Air Handling Units (AHU)

Air Handling Units (AHU): A Small Guide What is an AHU? An Air Handling Unit is a key component of many heating, ventilation, and air conditioning (HVAC) systems. It’s a large cabinet that conditions and circulates air through a building. An AHU typically handles outside air, room air, or a mix of both, passing it through filters, coils for heating or cooling, and fans before delivering it to occupied spaces via ductwork. Core components - Housing and fan: The cabinet contains one or more fans that move air through the system. - Filters: Remove dust, pollen, and other particulates to improve indoor air quality. - Heating and cooling coils: Coils (heated by hot water/steam or electrically, and cooled by chilled water or refrigerant) condition the air’s temperature. - Humidification (optional): Some AHUs include a humidifier for moisture control in dry environments. - Duct connections, dampers, and control systems: Dampers regulate the amount of outdoor air versus recirculated air, while controls (sometimes connected to a building management system) manage temperature, airflow, and timing. How it works Outside air (or return air) enters the AHU, passes through filters, gets conditioned by the heating or cooling coils, and is pushed into the building’s ducts by the supply fan. Depending on the design, the AHU can introduce fresh outdoor air to meet ventilation requirements, or recirculate more air to reduce energy use. Some units also recover energy from exhaust air to improve efficiency, using devices like heat exchangers or enthalpy wheels. Small AHUs: what’s special Small AHUs are compact versions designed for modest spaces, single zones, or specific rooms. They’re common in offices, small schools, clinics, apartments, and light commercial setups. They can be standalone units mounted on rooftops, in mechanical rooms, or inside dedicated alcoves. Despite their size, many small AHUs offer features found in larger systems, such as variable-speed fans, efficient filtration, and simple controls. Key features to consider - Filtration: Look for higher-efficiency filters (e.g., MERV ratings appropriate for the space) to improve IAQ. - Energy efficiency: Energy-efficient fans (often with variable-speed drive), and heat recovery options can reduce operating costs. - Ventilation and controls: If the space requires outdoor air, ensure the AHU can provide it without over-ventilating. Integrated controls or a building management system help maintain comfort and efficiency. - Heating/cooling options: Depending on climate, you may need electric heating, hot-water steam, or chilled-water coils, or even a direct expansion (DX) coil. - Space and accessibility: For small rooms, compact footprints and easy access for filter changes and maintenance are important. Applications and benefits - Applications: Small commercial spaces, multi-tenant buildings, classrooms, retail areas, and residential settings with centralized HVAC needs. - Benefits: Improved indoor air quality through filtration, better comfort with controlled temperature and humidity, and the potential for energy savings with efficient fans and heat recovery. Maintenance basics - Regular filter changes according to the manufacturer’s schedule. - Periodic coil cleaning to maintain heat transfer efficiency. - Inspect condensate drains to prevent water damage and mold. - Check seals and gaskets to prevent air leaks. - Schedule professional inspections for overall performance and controls. Common pitfalls - Undersizing for the space leading to inadequate ventilation or comfort problems. - Skipping maintenance, which reduces IAQ and efficiency. - Inadequate air filtration that lets particulates through or creates too much pressure drop. - Poor duct design or placement, which can cause noise and uneven air distribution. Bottom line An AHU is the backbone of many comfort and IAQ strategies, even in small spaces. By selecting the right size, ensuring good filtration and energy-efficient features, and keeping up with maintenance, a small AHU can deliver reliable comfort, clean air, and steady performance for years to come.