How to select the correct air filters for your HVAC system

Posted by Laurence Cockman on 07/06/16 12:22

CoolStream_filter.jpgAir filters are used in heating, ventilating and air conditioning (HVAC) systems to improve air quality in indoor working environments. They can also serve to protect the HVAC equipment.

Pollutants can originate from a source outside or inside the building. In this blog I will discuss pollutants which originate outside of the building.

So how fresh is 'fresh-air'? The answer is - it depends. It depends on your location and the prevailing outside conditions at any given moment. Filtration can be used to combat the uncertainty of the outside air conditions.

How are filters graded?

Air filters are available in a variety of grades depending on the size of particle you want to filter out.

BS EN 779:2012 provides a method for checking the performance of air filters provided in general ventilation systems. Filters are then ranked according to their particulate removal capability and given a classification accordingly. The standard specifies three filter groups: G (coarse), M (medium) and F (fine).

Coarse filters are classified according to the average arrestance of a synthetic dust used in the test, and range from G1 (50-65% arrestance) to G4 (90% arrestance). A course G4 filter is designed to capture pollen, fog and coarse dust particles (≥10μm), as well as leaves, insects, textile fibres, sand, ash, mist and hair.

Medium filters are classified according to the average efficiency (Em) of 0.4μm particles. 0.4μm and larger particles include mould & plant spores, pet dander, textile dust, skin flakes, yeast cells, insecticide dusts and most bacteria. There are two classifications for medium filters; M4 (40% ≤ Em < 60%) and M5 (60% ≤ Em < 80%).

Fine filters are also classified according to the average efficiency (Em) of 0.4μm particles, but these filters also have to meet a minimum efficiency level. Classifications range F7 (80% ≤ Em < 90% and a minimum efficiency of 35%) to F9 (95% ≤ Em and a minimum efficiency of 70%).

The minimum efficiency is the lowest % value during the complete installation cycle of the filter. The average efficiency is the average % value during the three stages of the filter installation cycle; initial efficiency, efficiency throughout the test’s loading procedure and efficiency after discharging.

BS EN 779:2012 also sets of maximum final test pressure drops to promote energy efficiency. For coarse (G) filters it's 450Pa and for medium (M) and fine (F) filters it's 450Pa, at the specified test volume of 0.944m³/s.

It is important that filtration has been tested to BS EN 779:2012, as it ensures that filters have had a demanding check of their quality and performance.

Which grade filter do you need?

To reiterate what was said earlier: filtration can be used to combat the uncertainty of the outside air conditions. In some industries it will be essential to ensure the internal climate meets the exacting demands for production (take the food industry for example, where F9 filters would be vital to maintain the required levels of hygiene and product quality) , but where the process is less sensitive it may only be a consideration to improve the working environment for building occupants.

Colt can offer grades of filtration from G4 to F9 depending on your requirements.


Laurence Cockman Laurence Cockman is a Senior Consultant for Colt UK and specialises in the design and product application of energy efficient HVAC systems.

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It is important that the right HVAC filters are selected to deliver the required indoor air quality, making it fit for both the building occupants and the production processes.


Topics: Climate Control, Evaporative cooling