Air Handling Unit (AHU)

The Full Picture

An air handling unit is the machine at the center of an air-based HVAC system. One university construction standard describes a casing containing a fan, a filter and cooling, heating and/or humidification coils, with mixing boxes, return fans and damper sections added as needed. Return air and outdoor air come in, the unit conditions the mix, and the supply fan sends it out through ducts to diffusers, VAV boxes or other terminals. The same standard notes that units installed inside are typically called AHUs, while exposed units on a roof are rooftop units (RTUs). Many AHUs are chilled-water and hot-water units fed by a central plant, so the unit sits at the end of piping that is priced separately.

On the drawings an AHU is tagged and located on the mechanical floor plan or roof plan, shown with its duct connections, and described in the equipment schedule on the mechanical schedule sheets. A typical schedule row gives the tag, the area served, supply airflow in CFM, outside air CFM, fan motor size, coil capacities, filter type, electrical characteristics and the weight. Division 23 of the specifications covers the unit itself, and Duke's standards, for example, file it under 23 70 00 Central HVAC Equipment. Cooling capacity is often shown in tons, and one ton is 12,000 Btu of cooling.

The takeoff unit is each (EA), one per tag, and the price depends on what the schedule says. Airflow and coil capacity drive the size of the unit, and filter sections, energy recovery, a return fan, a VFD, sound attenuation and a custom casing all change the price. An estimator therefore pulls the schedule row into the carry sheet rather than counting a symbol alone. Items attached to the unit are usually separate lines: the concrete pad or steel dunnage, vibration isolation, chilled and hot water piping connections with valves, condensate drain, electrical feed and disconnect, controls, and startup and balancing.

Pricing mistakes tend to come from the edges. Equipment is often furnished by one party and set by another, so a missing crane or rigging line is a common gap, particularly for units delivered in sections. Another is counting every AHU on the schedule when one is noted as existing or by owner. A third is pricing the unit from the plan view alone when the schedule calls for a larger fan, a different coil or an energy recovery section. Ductwork leaving the unit is its own takeoff and bid package, and the controls are usually carried by a specialty contractor.