Term of the Week: Pneumatic Control System

What is a Pneumatic Control System?

Pneumatic HVAC controls use compressed air (typically 0-20 psi) as the energy and signaling medium. Instead of electrical voltage or digital communication, these systems use air pressure changes to open valves, modulate dampers, and control heating or cooling.

They were the dominant form of commercial building control before electronic and digital systems (DDC) took over, and are still common in older buildings or in special environments where “spark free” components are required.

How a Pneumatic System Works (High Level)

1. Air Supply

A pneumatic system starts with a compressor and air dryer/filter.

• This “control air” is distributed through small copper or plastic tubing to thermostats, controllers, and actuators.
• Typical supply pressure: 18–20 psi.
• The first truly “Open” control system

2. Control Signal = Air Pressure

Instead of volts or milliamps, the control signal is a pressure band.?For example:

• 0 psi = off
• 5 psi = half open
• 15 psi = fully open
Devices interpret this pressure to position a valve stem or damper blade.

3. Actuation

A diaphragm actuator or pneumatic piston converts the incoming air pressure into movement.?The force produced is proportional to the pressure.

Common Pneumatic Components in HVAC

1. Pneumatic Valve Actuators

   Used on heating/cooling coils, reheat valves, steam valves, and similar applications.

   How they work

   • A diaphragm actuator receives a control signal (for example, 8–13 psi).
   • As pressure increases, the actuator pushes or pulls the valve stem.
   • Spring return determines fail position (fail-open or fail-closed).

   Where you’ll find them

   Boilers, AHU coils, FCUs, VAV reheat coils.

2. Pneumatic VAV Box Components

   Classic pneumatic VAV boxes (pre-DDC retrofit) typically include:

   • Pneumatic Thermostat (P-Stat)
     Senses zone temperature and outputs a proportional air signal.
   • VAV / Velocity Controller
     Common brands include KMC, Barber-Colman (Schneider), Johnson Controls, and Honeywell pressure-dependent controllers.

   Controller functions

   • Measures differential pressure from the airflow pickup.
   • Modulates a damper actuator based on thermostat signal.
   • Maintains minimum and maximum airflow using restrictors and setpoints.

   Damper Actuator

   • Receives an air pressure control signal to position the damper blade (examples: 3–8 psi, 5–10 psi, 8–13 psi).

   Typical sequence

   1. Room warms up.
   2. Pneumatic thermostat increases output pressure.
   3. Controller increases actuator pressure.
   4. Damper opens.
   5. Airflow increases and cools the space.

3. Pneumatic Thermostats

   Examples: Honeywell TP970, Schneider TK-1000/2212 series, Johnson Controls T-4000 series.

   How they work

   • Sense temperature using a bimetal element or fluid-filled bellows.
   • Modulate output air pressure proportionally to temperature deviation.
   • No electricity required.
   • Typical outputs: 3–13 psi or 5–15 psi modulating signals.

Pneumatic Signals vs Electric / Electromechanical / DDC

• Signal: Air pressure (various signals from 3–15 psi)
• Power source: Compressed air
• Speed: Slower response

• Spark-safe
• No electronics ? long life
• Simple and durable

• Drift due to leaks
• Harder to tune
• Air system maintenance required
• Limited monitoring/feedback

Electric/Electromechanical Controls

Examples:

• 24 VAC thermostats
• On/off relays
• Floating actuators

Signal types:

• On/off dry contacts
• 0–10 VDC, 2–10 VDC
• 4–20 mA
• Tri-state (open/close signals)

Pros: Simple, no air system?
Cons: Limited programmability compared to DDC

DDC / Modern Digital Controls

Examples: JCI FX, Honeywell Optimizer, Distech EC-Net

Signal Types

• BACnet MS/TP
• BACnet/IP
• Modbus
• High-resolution analog outputs (0–10 VDC, 4–20 mA)

Pros

• Fully programmable
• Trending and alarms
• Scalable

Cons

• Requires IT understanding
• Higher initial cost

Putting It All Together (Simple VAV Example)

Cooling-Only Pneumatic VAV Box Sequence

1. Thermostat senses the room is warm and increases its output pressure from ~8 psi toward 13 psi.
2. VAV controller receives the thermostat signal and calculates the necessary CFM.
3. Actuator receives increasing pressure, causing the damper to open proportionally.
4. As the room cools, thermostat output pressure decreases.
5. Damper modulates back toward minimum flow until the system stabilizes.

Stay Tuned!

Next week’s article will cover common failure and maintenance points in a pneumatic HVAC system.