What Are Air Changes?
You may have heard someone talking about air changes in relation to a ventilation system, or you may be planning a system and need to know what calculations to do. Read on for an introduction!
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 this room requires one air change per hour, the fan will have 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? Below are some common types of room and the air changes recommended for them:
Bathrooms and shower rooms 3 - 8.
Cafes 10 - 12.
Canteens 8 - 12.
Cellars 3 - 10.
Conference rooms 7 - 10.
Entrance halls, corridors 7 - 10.
Garages 6 - 10.
Commercial 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. Applying the above list to our room from the first example, we can see that a low usage bathroom would need 3 air changes per hour, so a single extract fan should be capable of moving 22.5 litres per second.