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Variable-Speed, Variable-Volume AHU

Understand how the variable-speed, variable-volume AHU works. Explore component overviews, measurements, and additional resources.

Found in:
Air Handling Plant

Overview

A variable-speed, variable-volume (VSVV) air handling unit (AHU) provides conditioning to mixed air (a combination of outdoor and return air), discharges the conditioned air into zones, returns air from the zones, and exhausts air to the outdoors. The served zones are typically equipped with variable air volume boxes (VAV) which allow for zone-specific control of supply airflow.

The overall airflow of the air handling unit (AHU) is modulated using variable frequency drives (VFDs) on the supply and return fan motors, typically based on the static pressure in the duct system (i.e., static pressure reset). For a more detailed description of the system, refer to the ASHRAE HVAC Systems and Equipment Handbook (2020), Section 4.

The major components of a VSVV system are the fans and motors, motor VFDs, heat exchanger coils, dampers, and VAV boxes.

Diagram of a VSVV AHU showing its components
Diagram of a VSVV AHU showing its components.

Components

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Fans and Motors and Variable Frequency Drives

The fan motors with VFDs provide pressure in the system to move air through the duct work. Typically, fans are placed on the return and supply side of the unit to maintain proper airflow. The air is blown through filters and heat exchanger coils to be conditioned. Fan speeds are varied based on the need for conditioned or ventilation air in the zones served.

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Heat Exchanger (Heating/Cooling Coils)

The heat exchanger includes a set of heating and cooling coils, which provide heating or cooling to the air before it is discharged from the AHU. Mechanical valves regulate the amount of heating or cooling water (or steam) introduced from the loop to the heat exchangers that control the energy supplied to the discharge air.

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Variable Air Volume (VAV) Box

A device that includes a damper to control zone specific airflow, a VAV box is sometimes equipped with a fan based on the overall system design. VAV boxes are typically equipped with reheat coils to prevent over-cooling of zones with low cooling loads and high ventilation air requirements.

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Dampers

Dampers are controlled to supply appropriate quantities of fresh air to the AHU and exhaust air to the outdoors. The dampers can also be set to maintain desired building pressures. Dampers in VAV boxes control zone airflows based on the space thermostat setpoint.

info System Operation
In general, the mixed air is conditioned by heating or cooling coils that use the output of the heating or cooling loops, using mechanical valves to regulate the amount of energy introduced to the coils. Supply and return fans with VFDs and dampers control airflows inside the main AHU. Dampers in the VAV boxes control zone airflows. Occasionally, a VAV box will also be equipped with a supply fan.

How do I…

Assess System Energy Usage

The energy consumption of a VSVV AHU is the electricity used at the fan motors, VFD, and VAV reheat boxes, if present.

Quantify Electricity Usage (kWh)

Measure the hourly average true RMS electricity of the fan motors, outdoor air temperature and relative humidity.

How to Measure

Click any measurement below for detailed setup, data collection, and troubleshooting instructions.

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True RMS Power (kW)

Use this technique to measure the fan motor hourly true RMS power draw (kW).

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Outdoor Air Temperature

Use this technique to measure hourly outdoor air temperature.

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Borrow Equipment and Download Calculator

Reserve loggers, download the calculator, and enter your measured data.

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VSVV Fan (kW) Measurement Kit

This tool kit includes:

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VSVV Fan Motor Energy (kW) Calculator

Provides estimated annual electricity consumption of a fan that operates at variable-speed, variable-volume using kW and OAT data.

For more details about the methodology behind this calculator, see Fan Motor Energy Consumption.

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Measurement Locations
Diagram of a VSVV AHU indicating measurement points.
Measurements needed to quantify electricity usage of a VSVV AHU.

Assess System Performance

The performance of the system can be determined by quantifying the heating/cooling load of the zones served by the system.

Quantify Amount of Heat/Cooling Provided to the Space (Btu)

Determine the heating/cooling load.

Equipment to Measure
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Heat Exchanger (Heating/Cooling Coils)

What to Measure

  • Average hourly air flow rate (cfm) of supply air.
  • Average hourly air temperature (°F) before the heating/cooling coil.
  • Average hourly air temperature (°F) after the heating/cooling coil.
Measurement Locations
Diagram of a VSVV AHU indicating measurement points.
Measurements needed to quantify heating/cooling load on building of an VSVV AHU.

Further Reading

ASHRAE (2014). “ASHRAE Guideline 14-2014 – Measurement of Energy, Demand, and Water Savings.” Annex A.

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.

Li, Y. (2015). “Variable Frequency Drive Applications in HVAC Systems”, in M. Chomat (ed.), New Applications of Electric Drives, IntechOpen, London. 10.5772/61782.