AHUs – Project Haystack




We define a wide range of equipment for air handling including:

  • ahu: air handling unit
  • doas: dedicated outside air system
  • mau: makeup air unit
  • rtu: roof top unit
  • fcu: fan coil unit
  • crac: computer room air conditioner
  • unitVent: unit ventilator
  • heatPump: heat pump

All of these equipment types follow the same fundamental modeling techniques and subtype from airHandlingEquip.


AHUs define a suite of choices that should be made on a per instance basis.

Heating Process

If the AHU provides heating, then it should add one of the following heatingProcess choices:

Or if the unit does not provide heating, then add the coolingOnly tag.

When applicable the heating choice marker should also be paired with the appropriate flow ref tag. For example if an AHU uses hot water heating, then the AHU should also define the hotWaterRef tag to refer to its hot water plant.

Cooling Process

If the AHU provides cooling, then it should add one of the following coolingProcess choices:

Or if the unit does not provide cooling, then add the heatingOnly tag.

If using chilled water, then the chilledWaterRef should be defined to reference the chilled water plant.

Air Volume Adjustability

One of the following airVolumeAdjustability markers should be defined:

This choice models the AHU's ability to adjust the volume of air flow. Typically this distinction is based on whether the AHU's fan is single speed or a VFD.

Zone Delivery

One of the following ahuZoneDelivery choices should be defined to model how the AHU delivers conditioned air to the zone:

  • vavZone: AHU supplies air to VAV terminal units
  • directZone: AHU supplies air directly to the zone
  • chilledBeamZone: AHU supplies air to chilled beam terminal units
  • multiZone: air is split into a duct per zone

A Variable Volume Temperature or VVT system is defined as a constant volume AHU with VAV terminal units. This is indicated by the presence of both the constantAirVolume and vavZone tags.


One of the following ductConfig markers should be defined:


Most points in an AHU are associated with one of the following ductSection of the unit:

  • discharge: air exiting the unit to be supplied to the zones/terminal units
  • return: air returning from the zone back into the unit
  • outside: fresh, outside air entering the unit for air quality and economizing
  • exhaust: air exiting the unit back outside
  • mixed: return and outside air mixed together before passing through the heating/cooling elements
  • cool: cooling elements/coils
  • heat: heating elements/coils
  • zone: conditioned space associated with the unit

If the AHU has one duct used for both fresh air and economizing then use the outside tag. If there are two dedicated ducts, then use the ventilation and economizing tags for the respective ducts.

The follow diagram shows the AHU sections and logical flow of air:

ahu sections


The following are examples of fully tagged AHUs:

// Built up AHU supplied by central plants with VAV zones
id: @abc
dis: "AHU"
siteRef: @xyz

// Rooftop unit direct to zones
id: @abc
dis: "RTU-2"
siteRef: @xyz