Air Changes Per Hour

Air changes are a simple way of demonstrating how much air needs to be moved into or out of a room to rid it of excess moisture, humidity and resulting problems with black mould. Read on to find out how it works.

Mould and condensation are caused by excess humidity in the air, so to prevent them we need to replace the air in the room with “clean” air, unladen with moisture. But when looking into the expansive world of ventilation, there are lots of options when it comes to power and throughput – so what’s right for you?

When talking about what’s required for a given space, you will hear ventilation experts refer to “air changes” or “air changes per hour”. This simply means how many times per hour the volume of air in the room is cycled through the ventilation system. Remember, you won’t see fans specifying their air changes! This depends on the size of your room.

Here’s a simple example:

Imagine a room measuring 3 metres in all directions, with a single extractor fan for ventilation.

The volume of your room is 3metres x 3metres x 3metres = 27 metres cubed. There are 1000 litres in a cubic metre, so another way to state the volume of your room is 27,000 Litres.

If one air change per hour is needed, the extractor will need to move 7.5 Litres of air a second. (27,000 / 60 / 60, or 27,000 Litres divided by 60 minutes in an hour, divided by 60 seconds in a minute)

If the room needed two air changes per hour, we’d need 15 Litres per second to pass through the fan. (27,000 x 2 / 60 / 60)

So when thinking of replacing or installing ventilation, you’ll need your tape measure and a calculator handy. Remember if you’re not shifting enough air, you’re not going to solve that mould or moisture problem.

So how many air changes does your room need? This list shows the recommended air changes for different types of room:

-Bathrooms and shower rooms 3 – 8.
-Cafes 10 – 12.
-Dining Rooms 8 – 12.
-Cellars 3 – 10.
-Conference rooms 7 – 10.
-Entrance halls, corridors 7 – 10.
-Garages 6 – 10.
-Production kitchens 20+.
-Laundries 10 – 15.
-Offices 4 – 6.
-Restaurants 8 – 12.
-Shower rooms (sports halls) 10 – 15.
-Toilets (public) 10 – 15.
-Workshops 6 – 10.
-Laboratories where gases and/or chemicals are used 10.

It follows that the more moisture that is produced in a room, the more air you will need to move. If we assume our room in the first example is a bathroom, we can see that for low usage the list above indicates 3 air changes per hour will be needed, indicating that our fan will need to be able to move 22.5 litres per second.