🔧 Air Handling Unit Components and Selection Guide

AHU Overview and Application Types

An air handling unit (AHU) is the central component of a ducted HVAC system, assembling fans, coils, filters, dampers, and accessory sections into a single factory-built or field-fabricated enclosure. AHUs range from small rooftop units (RTUs) serving a single zone to large built-up central station units serving an entire building. Understanding each component section and its selection criteria is essential for designing reliable, energy-efficient HVAC systems that meet ASHRAE 90.1, ASHRAE 62.1 ventilation, and ASHRAE 55 thermal comfort requirements.

Common AHU application types include single-zone constant-volume (CV), multi-zone variable air volume (VAV), 100% outside air (OAU/DOAS), and dedicated outdoor air + recirculation (hybrid). Each application has different component configurations and performance priorities.

Fan Selection and AMCA Performance Standards

The supply fan is the most energy-consuming component of an AHU, typically accounting for 30–50% of HVAC system energy in VAV applications. Fan selection involves matching the system curve (pressure drop vs. airflow relationship) to the fan performance curve (static pressure vs. airflow at rated speed):

• Fan types: Forward-curved (FC) centrifugal fans are low-cost but inefficient at higher pressures; backward-curved (BC) or backward-inclined airfoil (AF) fans offer higher peak efficiency (typically 80–85% at best efficiency point, BEP). Plug fans (plenum fans without a scroll housing) are increasingly popular in built-up AHUs because they allow direct connection to the AHU plenum and variable-speed drives without duct inlet effects.
• AMCA standards: Fan performance is rated per AMCA 210 (Laboratory Methods of Testing Fans for Certified Aerodynamic Performance Rating). Certified fan ratings carry the AMCA Certified Ratings Seal. Fan sound power is rated per AMCA 300.
• Fan Efficiency Grade (FEG): ASHRAE 90.1-2016 and later editions require fans in systems ≥ 5 hp to meet minimum Fan Efficiency Grade (FEG) per AMCA 205. FEG-67 is the minimum efficiency level; high-efficiency fans achieve FEG-79 or FEG-95. Wire-to-air efficiency (motor + drive + fan) must also meet minimum thresholds under ASHRAE 90.1 Section 6.5.3.1.
• Variable speed drives (VSDs): VAV systems require fan speed control via VFD (variable frequency drive). Fan power varies with the cube of speed — reducing airflow to 70% of design reduces fan power to approximately 34% (0.7³ = 0.343). Always specify VFDs on supply and return fans for VAV systems.

Cooling Coil Selection

Chilled-water cooling coils transfer heat from supply air to chilled water. Selection parameters include entering air conditions (dry-bulb and wet-bulb), leaving air conditions (required supply air temperature and humidity ratio), entering water temperature (EWT, typically 44–46°F), leaving water temperature (LWT, typically 54–56°F), face velocity (typically 400–550 FPM to limit moisture carryover), and rows × fins per inch (FPI):

• Higher face velocity (> 500 FPM) risks moisture carryover — always include a downstream moisture eliminator or drain pan with positive drainage.
• More coil rows and/or higher FPI increase heat transfer but also increase pressure drop and fan energy. Typically 6–8 rows, 12–14 FPI for standard chilled-water applications.
• Direct-expansion (DX) coils are rated at ARI 210/240 standard conditions; always obtain manufacturer selection software performance data for actual entering conditions.
• Coil bypass factor (CBF) affects the ability to simultaneously control temperature and humidity — lower CBF (more rows, higher FPI) provides more dehumidification but requires reheat to maintain supply temperature without overcooling.

Heating Coils

Hot-water heating coils (1–2 rows, 12 FPI) preheat outdoor air to prevent coil freezing and provide morning warm-up or reheat. Electric resistance coils are used in smaller units or for reheat (SCR or staged). Steam distributing coils must be used as the first coil section in freezing climates — standard steam coils can freeze and rupture if steam supply is interrupted; distributing coil designs maintain steam velocity in inner tubes to prevent freezing.

Filtration — ASHRAE 52.2 and MERV Ratings

ASHRAE Standard 52.2, Method of Testing General Ventilation Air-Cleaning Devices for Removal Efficiency by Particle Size, defines the Minimum Efficiency Reporting Value (MERV) scale from MERV 1 (coarse, > 10 μm capture) to MERV 16 (HEPA-like, ≥ 0.3 μm capture efficiency ≥ 75%). Design guidance:

• MERV 8: Minimum for commercial applications (captures pollen, dust mites, household dust). Requires 2-inch pleated filter.
• MERV 11–13: Standard for offices, schools, and most commercial buildings. Captures fine dust, mold spores, legionella. 4-inch deep pleated filters at MERV 11–13 offer significantly lower initial pressure drop (0.3–0.5 in. wg) than older 1-inch filters.
• MERV 14–16: Required for hospitals (general patient areas), cleanrooms, and sensitive occupancies. Higher pressure drop (0.6–1.2 in. wg clean) — must be accounted for in fan sizing.
• HEPA (MERV 17–20 equivalent): 99.97% efficiency at 0.3 μm. Required for cleanrooms, isolation rooms, and pharmaceutical manufacturing. Very high pressure drop (1.0–2.0 in. wg) requires dedicated fan sections.

Always design for filter loading: the clean filter pressure drop is only 40–60% of the dirty (change-out) pressure drop. Fan systems must be capable of delivering design airflow at dirty filter conditions, or BAS controls must compensate by increasing fan speed as filters load.

Outdoor Air and Return Air Dampers

AHU damper assemblies control outdoor air intake, return air recirculation, and exhaust air relief. Damper selection criteria include leakage class (AMCA 500-D leakage classification, Class I being the tightest at < 4 CFM/ft² at 1 in. wg), pressure drop at design velocity (typically 0.04–0.10 in. wg at 1,000 FPM), and actuator torque sizing. Parallel-blade dampers provide poor flow control at mid-range positions — specify opposed-blade dampers for modulating control applications. For economizer OA dampers, specify Class I low-leakage dampers to prevent infiltration energy losses when the damper is at minimum position.

Energy Recovery and Humidification Sections

Energy recovery wheels (rotary heat exchangers) and plate heat exchangers are covered in ASHRAE 90.1 Section 6.5.6 and rated per AHRI 1060/1061. Rotary enthalpy wheels transfer both sensible and latent heat (50–80% total effectiveness); fixed-plate sensible-only heat exchangers transfer only sensible heat (60–80% sensible effectiveness). Exhaust air bypass dampers are required to prevent frost in cold climates and to modulate effectiveness in mild weather.

Humidification sections use steam injection (cleanest, fastest response, best control — use clean steam or pure steam in healthcare settings), evaporative (adiabatic) humidifiers, or electrode/resistance steam humidifiers. All humidifiers require downstream absorption length (minimum 10–15 pipe diameters) before any changes in duct direction to prevent carryover condensation.