air

Air Handling Plant

Understand how the air handling plant works. Explore system overviews, measurements, and additional resources.

Overview

The air handling plant (AHP) uses input energy (e.g., electricity, heated water, or chilled water) to condition the air and maintain required indoor environmental conditions such as temperature, humidity, and fresh air supply. The AHP may consist of various types of air handling units (AHUs) at the system level, including rooftop units, fan coil units, and constant-speed units, which will be discussed in the system-level descriptions. For a more detailed overview of various system configurations for this plant, please refer to the ASHRAE HVAC Systems and Equipment Handbook (2020), Section 4 – Air Handling and Distribution.

Diagram of an air handling plant showing the direction of various air flows and components
Air handling plant.

Systems

For a given air handling plant, there may be a mix of air handling systems installed (e.g., constant-speed, constant-volume, variable air volume) and configurations (e.g., split heat pump, rooftop unit). Air handling systems provide heating, cooling and ventilation air to spaces in a facility.

air

Constant-Speed, Constant-Volume (CSCV) AHU

The CSCV AHU delivers a fixed amount of air volume at a constant airflow rate, while adjusting the temperature of that air to meet the heating or cooling demands.

Learn Morechevron_right
air

Variable Air Volume (VAV) AHU

A VAV AHU modulates airflow in response to changes in VAV box damper position. Fan motors are equipped with variable frequency drives (VFDs) that allow the fan speed (and associated air volume) to modulate as the building demand changes.

Learn Morechevron_right
sync_alt

Air-to-Air Energy Recovery

Air-to-air heat exchangers transfer energy between two airstreams that must be at different temperatures for sensible heat transfer and different moisture contents for latent heat transfer.

Learn Morechevron_right
air

Dedicated Outdoor Air System (DOAS)

A DOAS system is designed to condition and filter ventilation air independent of the space heating and cooling needs of a facility. A DOAS can generally provide limited space heating and cooling, but only as a byproduct of the required volumes of ventilation air provided to a space or zone.

air

Dual-Speed, Constant-Volume (DSCV) AHU

A DSCV AHU uses fan motors that have a high and low speed settings which allows the AHU to operate at a slower speed during setbacks during unoccupied times

air

Induction Unit (non-fan powered)

A system that uses a fast-moving primary air stream (usually created by a set of nozzles) to induce air from the space to be drawn through the unit and mixed with the primary air stream. The primary air stream is generated by a separate system such as a CSCV AHU or a DOAS.

air

Rooftop Unit (RTU)

A roof mounted packaged system that provides heating, cooling and ventilation and supplies air directly to the space below it or into a set of ducts.

air

Split Heat Pump (HP)

A HP is a split system that provides heating or cooling and is comprised of an indoor unit with a fan and evaporator coil and an outdoor unit with a fan and condenser coil.

hvac

Fan Coil Unit

A FCU is a smaller, factory-assembled device that is used to circulate and condition air.

mode_heat

Fuel Fired Furnace

A fuel burning device that heats air for space conditioning.

How do I…

Assess Plant Energy Usage

The energy consumption in an air handling plant is the sum of the energy used by each AHU system, including the electricity to power the supply and return fans and the energy needed to heat or cool the air through the AHU. Heating and cooling energy may come from a central heating or cooling plant or from an energy source processed directly at the AHU, such as natural gas for heating or electricity to drive compressor-based cooling.

info Energy Sources
Only energy sources that are processed directly at the AHU are covered in this section. Those that are processed upstream of the AHU are covered in the relevant plant guides (e.g. the energy to produce chilled water from a water-cooled chiller is discussed in the water-cooled chilled water plant guide).
Quantify Electricity Usage (kWh)

Determine the total amount of electricity used by the fan motors, heat pumps, packaged air handlers and heat recovery systems.

Equipment to Measure
air

Constant-Speed, Constant-Volume (CSCV) AHU

What to Measure:

  • Average hourly true RMS power draw (kW) of the supply and return fan motors.
Learn Morechevron_right
air

Variable-Speed, Variable-Volume (VSVV) AHU

What to Measure:

  • Average hourly true RMS power draw (kW) of the chilled water pumps.
Learn Morechevron_right
sync_alt

Air-to-Air Energy Recovery

What to Measure:

  • Average hourly true RMS power draw (kW) of the heat exchanger motor.
Learn Morechevron_right
air

Dual-Speed, Constant-Volume (DSCV) AHU

What to Measure:

  • Average hourly true RMS power draw (kW) of the supply and return fan motors.
air

Dedicated Outdoor Air System (DOAS)

What to Measure:

  • Average hourly true RMS power draw (kW) of the supply and return fan motors.
air

Rooftop Unit (RTU)

What to Measure:

  • Average hourly true RMS power draw (kW) of the rooftop unit.
  • Average hourly fuel usage by the burner (gal/h for fuel oil or propane, ccf/h for natural gas).
air

Induction Unit (non-fan powered)

What to Measure:

  • Average hourly true RMS power draw (kW) of the induction unit fan motor.
hvac

Fan Coil Unit

What to Measure:

  • Average hourly true RMS power draw (kW) of the fan coil unit.
air

Heat Pump

What to Measure:

  • Average hourly true RMS power draw (kW) of the heat pump.
mode_heat

Fuel Fired Furnace

What to Measure:

  • Average hourly fuel usage by the furnace (gal/h for fuel oil or propane, ccf/h for natural gas).
Measurement Locations
Diagram of an air handling plant indicating measurement points.
Measurements needed from air handling systems to quantify electricity usage in an air handling plant.

Assess Plant Performance

The heating or cooling energy that is delivered to the air by the AHU can also be assessed for both AHU-based heating and cooling sources as well as distributed sources of heating and cooling.

Quantify heat transfer (Btu)

Determine the amount of heating and cooling delivered by an air handling unit.

Equipment to Measure
air

Constant-Speed, Constant-Volume (CSCV) AHU

What to Measure:

  • Average hourly air temperature (°F) before the heating and cooling coils.
  • Average hourly air temperature (°F) after the heating and cooling coils.
  • Air flow rate (CFM) of the supply air.
Learn Morechevron_right
air

Variable Air Volume (VAV) AHU

What to Measure:

  • Average hourly air temperature (°F) before the heating and cooling coils.
  • Average hourly air temperature (°F) after the heating and cooling coils.
  • Air flow rate (CFM) of the supply air.
Learn Morechevron_right
sync_alt

Air-to-Air Energy Recovery System

What to Measure:

  • Average hourly return air temperature (°F), flow (CFM) and humidity (%).
  • Average hourly outdoor air temperature (°F) and humidity (%).
  • Average hourly supply air temperature (°F), air flow (CFM) and humidity (%).
  • Average hourly exhaust air temperature and humidity (%).
  • Hourly true RMS power draw of the supply and exhaust fan motors (kW).
Learn Morechevron_right
air

Induction Unit

What to Measure:

  • Average hourly air temperature (°F) before the heating and cooling coils.
  • Average hourly supply air temperature (°F) of the space.
  • Average hourly outdoor air temperature (°F).
  • Air flow rate (CFM) of the supply air.
  • Total operating runtime of the unit (hours).
air

Dual-Speed, Constant-Volume (DSCV) AHU

What to Measure:

  • Average hourly air temperature (°F) before the heating and cooling coils.
  • Average hourly air temperature (°F) after the heating and cooling coils.
  • Air flow rate (CFM) of the supply air.
air

Dedicated Outdoor Air System (DOAS)

What to Measure:

  • Average hourly air temperature (°F) before the heating and cooling coils.
  • Average hourly air temperature (°F) after the heating and cooling coils.
  • Air flow rate (CFM) of the supply air.
air

Rooftop Unit

What to Measure:

  • Average hourly air temperature (°F) before the heating and cooling coils.
  • Average hourly air temperature (°F) after the heating and cooling coils.
  • Average hourly outdoor air temperature (°F).
  • Air flow rate (CFM) of the supply air.
hvac

Fan Coil Unit

What to Measure:

  • Average hourly air temperature (°F) before the heating and cooling coils.
  • Average hourly air temperature (°F) after the heating and cooling coils.
  • Air flow rate (CFM) of the supply air.
hvac

Heat Pump

What to Measure:

  • Average hourly return air temperature (°F), flow (CFM) and humidity (%).
  • Average hourly outdoor air temperature (°F) and humidity (%).
  • Average hourly supply air temperature (°F), flow (CFM) and humidity (%).
  • Hourly true RMS power draw of the supply fan motor (kW).
mode_heat

Fuel Fired Furnace

What to Measure:

  • Average hourly supply air temperature (°F) of the space.
  • Average hourly flue temperature (°F).
  • Average hourly incoming outdoor air temperature (°F).
  • Average hourly furnace intake air temperature (°F).
Measurement Locations
Diagram of an air handling plant indicating measurement points.
Measurements needed from air handling systems to quantify the performance of an air handling plant.

Further Reading

ASHRAE (2020). “ASHRAE Handbook: HVAC Systems and Equipment,” Chapter 1. HVAC SYSTEM ANALYSIS AND SELECTION. I-P Edition.

ASHRAE (2020). “ASHRAE Handbook: HVAC Systems and Equipment,” Chapter 4. AIR HANDLING AND DISTRIBUTION. I-P Edition.

ASHRAE (2019). “ANSI/ASHRAE/IES Standard 90.1-2019 – Energy Standard for Buildings Except Low-Rise Residential Buildings”. ASHRAE.

Taylor, S (2014). “Return Fans in VAV Systems”. ASHRAE Journal, Vol. 56; pp. 54-58.