Casing performance - Achieving good indoor climate with lower operating costs

Six features define the quality and performance of air handling unit casings.

To achieve a good indoor climate with low operating cost the ventilation system must be designed and installed in a proper way so that energy losses are minimised and air quality preserved. This means that it must be air tight and well insulated but must be formed to allow cleaning. This applies to the whole duct system including the ventilation unit.

What about the casing?

For the duct system we have standards that describe how the ducting shall be insulated, what the maximum allowable leakage rates are and requirements for cleaning access. But what about the casing of the ventilation unit?

The casing of an air handling unit has an important role to play. It contains the vital ventilation components and it too needs to provide good insulation and minimise leakages. At the same time, we need to be able to easily access the fans, filters and heat exchangers for maintenance.

With a poor air handling unit casing there is a risk that the energy performance and the air quality are affected.

Behind a well-functioning casing lies a considerable amount of product development and testing.

 Air Handling Unit Classifications

The quality of an air handling unit casing is described by a series of characteristics that rate the thermal and air leakage performance. These ratings are defined in the International Standard EN1886; which also defines how they shall be measured.

The air handling unit classifications are Air leakage, thermal transmittance, thermal bridging, mechanical strength, filter bypass leakage and acoustic insulation.

Air leakage from the AHU casing impact the energy performance, the function and has a hygiene impact so it is important to minimise it.

Air leakage has three levels of classification for air handling unit casings. They are L1, L2 and L3 where L1 means the lowest leakage.  Broadly speaking, we can say that L3 is suitable for applications with low to medium filtration, L2 for medium to high filtration and L1 for very high or HEPA class filtration.

For most comfort applications then, L2 is the most appropriate.

Thermal transmittance, also known as the U-value, tells us how much heat energy will be lost through the casing. The units of measurement are W/m²K

Five classifications are defined for air handling units from T1 to T5 where T1 gives the lowest losses. Air handling units are subjected to the same temperature differences as the envelope of the building though the surface area is a lot less of course.  T2 gives a level of insulation similar to that of external doors; which is appropriate for air handling units. T5 means that the unit has such a bad U value that it is considered unclassified.

Thermal bridging causes some heat losses, but the main problem is that of condensation. Condensation needs to be avoided because it can lead to corrosion and hygiene problems. In cold climate it can even lead to the formation of ice on the casing.

Five classifications are defined from TB1 to TB5 where TB1 is the best and TB5 means unclassified. The TB value tells us what the temperature difference between the ambient and any point on the outside surface of the casing. It is the ratio of the surface to ambient temperature and the internal to ambient temperature. The theoretical maximum value of TB is 1.0

With a classification of TB2 the point with the lowest surface temperature would be at 8 C when the ambient air is 20 degrees and the air inside the unit is 0 C. With a normal plantroom humidity, the risk of condensation is low but if the TB value is lower then the risk is, of course higher.

The Mechanical strength of the casing is defined as the measured deflection of the outer surface of the casing when the pressure in the unit is 1000 Pa positive and negative pressure. To be classed as D1 the deflection must be under 4mm. Up to 10mm deflection gives a classification of D2.

The acoustic insulation is defined by the insertion loss of the casing. The insertion loss tells us how much the noise created inside the unit is reduced by the casing. It is given for each frequency band.

Last, but not least, we have the filter bypass classification. Leakage around the filter frame and through the casing between the filter and the fan will reduce the quality of the air supplied and it is important that the filters are installed in frames with a low leakage rate. The leakage rate should be aligned with the filter class intended to be used. A filter frame is classed by the leakage flow expressed as a percentage of the nominal airflow. To be classed F9, the leakage flow must be below 0.5%. By comparison, a filter frame classed as F7 may leak up to 2% of the nominal flow.

Eurovent Certification

A vital part of Eurovent Certification for air handling units is the measurement and classification of these casing characteristics. This is done by an independent laboratory that ensures testing is carried out in a fair and proper way.

Tests are carried out on a box with prescribed dimensions manufactured using the same design as the air handling unit casing so that the performance of different air handling unit types can be compared.

Real units are also tested, and the results can differ because real units have more doors, and there are penetrations through the casing wall for cables and pipes.

The classifications are published on the Eurovent web pages for both the model box and the real unit.

The Swegon GOLD and SILVER units are classified by Eurovent as follows:

Acoustic insulation of the model box casing:

If you want to deepen your knowledge about Eurovent certification, go to the Eurovent website 

Do you want to know more about how we can help you achieve good indoor climate at low operating costs? We have developed air handling units for more than 50 years.