Considerations for Air Handling Units

We spoke with Iain Lawrence, Senior Project Manager from our London office about a key aspect of infrastructure, Air Handling Units (AHU). Iain highlighted some key factors to consider when exploring the use of AHUs on your site.

What is an Air Handling Unit?

An Air Handling Unit (AHU) is used to re-condition and circulate air as part of a heating, ventilating and air-conditioning system. The basic function of the AHU is to take in outside air, re-condition it and supply it as fresh air to a building. All exhaust air is removed, which creates an acceptable indoor air quality. Depending on the required temperature of the re-conditioned air, the fresh air is either heated by a recovery unit, or heating coil, or cooled by a cooling coil.

In buildings where the hygienic requirements for air quality are lower, some of the air from the rooms can be re-circulated via a mixing chamber and this can result in significant energy savings. A mixing chamber has dampers for controlling the ratio between the return, outside and exhaust air.

AHUs connect to ductwork that distributes the conditioned air through the building and returns it back to the AHU. A heat/cooling recovery exchanger is normally fitted to the AHU for energy savings and increasing capacity. An AHU is designed for outdoor use, typically on roofs, is also known as a rooftop unit (RTU).

An AHU is a large metal box containing separate ventilators for supply and exhaust, heating coil, cooling coil, heating/cooling recovery system, air filter racks or chambers, sound attenuators, mixing chamber, and dampers.

What should be considered?

When installing an AHU, the following should be at the fore of your plans:


AHUs have specific space requirements and having a clear understanding of these from the onset will mitigate the need to for changes to installation or design plans. This is especially the case for Healthcare and Laboratory settings, where a higher degree of control or hygiene are required.

Prior to taking on such an installation, ensure that the correct energy calculations have been made, and that the Client and Stakeholders understand the impact of this change. This includes future-proofing, and establishing whether there is enough flexibility in the unit to enable changes of use, or an internal redesign of the serviced area.

When installing replacements units, further investigation of the existing ducting will be required, in order to confirm that it is fit for purpose, or whether it will require some updating, or a complete upgrade. In this case, confirmation of sufficient service space will be needed in order to accept the new ducting.


Working with an experienced team for these installations can save time, money and unnecessary stress on the installation of an AHU – components which are high on all Stakeholders’ lists! By considering the location of the AHU well in advance, the site can be tested for its suitability. For example, if the site is located on the ground level at the exterior, establishing the any noise or vibration issues to adjacent buildings well in advance can allow Design Teams to work with the Project Manager to mitigate these.

A common location for the installation of an AHU is on the site’s roof (Known as Roof Top Units, or RTUs). In advance of an installation on a rooftop, necessary structural investigations will need to take place. This applies to both new and replacement units, which may also differ in weight and size from the originals.

Regardless of the AHU’s location on the site, planning permission, security, safety and future accessibility for ongoing servicing and maintenance will also need to be considered.


A commonly used method for the installation of an AHU is via crane. While this can be the most efficient method of installation, factoring in the site’s logistical challenges may lead to reconsideration. An AHU unit may need lifting into position, either over other obstacles or onto the roof level. If a crane solution is not possible, alternatives such as delivering the unit in component form and assembling on site may be more achievable.

Final Note

Working with a team of experienced and competent Project Managers can make all the difference, particularly on costly and logistically-challenging tasks such as installing AHUs. Contact our teams today to find out how we can help to support the smooth delivery of infrastructure to your site.

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