What is the Difference Between Electric and Pneumatic Actuators?

Electric and pneumatic actuators play central roles in HVAC systems by controlling dampers and valves, yet they differ fundamentally in how they operate. Electric actuators respond to a control signal, typically between 2 and 10 volts DC or 4 to 20 milliamps, to adjust their position precisely. In contrast, pneumatic actuators rely on compressed air pressure to function, also known as an air signal. This requires dedicated air supply lines and supporting equipment. Although pneumatic systems were once widely used, electric actuation has become the standard thanks to its enhanced precision, easier installation, and streamlined design. Belimo’s direct‑coupled electric actuators, for example, eliminate the need for air lines, compressors, transducers, or positioners, components that are mandatory in pneumatic setups.

What is hysteresis?

Hysteresis refers to the difference between the control signal and the actual position of the actuator depending on the direction of movement. In pneumatic systems, internal friction from linkages, stem binding, seals, and other mechanical resistance contributes to this effect. Because the air control signal also acts as the power source, it must exceed the target to overcome resistance, often leading to overshoot. When the signal reverses direction, it must drop below the spring force to initiate movement, also contributing to positional error.

Why is electric actuation more accurate than pneumatic actuation?

Electric actuators like Belimo’s are designed for precision positioning, with a normal hysteresis of just 1%. Belimo minimizes this through internal feedback circuits that continuously compare the input signal to the actual shaft position. Pneumatic actuators, by contrast, typically exhibit around 1.5 PSI of hysteresis over a 5 PSI spring range. Additional mechanical loading increases this effect, further reducing accuracy.

What is the resolution of Belimo electric actuators compared to pneumatic hybrids?

Belimo actuators have a minimum resolution of 160:1, allowing them to respond to very small changes in the control signal. A typical DDC controller can output with 0.1 VDC resolution, and Belimo’s internal circuitry can match that. Pneumatic hybrid systems, which include a DDC controller, transducer, positioner, and actuator, can only achieve about 20:1 resolution in practice.

Why don’t pneumatic actuators always respond predictably to the control signal?

Pneumatic actuators often do not position based solely on the control signal. The final position depends on the net effect of air signal force, spring force, damper load, and resistance. These interacting forces can cause significant shifts in the effective spring range. For example, an actuator with an 8 to 13 PSI spring range could actually begin to move at 4 PSI if the damper load acts in the same direction as the air signal. The result is inconsistent positioning and reduced control resolution.

Can positioners improve the accuracy of pneumatic actuators?

Yes, but only to a limited extent. A pneumatic actuator alone may have accuracy of +/-15%. With a positioner, this can improve to about five percent, but this comes with added complexity and maintenance. Mechanical positioners degrade over time due to wear, spring fatigue, and calibration drift. Each component in the signal chain, including the transducer, positioner and actuator, introduces error. Over time, even with careful setup, accuracy continues to decline.

How does actuator performance affect temperature control and energy efficiency?

Precision actuation directly impacts thermal comfort and energy efficiency. Electric actuators with tight hysteresis and high-resolution track the control signal closely, keeping temperatures within range and minimizing energy waste. Pneumatic actuators, with their inherent inaccuracy, tend to hunt for position, especially under varying torque loads or high air velocities. This results in overshoot, undershoot, and instability in temperature and airflow control.

Are electric actuators more reliable than pneumatic systems over time?

Yes. While pneumatic actuators themselves may be mechanically durable, the surrounding components, such as transducers, compressors, driers, filters, and fittings, can introduce multiple points of failure. Maintenance for a pneumatic system can involve up to 23 components. Belimo’s electric systems require just three: wiring, the DDC panel, and a transformer. Belimo electronic actuators are expected to last a minimum of 60,000 full stroke cycles, and 15 to 20 years with minimal maintenance. Belimo actuators have a failure rate of just 0.3 percent over a 2-year warranty period.

What are the disadvantages of pneumatic actuators?

Pneumatic actuators typically perform poorly in modulating control applications that demand accurate intermediate positioning. Due to hysteresis and internal friction, only 20 to 50 percent of a pneumatic actuator’s full stroke may be usable for reliable control. They also rely on external components like compressors, transducers, and air stations, which introduce additional maintenance requirements and potential points of failure.

What are the disadvantages of electric actuators?

Electric actuators may require additional protection or design considerations in extreme high-load or process-specific applications where fast-acting pneumatic devices have traditionally been used. But for the vast majority of HVAC applications, electric actuation not only meets performance requirements but also delivers superior control flexibility and consistency.

What are the cost differences between electric and pneumatic actuation?

Initial installation costs are typically lower for Belimo electric actuators. When comparing a complete setup for a spring return damper, electric systems are less expensive than pneumatic systems once the costs of transducers, positioners, labor, and shared air station infrastructure are included. The difference becomes even greater over time. Pneumatic systems require ongoing maintenance across many more components, while Belimo’s electric solution involves only wiring, a DDC panel, and a transformer. With no recurring service needs and higher efficiency through better control, the life cycle cost of electric actuation is significantly lower. 

Which is better, pneumatic or electric?

Electric actuation offers better accuracy, reliability, resolution, and energy efficiency in most HVAC applications. While pneumatic systems can deliver higher torque in simple two-position control scenarios where precision is less critical, electric actuators often meet those torque demands while providing superior consistency and long-term cost savings. For modulating control, where accurate positioning and repeatability are required, electric actuation is a significantly better fit and avoids the drift, hysteresis, and hunting associated with pneumatic systems. In cold ambient environments, where air lines and pneumatic components are prone to freezing, electric actuation is the better fit, especially when position feedback is required. Belimo offers both fail-safe and non fail-safe models to meet these needs.