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 surfaces.

- 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 Grenoble”

In Grenoble, 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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