AHUs: anatomy and commissioning of an air handling unit

The Air Handling Unit (AHU) is the heart of a building's air distribution system. It filters, heats, cools, humidifies and distributes air to the rooms. For the commissioning engineer, the AHU is the most complex and complete piece of equipment to commission.

The sections of an AHU

• • Mixing box: mixes fresh air and recirculated air. The motorised damper sets the fresh air proportion.
• • Filter section: G4 (pre-filter) + F7/F9 (fine filter) filters according to NF EN ISO 16890. Initial ΔP and maximum final pressure drop must be checked.
• • Heating coil: hot water/air heat exchanger. Output = airflow × ΔT × 1.2 (in kW).
• • Cooling coil: chilled water/air heat exchanger. Watch out for condensation and the drip tray.
• • Humidifier: steam (the most common) or adiabatic (evaporative). The target humidity is 40-60% RH.
• • Fan: centrifugal or plug fan (no scroll). Variable speed drive to match the airflow.
• • Supply plenum: distributes the treated air into the ductwork.

Commissioning section by section

• 1. Fan: check the direction of rotation, measure the airflow (Pitot tube in the duct), set the drive.
• 2. Filtration: measure the ΔP with clean filters (baseline) and check the dirty filter alarm.
• 3. Heating coil: check the water flow rate (balancing valve), measure the heat output, test the controls.
• 4. Cooling coil: as above + check condensate drainage and frost protection (frost thermostat).
• 5. Humidifier: check steam production, the humidity sensor and the safety cut-out.
• 6. Dampers: test the opening/closing of motorised dampers and check the limit switches.
• 7. Heat recovery: measure efficiency (temperature ratio) and check the bypass.

Measurements to record

• • Supply and return airflow (m³/h).
• • Temperature and humidity at the inlet and outlet of each section.
• • Static pressure in the ductwork.
• • Fan current and speed.
• • Filter ΔP.
• • Coil outputs (heating and cooling).