Poor temperature control increases energy costs, reduces comfort, and affects HVAC reliability. Thermal actuators solve these problems by enabling accurate, automatic valve control.
A thermal actuator converts thermal energy or an electrical control signal into mechanical movement. In HVAC systems, it commonly controls valves regulating hot or chilled water. Main types include thermoelectric, wax, bimetallic, pneumatic, hydraulic, and motorized actuators, each suited to different control requirements and operating conditions.
Understanding their working principles and differences makes it easier to select the right actuator for an HVAC system.
O que é um atuador térmico?
A atuador térmico is a control device that produces mechanical movement in response to heat, temperature variation, or an electrical signal that creates heat internally.
In HVAC systems, thermal actuators are commonly installed on control valves used in:
- Unidades de ventilação e refrigeração
- Radiator systems
- Sistemas de aquecimento por piso radiante
- Heating and cooling manifolds
- Zoned HVAC systems
- Commercial building control systems
Their primary purpose is to regulate the flow of hot or chilled water according to actual heating or cooling demand.
For example, when a thermostat detects that a room requires heating, it sends a signal to the actuator. The actuator opens the associated valve, allowing hot water to flow through the heating terminal. Once the required room temperature is reached, the control signal changes and the valve closes.
This automatic process supports more stable indoor temperatures while reducing unnecessary energy consumption.
How Does a Thermal Actuator Work?
The operating principle depends on the actuator design.
A typical thermoelectric actuator contains three main components:
- Electrical heating element
- Temperature-sensitive expansion material
- Piston or spindle mechanism
When electrical power is supplied, the heating element increases the temperature of the internal expansion material.
As the material heats up, it expands and generates mechanical force. This force pushes the actuator spindle, which then opens or closes the connected HVAC valve.
When the electrical signal is removed, the material gradually cools and contracts. A spring or return mechanism moves the actuator back to its original position.
Thermal actuators generally operate more slowly than conventional electric motors. However, rapid movement is usually unnecessary in hydronic HVAC applications.
Their gradual movement can help maintain stable water flow and reduce sudden changes within the system.
What Are the Main Types of Thermal Actuators?
Thermal actuators can be classified according to their operating mechanism and energy source.
1. Thermoelectric Actuators
Thermoelectric actuators are among the most widely used actuator types in HVAC systems.
They use electrical energy to heat an internal expansion element. As the material expands, it creates linear movement that operates the valve.
Common applications include:
- Válvulas de zona
- Unidades de ventilação e refrigeração
- Radiator valves
- Underfloor heating manifolds
- Heating and cooling distribution systems
Thermoelectric actuators are generally compact, quiet, and suitable for automatic on/off control.
Depending on system requirements, they are commonly available in normally closed e normally open configurations.
2. Wax Thermal Actuators
Wax thermal actuators use the expansion and contraction of specially formulated wax to generate mechanical movement.
When the wax is heated, its volume increases and pushes a piston outward. When the temperature decreases, the wax contracts and the piston returns.
Key characteristics include:
- Compact construction
- Relatively simple operating mechanism
- High mechanical force for their size
- Smooth and gradual movement
Wax actuators can be found in HVAC valves, thermostatic control devices, automotive cooling systems, and other temperature-sensitive equipment.
3. Bimetallic Thermal Actuators
A bimetallic actuator uses two different metals bonded together.
Because the two metals have different thermal expansion rates, temperature changes cause the combined strip to bend.
This movement can be used to operate:
- Electrical switches
- Small valves
- Dampers
- Thermostatic control mechanisms
Bimetallic actuators have relatively simple structures and may operate without complex electronic control systems.
However, their movement range and positioning accuracy may be more limited compared with electronically controlled actuators.
4. Pneumatic Actuators
Pneumatic actuators use compressed air to generate mechanical movement.
Air pressure acts on a diaphragm or piston, which changes the position of a connected valve or damper.
Pneumatic actuators have traditionally been used in:
- Large commercial buildings
- Institutional facilities
- Industrial HVAC systems
- Older building automation systems
Although electronic control technologies are now widely adopted, pneumatic actuators remain in operation in many existing installations.
They can also remain useful where compressed-air infrastructure is already available.
5. Hydraulic Actuators
Hydraulic actuators use pressurized liquid instead of compressed air.
The hydraulic pressure moves a piston or other mechanical element, generating force for controlling equipment.
Their main characteristics include:
- High operating force
- Stable mechanical movement
- Suitability for heavier loads
- Strong performance in selected industrial applications
However, hydraulic actuators require additional components and fluid-management systems.
For this reason, they are less common in standard room-level HVAC applications than thermoelectric actuators.
6. Motorized Valve Actuators
Motorized actuators are not always categorized as purely thermal actuators, but they perform similar temperature-control functions in HVAC systems.
These devices use an electric motor to position a valve according to signals from:
- Thermostats
- Sensores de temperatura
- HVAC controllers
- Building Management Systems (BMS)
Motorized actuators may provide either on/off control or modulating control.
With modulating control, the valve can remain partially open rather than simply fully open or fully closed.
This allows water flow to be adjusted more precisely according to actual heating or cooling demand.
Normally Open vs. Normally Closed Thermal Actuators
Another important classification is the actuator’s default operating position.
Normally Closed (NC)
A normally closed actuator keeps the valve closed when electrical power is not supplied.
When energized, the actuator opens the valve.
Normally closed designs are commonly selected when water flow should automatically stop if the control signal or electrical supply is interrupted.
Normally Open (NO)
A normally open actuator keeps the valve open when electrical power is unavailable.
When energized, the actuator closes the valve.
This configuration may be selected when system design requires continuous flow during a power interruption.
A opção correta depende de:
- HVAC system configuration
- Requisitos de segurança
- Estratégia de controlo
- Energy-management requirements
- Fail-safe operation
How to Select a Thermal Actuator
Selecting the correct actuator requires more than choosing between normally open and normally closed models.
Important technical factors include:
- Valve compatibility
- Operating voltage
- Actuator stroke length
- Closing or operating force
- Response time
- Control signal type
- Normally open or normally closed configuration
- Ambient operating conditions
- Connection dimensions
- BMS compatibility
Engineers should also determine whether the application requires simple on/off operation or proportional control.
For commercial and industrial projects, actuator selection should be coordinated with the overall HVAC control strategy rather than treated as an isolated component decision.
Jinyi HVAC provides industrial and commercial HVAC solutions covering equipment selection, customized control strategies, commissioning support, and BMS integration for different project requirements.
Conclusão
Thermal actuators provide reliable automatic valve control, helping HVAC systems improve temperature regulation, energy efficiency, system stability, and overall operational performance.
