Thermal actuators have become part of many devices and machines that require motion energy. The various thermal actuator types are mechanical systems that create motion through the expansion and contraction of materials, using a thermal induction process.
We can easily find various types of thermal actuators in applications that need a small footprint, high force output, and low voltage. All types of thermal actuators have piston components and contain thermally sensitive materials such as wax.
This wax material expands as the temperature increases. It is precisely calculated so that it pushes the piston out of the actuator. When the thermal actuator encounters a decrease in temperature, the thermally sensitive material contracts, and the piston returns to its initial position. The combination of all its components produces linear movement in response to temperature changes. For a deeper explanation of this mechanism, see our article on thermal actuator working principles.
These non-electric motors are an important part of the automotive, agricultural, aerospace, and many other industries. Most wax-based thermal actuators rely on thermal expansion rather than complex motorized drives. They rely only on changes in temperature to do what the system needs, such as opening or closing valves, releasing latches, and operating switches.
Contents
- 1 Thermal Actuator Types by Function (Most Common Classification)
- 2 Comparison Table: Thermal Actuator Types by Function
- 3 Thermal Actuator Types by Internal Mechanism (Engineering Classification)
- 4 Comparison Table: Thermal Actuator Types by Internal Mechanism
- 5 Relationship Between Functional and Mechanical Types
- 6 Which Thermal Actuator Type Should You Use?
- 7 Frequently Asked Questions
- 7.1 What are the main types of thermal actuators?
- 7.2 What is the difference between an ON/OFF and a proportional thermal actuator?
- 7.3 Do I need to choose a thermal actuator based on its internal mechanism?
- 7.4 What does the 920074PL water pump control actuator do differently?
- 7.5 Which Legom thermal actuator should I choose for underfloor heating?
Thermal Actuator Types by Function (Most Common Classification)
In practical applications and market usage, thermal actuator types are most commonly classified based on how they operate and how they are controlled.
ON/OFF Wax Thermal Actuators
ON/OFF wax thermal actuators are the most widely used type. They operate in a simple open or close manner. When the electrical heating element reaches a specific level, the wax expands and drives the piston to its working position. When the temperature drops, the actuator returns to its original position with the help of an internal or external spring.
This type is commonly used in underfloor heating systems, radiator valves, fan coil units, and hydronic HVAC applications where precise modulation is not required.
Proportional Thermal Actuators
Proportional thermal actuators are designed to provide gradual or proportional movement instead of simple ON/OFF action. The actuator stroke varies according to the control signal, typically 0 to 10V or similar analog control.
This type is suitable for advanced HVAC systems that require better temperature regulation, energy efficiency, and smoother valve control. To see how this solves real comfort problems, read our article on the Proportional Thermal Actuator (0-10V).
Smart Thermal Actuators
Smart thermal actuators integrate wireless or digital control technologies such as Wi-Fi, Zigbee, or Tuya systems. While the internal working principle remains based on thermal expansion, the control and communication are handled through smart controllers or building management systems. These actuators are commonly used in smart homes and modern commercial buildings.
Comparison Table: Thermal Actuator Types by Function
| Type | Control Method | Movement Style | Typical Applications |
|---|---|---|---|
| ON/OFF Wax Thermal Actuator | Fixed ON/OFF | Linear, fixed stroke | Underfloor heating, radiator valves |
| Proportional Thermal Actuator | 0 to 10V / analog | Variable, proportional | Advanced HVAC, energy-efficient systems |
| Smart Thermal Actuator | Wireless / digital | ON/OFF or proportional | Smart homes, BMS systems |
Thermal Actuator Types by Internal Mechanism (Engineering Classification)
From an engineering and design perspective, thermal actuators can also be classified according to their internal construction.
Pusher Thermal Actuator
The pusher thermal actuator is a type that does not use a diaphragm or elastomeric bag. It delivers relatively high pushing force because there is no flexible membrane limiting movement. The main components include wax material, a piston, a vessel, and sealing elements.
Because there is no elastomer bag to contain the melted wax, this type requires very precise sealing and manufacturing accuracy. Any leakage may directly affect performance and reliability.
Flat Diaphragm Thermal Actuator
This is one of the commonly used thermal actuator designs in general applications. The diaphragm has controlled deformability and returns to its original shape when external force is removed. Among common designs, the flat diaphragm type usually has a shorter output stroke, typically around 3 to 4 mm.
In this design, the piston is positioned above the diaphragm, which requires a longer piston guide for stable movement. When the temperature rises, the wax expands and pushes against the diaphragm. The diaphragm transfers this force to the piston. When the temperature decreases, an external spring helps return the piston to its original position. This type is suitable for applications that do not require long stroke lengths but demand stable and repeatable operation.
Press-On Thermal Actuator
Press-on thermal actuators use an elastomeric bag to contain the wax material. The piston is surrounded by wax inside this flexible bag. When the wax expands due to heat, the elastomer bag presses against the piston and generates movement.
This design allows for a more compact structure and often supports longer stroke lengths compared to flat diaphragm types. An external spring is usually required to reset the actuator once the temperature decreases.
Comparison Table: Thermal Actuator Types by Internal Mechanism
| Mechanism Type | Stroke Length | Force Capability | Structural Characteristics |
|---|---|---|---|
| Pusher Type | Medium | High | No diaphragm, high sealing accuracy required |
| Flat Diaphragm Type | Short (≈3 to 4 mm) | Medium | Stable, widely used, simple structure |
| Press-On Type | Medium to long | Medium to high | Compact, elastomer bag design |
Relationship Between Functional and Mechanical Types
In real products, functional types and internal mechanisms are closely related. A smart or proportional thermal actuator may internally use a flat diaphragm or press-on structure. Therefore, classification by function helps users choose the right product, while classification by internal mechanism helps engineers understand design and performance characteristics.
Which Thermal Actuator Type Should You Use?
When selecting a thermal actuator, users do not need to choose based on the internal mechanical structure of the actuator. Internal designs such as diaphragm or wax capsule forms are determined by the manufacturer and do not affect installation compatibility or system operation in most HVAC applications.
Selection should instead be based on functional and application requirements, including control method (ON/OFF, proportional, or smart control), operating voltage, normally open or normally closed behavior, stroke length, output force, valve connection size, and the intended working environment. By focusing on these parameters, users can reliably select the appropriate thermal actuator for their system without needing to consider internal construction details. For a detailed quality and durability checklist, see our guide on how to choose a high-quality thermal actuator.
“Customers sometimes ask which internal mechanism they should specify, pusher, flat diaphragm, or press-on. My answer is usually that for a standard HVAC installation, that is our job to decide, not theirs. What the buyer actually needs to get right is the functional fit: the control signal their system sends, whether they need normally closed or normally open, the valve thread, and the force required to drive that valve. Get those four right and the actuator works. The internal construction matters to us as the manufacturer because it determines how we hit the force and stroke targets, but it should be invisible to the person installing it.”
— Maggie Shen, Founder of Legom
Product Classification by Thermal Actuator Type (Legom Products)
Based on functional classification, the following table shows how Legom thermal actuator products are categorized according to control method and application.
| Product Model | Functional Type | Typical Application |
|---|---|---|
| 920018PL | ON/OFF Wax Thermal Actuator (manual opening & position indicator) | Underfloor heating and hydronic valve control |
| 920039PL | ON/OFF Wax Thermal Actuator | Underfloor heating and hydronic valve control |
| 920059PL | ON/OFF Wax Thermal Actuator | Underfloor heating and hydronic valve control |
| 920062PL | ON/OFF Wax Thermal Actuator | Underfloor heating manifold valve control |
| 920066PL | ON/OFF Wax Thermal Actuator | Underfloor heating and hydronic valve control |
| 920067PL | ON/OFF Wax Thermal Actuator | Underfloor heating manifold valve control |
| 920071PL | Smart Thermal Actuator (WiFi/Zigbee/Tuya) | Smart underfloor heating and building automation |
| 920074PL | Water Pump Control Type (integrated micro switch) | Pump activation in underfloor heating circulation systems |
| 920075PL | Smart Thermal Actuator (WiFi/Zigbee/Tuya) | Smart underfloor heating manifold valve control |
| 920080PL | Proportional Thermal Actuator (0 to 10V) | Modulating underfloor heating and hydronic valve control |
| 920083PL | ON/OFF Wax Thermal Actuator (manual opening & position indicator, NC/NO) | Underfloor heating and hydronic valve control |
Thermal Actuator Manufacturing Process
To better understand how thermal actuators are built and how their internal components work together, the following video shows the manufacturing process of a typical wax-based thermal actuator. The example shown is the Legom 920062 thermal actuator, which represents a common design used in HVAC valve control applications.
Note: The video is provided for reference to illustrate general manufacturing and assembly processes. Internal structures and processes may vary between models and configurations.
Product Introduction Example: 920062PL Thermal Actuator
To provide a clearer understanding of how a finished thermal actuator is applied in real HVAC systems, the following video introduces the Legom 920062PL thermal actuator. The video highlights the product’s design, intended applications, and role in precise temperature control for heating and valve systems.
Frequently Asked Questions
What are the main types of thermal actuators?
Thermal actuators are most commonly classified by function into three types: ON/OFF wax actuators that simply open or close a valve, proportional actuators that modulate valve position based on a 0 to 10V signal, and smart actuators that add wireless control such as WiFi, Zigbee, or Tuya. From an engineering perspective, they can also be classified by internal mechanism into pusher, flat diaphragm, and press-on types. For most buyers, the functional classification is what matters when selecting a product, while the internal mechanism is a manufacturer design decision.
What is the difference between an ON/OFF and a proportional thermal actuator?
An ON/OFF actuator has only two states: fully open or fully closed. It switches the valve completely on or off in response to a thermostat signal, and is the standard choice for most underfloor heating zone control. A proportional actuator accepts a 0 to 10V control signal, allowing the valve to be positioned anywhere across its travel. This enables finer flow regulation and smoother temperature control, making it suitable for advanced HVAC systems and modulating controllers. In the Legom range, the 920080PL is the proportional model, while most other models are ON/OFF.
Do I need to choose a thermal actuator based on its internal mechanism?
No. For most HVAC applications, the internal mechanism (pusher, flat diaphragm, or press-on) is determined by the manufacturer and does not affect installation compatibility or system operation. You should select based on functional and application requirements instead: control method, operating voltage, normally open or normally closed behavior, stroke length, output force, and valve connection size. These functional parameters determine whether the actuator fits and works in your system, while the internal construction is the manufacturer’s way of meeting those targets.
What does the 920074PL water pump control actuator do differently?
The 920074PL is a specialized model with an integrated micro switch rated up to 700mA. In addition to controlling a manifold valve, it can start a circulation pump when there is a heat demand. This makes it well suited to single-zone and distributed heating layouts where the pump should only run when a zone is actually calling for heat, improving energy efficiency. It operates at 220V AC with a normally closed direction and an M30 × 1.5mm adapter, like the standard models, but the micro switch gives it the additional pump-control function.
Which Legom thermal actuator should I choose for underfloor heating?
For standard underfloor heating zone control, an ON/OFF model such as the 920039PL, 920059PL, 920066PL, or 920067PL is the typical choice, with normally closed operation as the safe default. If you want smart-home or app control, the 920071PL or 920075PL add WiFi, Zigbee, and Tuya connectivity. For proportional, modulating control in advanced systems, the 920080PL is the right model. Models such as the 920018PL and 920083PL add a manual opening feature and position indicator for easier installation. Confirm your controller signal, voltage, valve thread (M30 or M28 × 1.5mm), and required force before selecting, or contact the Legom team for guidance.
Reviewed and updated by the LEGOM Technical Team on June 26, 2026. This article explains thermal actuator types by function (ON/OFF, proportional, smart) and by internal mechanism (pusher, flat diaphragm, press-on), how to select the right type, and how the Legom thermal actuator range is classified by control method and application.