Air handling unit components and configurations

An air handling unit (AHU) conditions and moves air for a building or part of one. It brings in outdoor air, mixes it with return air, filters it, heats or cools it, sometimes humidifies or dehumidifies it, and pushes it through ductwork to the spaces it serves. AHUs range from small packaged units to large custom-built field-erected units serving entire hospitals or campuses.

Understanding the sections of an AHU helps owners make good decisions on specification, maintenance and replacement, and helps engineers anticipate problems such as freezing coils, wet filters and poor airflow. This guide walks through the main components and common configurations, with particular attention to Canadian winter conditions.

Main components

Outdoor air, return air and exhaust dampers

Dampers control how much outdoor air enters, how much return air is recirculated and how much is exhausted. In an economizer arrangement, the dampers modulate to use cool outdoor air for free cooling. Low-leakage dampers with insulated blades and good seals are important in cold climates, both for energy and to prevent coil freezing when units are off. See economizers: types, controls and common failures.

Mixing box or plenum

The mixing section combines outdoor and return air. Poor mixing creates layers of cold and warm air (stratification) that can freeze coils or trip freezestats even when the average mixed air temperature is above freezing. Blenders, baffles and careful damper arrangement help. See mixing boxes, stratification and freeze protection.

Filters

Filters protect coils and fans from dirt and improve indoor air quality. A common arrangement is a pre-filter (for example MERV 8) followed by a final filter (often MERV 13 or higher). ASHRAE Standard 62.1 sets minimum filtration requirements upstream of wetted surfaces such as cooling coils, and many owners now specify MERV 13 as a baseline for occupied buildings. Higher-efficiency filters increase pressure drop, which affects fan energy. See filtration, MERV ratings, HEPA and pressure drop.

Energy recovery

Many AHUs include an energy recovery device between the exhaust and outdoor airstreams: a heat wheel (sensible or enthalpy), a fixed-plate heat exchanger, a heat pipe or a run-around glycol loop. In Canada, energy recovery is often required by NECB for systems with significant outdoor air, and it substantially reduces heating loads. Frost control is a key design issue in cold climates.

Preheat coil

In cold climates, a preheat coil warms incoming outdoor air before it reaches other components. It is often a glycol or steam coil, or a hot water coil with a recirculation pump to keep water moving. Preheat protects downstream coils and filters from freezing.

Cooling coil and drain pan

The cooling coil (chilled water or direct expansion) cools and dehumidifies the air. Coil face velocity is typically kept to around 2.5 m/s (500 fpm) or less to prevent moisture carry-over from the coil into downstream sections. The drain pan must slope to drain fully, be made of corrosion-resistant material and be accessible for cleaning, as ASHRAE Standard 62.1 requires.

Heating coil

A heating coil (hot water, steam, electric or gas-fired) heats air for space heating. In some configurations, the heating coil also provides reheat after dehumidification.

Humidifier

Steam humidifiers, often steam-to-steam or electric, add moisture in winter. Proper absorption distance is needed so that moisture is fully absorbed before reaching filters or fans.

Supply and return fans

Fans move air through the system. Options include centrifugal fans with housed or plenum (plug) wheels, and fan arrays that use multiple small direct-drive fans. Variable speed drives are standard for VAV systems. See fan arrays and fan wall technology.

Sound attenuators and discharge plenum

Attenuators reduce fan noise transmitted into ductwork. Discharge plenums provide a smooth transition into the duct and can reduce system effect losses.

Casing

The casing holds everything together. Double-wall construction with insulation, thermal breaks and low air leakage reduces heat loss, condensation on the exterior and contamination. Thermal break construction is particularly important for units located outdoors or in unheated spaces in Canada.

Typical component sequence

A common sequence for a mixed-air unit in a cold climate is:

1. Outdoor air and return air dampers
2. Mixing section
3. Pre-filter
4. Energy recovery device (if outdoor air and exhaust are both handled)
5. Preheat coil
6. Final filter
7. Cooling coil and drain pan
8. Heating or reheat coil
9. Supply fan
10. Humidifier and attenuator

Actual order varies with application. Some designs place the final filter after the fan, especially in healthcare, to catch particles from the fan and coil sections.

Configurations

What to check during design and commissioning

• Coil face velocities and filter velocities within manufacturer and design limits.
• Freeze protection: preheat, glycol, mixing, damper sequences and low-limit controls.
• Accessible doors and service space for every section, including coils and drain pans.
• Drain trap height correct for the fan pressure, so condensate drains under negative or positive pressure.
• Casing leakage and thermal performance, especially for outdoor units.
• Airflow measurement for outdoor air, required in many designs for ventilation verification.
• Fan performance against the actual system curve, verified during air balancing.

Frequently asked questions

Why do AHU coils freeze in Canada?

Common causes include stratified mixed air, leaking outdoor air dampers when the unit is off, failed preheat coils, low water flow and inadequate glycol concentration. Freeze protection must be addressed in both design and sequences of operation.

Should the final filter be before or after the fan?

Either can be appropriate. Locating it after the fan captures particles generated by the fan and coils and is common in healthcare. It also places the filter under positive pressure. The choice depends on cleanliness requirements and layout.

How often should AHUs be inspected?

ASHRAE Standard 180 provides minimum inspection and maintenance tasks for AHU components. Filters, drain pans, coils, belts and dampers all need regular attention.

Key takeaways

• AHUs combine dampers, filters, coils, fans and optional recovery and humidification sections.
• Freeze protection and mixing are critical design issues in Canadian climates.
• Coil face velocity and drain pan design prevent moisture problems.
• Draw-through and blow-through configurations each have trade-offs.
• Access for maintenance and verification of airflow are as important as initial performance.