AHU (Air Handling Unit)

The air handling unit (AHU) is the component that draws in, filters, conditions, and distributes the air in a building. It ensures hygienic ventilation, indoor air quality (IAQ), thermal comfort, and, through heat recovery, significantly reduces heating/cooling kWh.

An AHU integrates the following components into a single unit: fans (often EC), filters (ISO ePM1/ePM2.5), a heat exchanger (counter-flow plate or enthalpy wheel), coils (hot/cold water or direct expansion), sometimes humidification/dehumidification, a mixing damper, a bypass for free/night cooling, acoustic attenuators, condensate trays, and a controller connected to the building management system.

CTA: operational definition, components and modes

An all-air air handling unit (AHU) can operate with entirely new air, entirely recycled air (exceptional and temporary), or a mixture. From a construction standpoint, considerations include airtightness class, thermal breaks, double-skin insulation, and the mechanical resistance of the panels (comfort and hygiene). Key points:

- Filtration : gradation adapted to the context (urban, pollen, workshops) and ΔP filters monitored to plan replacements.

- Recovery : η 75–90% typically (plates/wheel); automatic bypass for free-cooling phases.

- Fans : EC motors + VFD → variable flow based on CO₂/TVOC/presence or static pressure.

- Batteries : hot/cold water (or direct expansion), 2-way valves and target ΔT; anti-frost management.

- Humidity : enthalpy wheel or vapor/adiabatic coil depending on hygrometric need.

- Acoustics : attenuators, moderate speeds, careful placement.

- Hygiene : maintenance access, slopes towards condensates, watertight seals, cleanable floor areas.

- Variants : rooftop (compact roof group), hygiene AHU (health), decentralized AHU by zone.

Advantages, limitations and points of attention of a high-performing CTA

Why install/upgrade it?

• IAQ & comfort: filtered air, stable temperatures and RH.
• Energy: heat recovery + variable flow = kWh decrease; peak shaving.
• Operation: BMS supervision, useful alarms (filters, frost, fans), reports for trustees/asset managers.
• Acoustics: balanced networks → controlled sound levels at the outlets.

Limitations to anticipate

• CAPEX/footprint (technical room, roofs, networks).
• Demanding networks: sealing, thermal insulation, pressure losses.
• Essential maintenance (filters, condensates, wheels, probes).

Key points to consider for a good result

• Size the flow rates according to the actual use (offices/schools ≠ shops).
• Choose the filtration (ISO ePM1/ePM2.5) according to external pollution, plan ΔP and schedule.
• Aim for high recovery (+ effective bypass) and EC fans with low SFP.
• Regulate on demand (CO₂/presence/TVOC) and schedules via GTB; monitor ΔT batteries.
• Address acoustics (speeds, attenuators) and maintenance access (doors, clearances).
• Hygiene: siphoned condensate trays, washable materials, cleaning protocols.
• Commissioning: network balancing, leak testing, flow-pressure curves and BMS trends.

Anecdote — “The fresh air of Pennes Mirabeau”

In Pennes-Mirabeau, an office building complained of stuffy air in the afternoon and noise from the vents. The audit revealed clogged filters, inefficient heat recovery, and fixed-speed fans. The renovation included: a high-efficiency plate heat exchanger with bypass, variable-flow EC fans controlled by CO₂, additional attenuators, and adjustments to the building management system (pressure reset, scheduling). Three months later: CO₂ < 900 ppm under load, a significantly reduced SFP, lower noise levels, and an overall feeling of "fresh air"... without increased energy consumption.

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