What face velocity should I use for AHU cooling coil sizing?

Standard face velocity for chilled-water cooling coils is 400–550 FPM. At velocities above 500 FPM, condensed moisture can be entrained in the airstream (carryover) and travel downstream, causing duct corrosion, insulation damage, and ceiling staining. If your layout forces higher face velocity, specify a high-efficiency moisture eliminator downstream of the coil and ensure the drain pan slopes to drain properly. Lower face velocities (350–400 FPM) improve coil efficiency but increase coil face area and AHU footprint. DOAS and outdoor air units serving dehumidification loads often benefit from face velocities as low as 300 FPM.

What is the ASHRAE 90.1 requirement for fan efficiency?

ASHRAE 90.1-2022 Section 6.5.3.1 requires that fans in HVAC systems with a combined motor nameplate horsepower ≥ 5 hp shall have a minimum Fan Efficiency Grade (FEG) of 67, evaluated per AMCA 205 at the design operating point. Additionally, the motor plus drive efficiency must meet DOE efficiency standards for the motor nameplate size. For single-zone VAV systems, the fan power limit (Section 6.5.3.2) is expressed as a maximum watts per CFM or bhp per CFM based on system type and pressure class.

What MERV rating should I specify for a standard commercial office building?

ASHRAE 62.1-2022 and ASHRAE 170 provide minimum filtration guidance. For general office occupancy, MERV 8 is the code minimum but MERV 11 or MERV 13 is recommended practice in 2025+ for improved indoor air quality (IAQ), COVID-19 mitigation per CDC guidance, and LEED EQ credits. Use 4-inch pleated MERV 13 filters as the final filtration stage — they offer significantly lower clean pressure drop (0.20–0.35 in. wg) compared to 1-inch MERV 13 filters (0.50–0.80 in. wg), reducing fan energy. Always include a MERV 8 prefilter upstream to extend MERV 13 service life.

How do I prevent steam coil freeze-up in cold climates?

Standard single-distributing steam coils are vulnerable to freeze-up if steam pressure drops and condensate fills the tubes during cold weather. Prevention strategies: (1) specify steam distributing coils (with inner tubes that maintain steam distribution even at low pressure); (2) ensure condensate drains freely with no condensate back-pressure — use properly sized steam traps and check valves; (3) provide preheat control sequences that open the steam valve to minimum position before the OA damper opens in below-freezing conditions; (4) install a low-temperature safety thermostat (set at 35°F) downstream of the coil that closes the OA damper and opens the steam valve fully on freeze alarm; (5) ensure the steam system maintains minimum 2 psig supply pressure to the preheat coil at all times during cold weather.

Engineering·9 min read·May 10, 2025

🔧 Air Handling Unit Components and Selection Guide

A detailed engineering guide to AHU components — fans, coils, filters, dampers, heat wheels, humidifiers — covering selection criteria, AMCA performance standards, MERV ratings, and common design pitfalls.

📖Belt Conveyor System Design Drawing Set (Full Access)

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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.

Topics covered

air handling unitAHU selectionAMCA standardMERV filter ratingcooling coil selectionheating coilVAV AHUfan curvesfan efficiency gradeFEGAHU commissioningenergy recovery wheelASHRAE 52.2