ventilation is one of the most important and most invisible parts of a house. It determines how the air is exchanged and how moisture and odours are carried away. Here we go through the most common ventilation systems and how to tell whether yours is working as it should.
Short answer: Ventilation in a house is about exchanging the indoor air for fresh outdoor air. The most common systems are natural (stack) ventilation, mechanical extract ventilation and mechanical supply and extract with heat recovery (MVHR). MVHR recovers heat from the outgoing air and is most common in newer homes.
Why is ventilation important in a house?
Ventilation continuously exchanges the old indoor air for new outdoor air. This carries away moisture, odours and carbon dioxide while fresh air comes in. If ventilation works poorly the moisture stays put, which increases the risk of condensation and, over time, moisture problems.
Natural (stack) ventilation
Natural ventilation is the oldest system and relies on natural pressure differences. Warm indoor air rises and leaves the house through extract ducts, while new air enters through vents and gaps. It is silent and needs no electricity, but the airflow varies with weather and season and is often weak when it is mild outside.
Mechanical extract ventilation
In an extract system a fan draws air out of the kitchen and wet rooms, while supply air enters through vents. The airflow is steadier than with natural ventilation because the fan controls it. The drawback is that the outgoing, heated air leaves the house without the heat being recovered.
MVHR: supply and extract with heat recovery
MVHR is the most modern system and has both supply and extract fans plus a heat exchanger. The heat in the outgoing air is used to warm the incoming outdoor air, which saves energy. MVHR is most common in newer, well insulated homes and gives a steady, controlled air exchange.
| System | How it works | Heat recovery |
|---|---|---|
| Natural (stack) | Natural draught | No |
| Mechanical extract | Fan draws air out | No |
| MVHR | Supply and extract fans plus heat exchanger | Yes |
Which ventilation system does my house have?
You can often work out which system the house has by looking and listening. Some clues:
- Natural: common in older houses. No fans, but round extract vents in the kitchen and bathroom and vents or gaps for outdoor air.
- Extract: an extract fan often runs constantly, and you feel a steady pull at the extract vents but have no supply fans.
- MVHR: a unit, often in the utility room or a service space, with both supply and extract ducts. Supply air comes in through vents in bedrooms and living rooms.
The build year is also a hint: natural ventilation is most common in houses built before the mid 1900s, extract ventilation became common after that, and MVHR dominates in modern, well insulated homes.
Signs of poor ventilation
Some common signs that the air exchange is too weak:
- Fog and condensation on windows.
- Stuffy air and odours that linger.
- High humidity and moisture in wet rooms.
Several of these are linked to moisture. Read more about moisture in the home and black mould in the bathroom, which often has poor ventilation as a contributing cause.
How to improve the ventilation
Start with the simple things: clean and wipe extract vents, make sure supply vents are not blocked or taped over, and replace any filters as recommended by the manufacturer. Do not place furniture too close to vents. An air purifier or dehumidifier can complement for particles and moisture respectively, but they do not replace a working air exchange. If you want to improve the indoor climate overall, there is more advice in the guide on air quality at home.
Frequently asked questions
What types of ventilation are there in houses?
The most common are natural (stack) ventilation with a natural draught, mechanical extract ventilation where a fan draws air out, and MVHR which has both supply and extract fans with heat recovery.
What is MVHR ventilation?
MVHR stands for mechanical ventilation with heat recovery. A heat exchanger captures the heat in the outgoing air and warms the incoming outdoor air, which saves energy. It is most common in newer homes.
How do you notice poor ventilation?
Common signs are condensation on windows, stuffy air, odours that linger and high humidity in wet rooms. It often comes down to the air not being exchanged enough.
Can an air purifier replace ventilation?
No. An air purifier captures particles in the air but does not exchange the air or carry away moisture. It can complement ventilation, but a working air exchange is still needed.
