Clearing the Air: Pneumatic HVAC Controllers clarified

receiver controllers velocity controlle reset volume controllers rs

This blog breaks down three of the most common pneumatic HVAC controllers—Receiver Controllers, Velocity Controllers, and Reset Volume Controllers—and explains what each one does, where it’s typically used, and how it operates within older pneumatic systems.

Readers will learn how these controllers manage airflow, temperature, and VAV box operation, along with practical insights into replacement options and modern upgrade paths. Whether you’re troubleshooting an older building or identifying replacement components, this guide helps clarify the differences between these commonly confused pneumatic control devices.

In pneumatic HVAC systems, controllers use compressed air signals (typically 3–15 PSI) to control dampers, valves, and actuators.

Three common controller types you may encounter are:

Receiver Controllers
Velocity Controllers
Reset Volume Controllers

While different manufacturers used slightly different terminology, each controller served a different purpose in an old pneumatic control system. Knowing what’s what is important for replacement!

Common CONTROLLER types

Receiver Controller

A receiver controller is the main decision-making controller in a pneumatic loop. It receives a signal from a sensor or transmitter, compares it to a setpoint and adjusts output air pressure to control equipment.

What it controls typically:

Heating valves
Cooling valves
Outdoor air dampers
Humidifiers
Fan systems

Where you’ll find it:

Inside older AHU control panels
Near mechanical equipment
In central pneumatic control cabinets
Common applications: discharge air temperature control, chilled water valve control, boiler water temperature control, static pressure control

Example operation : A discharge air sensor sends a pneumatic signal to the receiver controller.?The controller adjusts output pressure to modulate a chilled water valve and maintain discharge air temperature.

Think of it as: The “brain” of the pneumatic control loop

NOTE: Most brands no longer manufacture these units and the majority of them are replaced by a modern DDC programmable controller.

Common CONTROLLER types

Velocity Controller

A velocity controller is designed to control airflow velocity or airflow quantity in a duct system. It is most commonly associated with pneumatic VAV boxes & airflow control terminals.

What it controls typically:

VAV box dampers
terminal unit airflow

Where you’ll find it:

On or inside pneumatic VAV terminal boxes
above ceilings in office buildings
in duct distribution systems
Common applications: maintaining minimum and maximum airflow, pressure-independent VAV control, zone airflow regulation

Example operation : A velocity sensor measures airflow in the duct.?The velocity controller modulates the damper actuator to maintain proper airflow regardless of duct static pressure changes.

Think of it as: The “airflow regulator” of the VAV system.

NOTE: These units are still manufactured by a few brands.

Common CONTROLLER types

Reset Volume Controller

A reset volume controller is a specialized pneumatic VAV controller that adjusts airflow limits based on another control signal. It is most commonly used in pneumatic VAV systems & multi-zone air distribution systems

What it does:

Receives a reset signal from a thermostat or master controller
Changes airflow set points
Modulates airflow between minimum and maximum limits
How is this different than a Velocity controller?: Includes a reset function based on the output from a master controller (typically a pneumatic thermostat)

Where you’ll find it:

Inside pneumatic VAV boxes
At terminal units serving occupied spaces
Common applications: room temperature control, energy-saving VAV systems, occupied/unoccupied airflow reset, perimeter zone control

Example operation : A room thermostat sends a reset signal to the reset volume controller.?The controller adjusts the VAV damper to increase or decrease airflow as the room temperature changes.

Think of it as: The “translator” between room demand and airflow control.

NOTE: KMC’s CSC units are great replacements for these units if a customer is unable to retrofit to a more modern controls system on their VAV boxes

Simple comparison