Homeowners Professionals

How to calculate extraction rate

It’s important to understand what extraction rate is before buying an extractor fan for your bathroom. But how do you calculate it? Viessmann can show you how.

Installing an extraction fan is one of the best ways to prevent your home from becoming a place of  mould and damp  - no one wants to take a shower in a room that smells of mildew or watch television in a living room that has mould growing up the walls. But did you know that you should size the fan correctly to ensure that it can successfully take enough moisture out of a room?

The best way to do this is to calculate the extraction rate.


What is an extraction rate?

Extraction rate is a figure that tells you how much air can be removed from a room in one hour. The figure is measured in metres cubed per hour, or m^3/hr. It’s important to get this figure correct, particularly in environments such as industrial kitchens, where there are a lot of fumes, smoke or water vapour being produced from ovens and pans of boiling water. Therefore, you need to calculate the size of the room and how much air the fan can remove in one hour. Buying a fan that’s too small won’t do the job efficiently, whereas a fan that’s too big will only waste electricity.

How to work out extraction rate

To work out the extraction rate, you need to calculate the cubic metres of the room you want the fan to go in. Do this using a measuring tape by noting down the height, width and length of the room in metres.

Once you have this information, multiply the three figures together to get the size in cubic metres. For instance, say that your bathroom has a height of 2.8 metres, a width of 3.2 metres and a length of 3 metres, multiply 2.8 by 3.2 by 3, which gives you 26.88 m^3.

Air change is a measurement that determines that amount of air that should be removed from a room per hour. It’s recommended that a bathroom has an air change per hour (ACH), often referred to as an air change rate, of 10. Therefore, take the volume of your bathroom that you’ve just calculated and multiply it by 10. In our example, this would be 26.88 by 10, which equals 268.80 m^3/hr.

When it comes to purchasing a new extractor fan, you will usually be provided with the diameter in millimetres, the extractor rate and the power voltage supply. Use the extractor rate to determine whether the fan will be sufficient or if you need a more powerful one.

If you’re struggling to find the extraction rate, you may need to look out for a litres per second (l/s) measurement. One l/s is around 3.6 m^3/hr. Therefore, to find the l/s from your extraction rate, divide it by 3.6. So this would be 268.80 divided by 3.6, which equals 74.67 l/s.

What extraction rate do I need for my bathroom fan?

The extraction rate you need for your bathroom will depend on the room’s size. However, building regulations state that the minimum fan extraction rate your bathroom should have is 50 m^3/hr (or 15 l/s) if the room doesn’t have an openable window, so do ensure you at least meet this level. 

Do I need a bathroom fan?

As we’ve stated above, unless you have an openable window, a bathroom fan is required under building regulations, but what else can they be good for?

  • Removing humidity

The main job of an extractor fan is to remove the humidity that builds up inside a bathroom when the bath or shower is running. As you run the hot water, some of it evaporates and creates steam. If this steam is left to linger in the room without being expelled, excess moisture and condensation will build up. An extractor fan will pull this steam out of the room.

  • Preventing mould

If the moisture in your bathroom that we’ve mentioned above has nowhere to go, mould will begin to grow. Mould particularly likes warm and damp environments, making the bathroom the perfect breeding ground. You may find black mould in the grout between your tiles or sealant around the bath and sink. Once it’s started to grow, it can be really  tough to remove.

Mould can be a health hazard for those with respiratory problems, such as asthma. Even if you don’t have any respiratory conditions, the allergens that it produces can cause symptoms like sneezing, coughing and skin rashes.

  • Avoiding musty smells

If you’ve ever forgotten about damp clothes in the washing machine or smelt your dog when they’ve just come in from a walk, you’ll know the scent we’re talking about. This musty odour is what your bathroom may smell like if you don’t use sufficient ventilation to remove the moisture.

Keep your bathroom smelling fresh by leaving your bathroom fan on for at least 20 minutes after a shower, washing your towels regularly and cleaning out the sink and bath’s drains.  

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