What Information Is Needed to Select an AHU?

Airflow and system pressure

Start with the design supply and extract airflows in m³/s, and the minimum airflow if the system will run at variable volume. The range between maximum and minimum flow affects fan selection and how coils and heat recovery are controlled at low airflow. If a minimum outdoor air quantity must be maintained at all times, state it separately, because it may need its own control at reduced supply airflow.

The fan must also overcome the external static pressure of the ductwork, terminals, dampers and attenuators outside the unit. That figure should come from a duct pressure calculation rather than an assumed allowance, and it should state whether filter loading has been included, because the difference between clean and dirty filters can change the fan duty considerably.

Design conditions and coil duties

Coils are sized from the summer and winter outdoor design conditions, the required supply air condition and the room conditions the system must hold. Where humidity matters, the cooling coil has to be selected to dehumidify, which usually means a lower coil temperature and a source of reheat.

Confirm the heating and cooling media and their temperatures: chilled water, low temperature hot water, direct expansion from a VRF or condensing unit, or electric heaters. In UK and Benelux climates, coils exposed to cold outdoor air need frost protection through preheat, glycol or control strategy, and the humidifier type and water supply need deciding if humidification is required.

Filtration and air quality

Filtration depends on the outdoor air quality at the intake and the air quality the space needs. Urban roadside, coastal and desert locations each load filters differently. General ventilation filters are now usually specified by ISO 16890 group, such as ePM1 or ePM10, with HEPA filters classified separately for critical applications.

Each filter stage adds pressure drop that rises as the filter loads, so the number of stages, the design final pressure drop and the access for changing filters all affect unit length, fan power and maintenance.

Heat recovery and energy performance

The right heat recovery device depends on how the airstreams are arranged and what the extract air contains. Thermal wheels and plate heat exchangers need the supply and extract airstreams brought together at the unit. Run-around coils suit separated airstreams and applications where cross-contamination must be avoided, at a lower recovery efficiency. In humid climates, an enthalpy wheel that also transfers moisture can reduce the latent load of the incoming fresh air, where carry-over is acceptable for the application.

Frost protection for plate exchangers, a summer bypass and the fan efficiency required by energy regulations should be agreed at selection stage. Requirements differ between the UK, the EU and the Gulf, so confirm which apply to the project location.

Space, acoustics and controls

Record where the unit will sit, indoors or outdoors, the available footprint and height, and the route into position. Units can be delivered in sections to pass through doors or lifts, but every split adds site assembly. Allow access space for filter changes, coil cleaning and fan replacement, and confirm the handing of connections. For outdoor units, confirm the weatherproofing, the roof support arrangement and safe access for maintenance at height.

Acoustic criteria for the rooms served and for nearby neighbours determine whether attenuators or acoustic casings are needed. For controls, define who supplies them, the protocol for BMS integration, such as BACnet or Modbus, and the control sequences, including airflow and supply temperature control, CO₂ or occupancy-based ventilation and interfaces with the fire alarm and smoke dampers.

A practical checklist

Gather as much of the following as the design stage allows. Where an item is not yet fixed, a selection can proceed on stated assumptions, provided they are recorded and revisited as the design develops. Henrik can help assemble this brief for an initial selection. For critical engineering decisions, our team reviews requirements before final specification.

• Supply and extract airflows
• Minimum airflow for variable volume
• External static pressure
• Summer and winter design conditions
• Supply and room conditions
• Heating and cooling media
• Filtration requirement
• Heat recovery type
• Location, dimensions and access
• Acoustic criteria
• Controls and BMS protocol