Pneumatic control system

Pneumatic Electro-Mechanical and DDC systems control valves hvac systems industrial valves

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

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

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.
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.
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

Pneumatic Controls

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

Advantages:

Spark-safe
No electronics ? long life
Simple and durable

Disadvantages:

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)

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

Thermostat senses the room is warm and increases its output pressure from ~8 psi toward 13 psi.
VAV controller receives the thermostat signal and calculates the necessary CFM.
Actuator receives increasing pressure, causing the damper to open proportionally.
As the room cools, thermostat output pressure decreases.
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.

Pneumatic Control System