How to obtain consistently good Indoor Air Quality from Mechanical Ventilation

The importance of Air Filters

airmaster-filterStandard filters in AirMaster heat recovery ventilation units are Class M5. As well as helping to optimise indoor air quality, they ensure that heat exchanger surfaces are kept clean, thus maintaining a high level of heat exchange efficiency. Filters are provided on both the air intake and room extract pathways of the air handling units.

Filters come within a sturdy frame with a pleated (zig-zag) element.  The pleating effect increases the actual filtered surface to 6 times the duct cross-sectional area. Consequently, pressure loss across the filter – caused by trapped impurities – will be slower, particularly when compared to a flat-surface filter. The rigid frame around the filter keeps it shape during service, holding bypass leakage to the absolute minimum. Filter replacement is normally at intervals of between 6-12 months.

Classification of Indoor Air Quality According to EN13779


Where external air quality is expected to be poor, Class F7 filters can be offered as a more rigorous alternative. Filter classification conforms to EN 13779 – the European Norm on filter selection for ventilation equipment in non-residential buildings.

Filter quality is one of the key considerations when comparing heat recovery ventilation alternatives. If filter selection is made from Class F or lower, the associated heat exchangers are not as efficiently protected and therefore will show deposit build-up within a year of use. This will have adverse effects on heat exchanger efficiency and specific fan power.

Heat Recovery by Counter Flow Heat Exchanger

The counter flow heat exchanger is at the heart of the heat recovery ventilation unit. In the counter flow process, the fluids enter at opposite ends, flow in opposite directions, and leave at opposite ends.


AirMaster air handling units have heat exchangers made of aluminium, with the exception of the floor-mounted AM 900. This has multiple plastic heat exchangers to accommodate its very compact design.

All AirMaster direct ventilation designs compare favourably in terms of cell “Specific Circumference”, which is defined as cell circumference / area. The key threshold for this variable is 1.0; above this, occlusion by condensation in the extract pathway becomes significant once the external temperature falls to around 8°C.


AirMaster heat exchangers typically have a plate-to-plate separation of 3.6 mm and cell side length of 20mm. Each cell area is therefore 3.6 x 20 = 72 mm², with a circumference of 2 x (20 + 3.6) = 47.2 mm. This gives a ‘Specific Circumference’ of 47.2 / 72 = 0.65, which is appreciably less than the threshold figure of 1.0.

Care is needed when defining the thermal efficiency of exchange. The higher the relative humidity (RH), the more effective the heat exchange process will be. AirMaster heat recovery ventilation unit efficiencies are usually quoted on a dry bulb basis, which eliminates all doubt concerning relative humidity, but gives a relatively low result: typical values are around 84%. For comparative purposes, thermal efficiencies with moist air have also been issued. For example, with a relative humidity of 80%, thermal efficiency rises to 91%. In the UK, ambient humidity varies between 75% and 90% RH approximately.

An important factor in the operation of the heat exchanger is the production of condensate; this is a continuous process in the extract pathway. For example, air extracted from a room at a temperature of 22°C will lose its ability to carry moisture as it cools. The moisture lost in this process will be left behind in the heat exchanger as condensate.

AirMaster heat recovery ventilation units are designed to drain naturally into the condensate tray. By having wide pathways through the exchanger, condensate is moved on rapidly as it forms, keeping occlusion of the air pathways to a minimum. The exchanger is made from aluminium, which is non-corrosive and can endure constant wetting without deterioration in performance.

Everything on this page is designed to keep AirMaster air handling units operating at peak efficiency over many years. Other units without such features can expect, over time, to incur a fall off in thermal performance, deterioration in IAQ and increased noise levels. In our view, you should not have to have such concerns. Consistent performance by AirMaster units will reflect well on any decision to specify them.

For further information on heat recovery ventilation systems, please contact us:

by phone: 01483 771910

by e-mail: [email protected]

We look forward to hearing from you!

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