Thermal actuators are all around us, usually working unnoticed. They sit in car engines, dishwashers, washing machines, showers, greenhouses, and heating manifolds. Any device that opens or closes a valve in response to temperature, without a motor or an electrical control signal, is likely using one.
This guide gives concrete examples of where thermal actuators are used, explains what they do in each case, and covers the element types that suit different applications.
What a Thermal Actuator Does
A thermal actuator converts heat energy into mechanical motion. Inside it sits a temperature-sensing material, almost always a specially formulated wax, sealed within a housing. As the wax warms it expands, and because the housing constrains it in every direction except one, that expansion drives a piston outward. When the temperature falls, the wax contracts and a spring returns the piston.
The significance is that no external power source is required. The device senses the condition and responds to it mechanically, in a single step. That is why thermal actuators appear so widely: they are simple, need no wiring, cannot be switched off accidentally, and continue working during a power failure.
Everyday Thermal Actuator Examples
The clearest way to understand the technology is through specific cases.
Car engine thermostat
Probably the most numerous thermal actuator in existence. When a cold engine starts, the thermostat stays closed, blocking coolant from reaching the radiator so the engine warms quickly. Once the coolant reaches the design temperature, the wax expands, opens the valve, and coolant begins circulating through the radiator.
The whole system holds the engine at its optimum operating temperature with no sensor, no wiring, and no electronic control. This is also the application that made wax elements commercially widespread.
Dishwashers and washing machines
Thermal actuators control water temperature and, in some designs, the release of detergent at the correct point in the cycle. A wax element responds to the water temperature directly, which is more robust in a wet, vibrating environment than an electronic sensor and switch would be.
Shower and tap mixing valves
A thermostatic mixing valve blends hot and cold water to deliver a stable temperature at the outlet. Inside, a wax element senses the mixed water and shifts the valve toward hot or cold to hold the set point.
This is what prevents a shower running cold when a tap is opened elsewhere in the house, and what stops it running dangerously hot if the cold supply fails. A domestic shower typically delivers water around 38 to 41°C, and the valve holds it there despite fluctuations in either supply.
Underfloor heating manifolds
This is the largest application in building services. A thermal actuator is fitted to each branch valve on a manifold, and it opens or closes that heating circuit in response to the room thermostat for that zone.
Here the actuator is electrically operated rather than purely self-acting: a PTC heating element warms the wax on command from the thermostat. The wax then does the mechanical work. This is what allows room-by-room temperature control in a hydronic heating system, and it is why a large house may contain a dozen or more of them.
Radiator valves
A thermostatic radiator valve uses a wax element to sense room air temperature and throttle the flow of water into that radiator accordingly. It is entirely self-acting, needing no wiring at all, which is why TRVs can be retrofitted to existing radiators so easily.
Greenhouse vent openers
An elegant example of the principle at its simplest. As a greenhouse warms in sunlight, a wax cylinder expands and pushes the roof vent open. As it cools in the evening, the vent closes. No electricity, no sensor, no timer, and nothing to fail except the element itself.
Livestock watering systems
In agriculture, thermal actuators regulate water flow and provide freeze protection in outdoor drinkers, keeping water available in cold weather without any power supply reaching the field.
Solar water heating
Solar collectors produce very high and unpredictable storage temperatures, since output is set by the weather rather than a thermostat. Thermal actuators inside mixing and safety valves manage that variability, tempering the delivered water and shutting off flow if it becomes dangerous.
Freeze protection on pipework
A freeze protection valve opens as water temperature approaches freezing, draining the pipe before ice can form and burst it. Because it is mechanical, it works during the power failures that so often accompany severe cold weather.
Thermal Actuator Applications by Industry
| Industry | Application | What the actuator does |
|---|---|---|
| HVAC and building services | Underfloor heating manifolds, radiator valves, mixing valves | Opens and closes zone circuits, blends water temperature |
| Automotive | Engine thermostats, transmission cooling, oil temperature control | Diverts coolant or oil to a cooler at the right temperature |
| Domestic appliances | Dishwashers, washing machines, water heaters | Controls water temperature and cycle timing |
| Plumbing and sanitary | Showers, taps, scald protection, TP relief valves | Holds safe delivery temperature, shuts off in danger |
| Agriculture | Greenhouse vents, livestock waterers | Opens vents on heat, protects water lines from freezing |
| Aerospace | Engine lubrication, safety shutdown, temperature control | Operates valves where long-term reliability is critical |
| Industrial process | Heat exchangers, oil circuits, cooling systems | Maintains process temperature without control wiring |
| Renewable energy | Solar thermal, heat pump circuits | Manages variable temperatures and protects components |
What connects these very different fields is a common requirement: something must respond to temperature reliably, in a location where running power and control wiring would be impractical, expensive, or a point of failure.
Why So Many Industries Use Them
Thermal actuators respond to even small temperature changes, and several characteristics explain their spread across sectors.
No external power source. The actuator uses the temperature change itself to perform the work, whether that is opening a valve or operating a switch. No wiring, no controller, no battery.
Simplicity. Few components, straightforward integration, and light designs. There is very little to go wrong compared with a sensor, controller, and motorised valve performing the same task.
Resistance to shock and vibration. Testing consistently shows good tolerance of mechanical stress, which is why they suit automotive, agricultural, and industrial environments where electronics struggle.
Material durability. Housings in stainless steel, aluminium, brass, or reinforced polymer withstand demanding conditions and install easily.
Works with gases, liquids, and vacuum. The sensing principle does not depend on the medium being electrically conductive or optically clear, so the same technology applies across a wide range of fluids.
Element Types and Which Application Suits Each
Wax elements come in three main constructions, and the choice depends on the stroke required, the space available, and the expected service life.
Press-on type
The piston is inserted into a bag surrounded by wax. As the wax expands, the bag presses on the piston and drives it out. This construction does not require a long piston guide, which allows a compact design.
It suits applications needing long-term reliability, smaller physical size, and a high stroke. Typical uses include livestock watering systems and automatic greenhouse ventilation, where the device must operate season after season without attention. The construction is also used in aerospace applications such as engine lubrication systems, safety shutdown, and temperature control, where durability over many years matters more than cost.
Pusher type
There is no diaphragm or bag in this construction. Wax and piston sit together within a container, which allows the wax to deliver its highest force. This suits applications where the actuator must overcome a stiff return spring or a valve requiring substantial closing force.
Flat diaphragm type
The most common construction in general applications. The piston sits directly on the diaphragm, which means a longer piston guide is required, and stroke is typically limited to around 4mm.
The automotive industry uses this type extensively for transmission cooling, oil temperature control, and engine thermostats. It is also the usual choice in HVAC applications where a short, precise stroke is all that is needed. For a fuller treatment, see our guide to the wax thermostatic element.
| Type | Stroke | Strength | Typical use |
|---|---|---|---|
| Press-on | High | Compact, long service life | Agriculture, aerospace, greenhouse vents |
| Pusher | High | Highest force output | Stiff valve assemblies |
| Flat diaphragm | Around 4mm | Precise, widely available | Automotive, HVAC, general valve control |
Self-Acting and Electrically Heated Actuators
One distinction is worth drawing because it separates two quite different modes of use.
A self-acting actuator responds to the temperature of the medium it sits in. A car thermostat and a greenhouse vent opener both work this way: nothing tells them what to do, they simply react to conditions.
An electrically heated actuator responds to a command. A PTC element inside warms the wax when the device is energised, which drives the piston. The thermal actuators on an underfloor heating manifold work this way, opening a zone when the room thermostat calls for heat.
Both use the same wax expansion principle. The difference is what supplies the heat that triggers it. This also explains why manifold actuators move gradually, typically taking three to five minutes to open or close fully: the wax must warm before it expands, and that gradual movement has the useful side effect of avoiding water hammer and pressure spikes in the pipework.
The Legom Thermal Actuator Range
Legom manufactures thermal actuators at its own facility in Jiaxing, Zhejiang Province, with an eleven-model range covering standard on/off zone actuators, manual-override units with position indicators, WiFi, ZigBee and Tuya smart actuators, water pump control models, and 0 to 10V proportional types.
| Specification | Legom range |
|---|---|
| Standard | EN 60730, with CE and RoHS certification |
| Housing material | PA66 with 30% glass fibre |
| Power consumption | 2W |
| Voltage options | 12V to 230V |
| Valve threads | M30 × 1.5mm and M28 × 1.5mm |
| Configurations | Normally closed and normally open |
| Protection class | IP41 to IP54 |
| Wax element | Manufactured in-house, rated for 100,000 cycles |
Because the wax elements are produced in the same facility rather than sourced externally, switching characteristics can be specified at the source rather than selected from what the market offers. Our guides to thermal actuator working principles and actuator types cover the technology and range in more detail.
“People are often surprised how many of these they already own without knowing it. The thermostat in your car is one. The valve in your shower is another. If you have underfloor heating there is one on every loop at the manifold. What they all have in common is that somebody looked at a problem and decided the most reliable answer was not a sensor and a controller and a motor, but a material that changes size when it gets warm. That is a hundred-year-old idea and it is still winning in applications where you cannot run a cable or cannot afford the thing to stop working when the power goes out.”
— Maggie Shen, Director of Legom
Working with Legom
As a thermal actuator manufacturer, Legom supports partners in applying the working principle correctly for their application. We supply OEM manifold builders, underfloor heating system integrators, and HVAC distributors in more than 90 countries through over 400 active partners.
OEM and ODM services cover voltage, stroke, force, response time, housing colour, and branding, with production batch test records available and all projects handled under confidentiality agreement. If you are looking for a partner or OEM supplier for a thermal actuator application, contact the technical team to discuss specification.
Frequently Asked Questions
What is an example of a thermal actuator?
The most common example is the thermostat in a car engine, which keeps coolant from reaching the radiator until the engine has warmed to its operating temperature. Other everyday examples include the mixing valve in a shower that holds water at a safe temperature, the actuators on an underfloor heating manifold that open and close each room’s circuit, thermostatic radiator valves, greenhouse vent openers that push a window open as the air warms, and the temperature controls inside dishwashers and washing machines.
How does a thermal actuator work?
It converts heat into motion using a wax sealed inside a housing. As the wax warms it expands, and because the housing constrains it in all directions but one, that expansion drives a piston outward. When the temperature falls, the wax contracts and a return spring resets the piston. No external power, wiring, or controller is required for a self-acting actuator, which is what makes the technology so widely applicable.
What industries use thermal actuators?
HVAC and building services use the most, in underfloor heating manifolds, radiator valves, and mixing valves. Automotive uses them in engine thermostats, transmission cooling, and oil temperature control. Domestic appliances use them for water temperature control. Agriculture uses them for greenhouse ventilation and livestock watering. Aerospace uses them where long-term reliability is critical, in engine lubrication and safety shutdown systems. Industrial process, plumbing, and renewable energy applications all use them as well.
Do thermal actuators need electricity?
Self-acting ones do not. A car thermostat, a greenhouse vent opener, and a thermostatic radiator valve all respond directly to the temperature around them with no power supply at all. Electrically heated actuators, such as those on an underfloor heating manifold, use a small PTC element to warm the wax on command from a thermostat, and these draw around 2W. Even in that case, the mechanical work is done by wax expansion rather than by a motor.
What is the difference between the three element types?
Press-on type places the piston in a bag surrounded by wax, allowing a compact design with high stroke and long service life, which suits agriculture and aerospace. Pusher type has no diaphragm or bag, so the wax delivers its highest force, which suits stiff valve assemblies. Flat diaphragm type places the piston directly on the diaphragm, giving a shorter stroke of around 4mm with a longer piston guide, and it is the most common construction in automotive and HVAC applications.
Why do underfloor heating actuators take several minutes to open?
Because they work by heating wax rather than by driving a motor. When the thermostat calls for heat, a PTC element inside the actuator warms the wax charge, and only as the wax expands does the piston move. That takes typically three to five minutes to open or close fully. The gradual movement is a design feature rather than a limitation: it avoids the water hammer and pressure spikes that a fast-acting solenoid valve would cause in the pipework.
How long does a thermal actuator last?
Service life depends mainly on the wax element, which is the component that ultimately wears through repeated expansion and contraction. A quality element retains a high proportion of its original performance after 100,000 operating cycles, while a poorly made one degrades noticeably sooner and gradually fails to open the valve fully. When comparing suppliers, ask about the cycle rating and the accuracy retained at that point rather than the headline specification when new.
What is the difference between normally closed and normally open?
A normally closed actuator keeps the valve shut when no power is applied and opens it when energised by a thermostat signal. A normally open actuator does the reverse, holding the valve open at rest and closing it when energised. Normally closed is standard for underfloor heating zone control, because it stops water flow automatically during a power outage. Normally open configurations are used where maintaining flow is the safer default condition, such as some district heating positions.
Reviewed by Maggie Shen, Director at Legom, on August 4, 2026. This guide to thermal actuator examples and applications was reviewed for technical accuracy, including element construction types and the distinction between self-acting and electrically heated actuators.