Hotels, office towers, and apartment blocks rely on fan coil units to maintain comfort room by room. What determines whether each room actually holds its set temperature is a small component mounted on the FCU valve: the thermal actuator.
This guide covers how thermal actuators work in fan coil applications, the Normally Closed and Normally Open decision and what it means during a power outage, and the compatibility parameters that must match before anything is ordered.
How a Thermal Actuator Controls a Fan Coil Unit
The operating principle is straightforward. A PTC heating element warms a sealed wax charge inside the actuator. As the wax warms it expands, and because the capsule constrains it in every direction except one, that expansion drives a piston. The piston operates the valve beneath.
In a fan coil unit, that valve regulates the flow of hot or chilled water through the coil. The fan runs continuously or on demand, and controlling how much water reaches the coil is what controls the air temperature leaving it.
Two operational characteristics make this design well suited to occupied buildings.
Silent operation. There is no solenoid click, no motor, and no hydraulic shock. In hotel bedrooms and offices, where acoustic comfort matters and equipment sits close to occupants, this is a genuine advantage over fast-acting alternatives.
Low power draw. Legom actuators consume 2W across the entire range. In a building with hundreds of fan coil units, that figure multiplied across every zone is a meaningful part of the electrical design.
Why the slow action is a feature. Standard thermal actuators take three to five minutes to complete their opening or closing cycle, because the wax must warm before it expands. Installers new to them sometimes assume a fault. The gradual movement protects the valve seat and avoids the water hammer and pressure surges that a fast-acting solenoid creates in a building with many zones opening and closing throughout the day.
Installation and Control
The actuator mounts directly onto the FCU valve, typically via a threaded ring nut connection, and a room thermostat provides the control signal.
In most installations that signal is simple on/off: the thermostat energises the actuator when the room calls for heating or cooling, and de-energises it when satisfied. Legom actuators accept supply voltages from 12V to 230V, so the choice is determined by what the thermostat or controller outputs.
For applications requiring proportional control rather than on/off, modulating actuators accepting a 0 to 10V signal position the valve across its travel in response to a varying control voltage. These integrate with building management systems in larger commercial installations where finer flow regulation is required.
Confirm the valve you intend to use supports the control type you need, since not every fan coil valve is designed for modulating actuation.
Normally Closed or Normally Open?
This is the decision that shapes how the system behaves when something goes wrong, and it cannot be changed after the actuators are installed without replacing them.
| Condition | Normally Closed (NC) | Normally Open (NO) |
|---|---|---|
| No power applied | Valve closed, no flow | Valve open, flow continues |
| Energised by thermostat | Valve opens | Valve closes |
| During a power outage | Flow stops | Flow continues uncontrolled |
| Energy behaviour | Energy saved by default | Energy consumed by default |
| Typical application | Hotel and office FCU zone control | Freeze protection, district heating positions |
Why NC dominates fan coil applications
Most hotel and office FCU installations use Normally Closed, and the reasoning is sound.
When power fails, the valve closes and water stops flowing through the coil. The room does not become progressively colder or hotter from uncontrolled circulation while nobody is monitoring it. Energy is not consumed by default. And in a building where a floor of rooms may be unoccupied for periods, closed is the sensible resting state.
Legom Fan Coil Valves are specified for compatibility with NC two-wire thermal actuators, which reflects this being the established convention for the application.
Where NO makes sense
Normally Open holds the valve open at rest, so flow continues when power is lost. This suits situations where maintaining circulation is the safer default: some district heating connections, gravity circulation layouts, and positions where stopping flow creates a risk rather than avoiding one.
Does NC always give the best protection?
This is worth examining rather than assuming, because the answer is more nuanced than the general rule suggests.
In a normal outage lasting minutes or hours, NC is clearly correct. The building retains its thermal mass, temperatures drift slowly, and stopping uncontrolled flow avoids the room overshooting in either direction.
The case that deserves thought is an extended outage in cold conditions with unoccupied rooms. With NC actuators, every valve in the building closes. No warm water circulates through any coil. In a cold climate, a building with no heat and no circulation is a building where pipework can eventually freeze, and burst pipes cause damage far exceeding any comfort concern.
This does not make NC the wrong choice for fan coil zone control. It means freeze protection should be a separate strategy rather than something you expect actuator selection to provide. Options include maintaining circulation on the primary circuit independently of zone valves, glycol in the water where the system permits it, low-temperature alarms, or backup power to critical circulation equipment.
Legom Fan Coil Valves are compatible with ethylene glycol solutions up to 50%, which covers standard freeze protection concentrations without requiring an intermediate heat exchanger.
Compatibility: What Must Match
Three parameters determine whether an actuator will work with a given fan coil valve, and confirming them before ordering avoids the most common procurement error in this category.
1. Thread and connection
The actuator attaches to the valve via a threaded connection, and the two must match. M30×1.5mm is the most widely used standard across fan coil and manifold valves, and Legom supplies both M30×1.5mm and M28×1.5mm.
Legom Fan Coil Valves use an M30×1.5 threaded ring nut, which pairs directly with standard actuators using that thread.
One installation note: hand-tighten the ring nut. Using tools to overtighten damages the actuator housing, and a cracked housing on a component controlling a water valve is a problem that appears later rather than immediately.
2. Stroke
The actuator must be able to move the valve through its full travel. Legom Fan Coil Valves have a plug stroke of 2.5mm, while standard Legom actuators deliver ≥4mm or ≥3.4mm depending on model, comfortably exceeding what the valve requires.
Where actuator stroke falls short of valve travel, the valve never fully opens or closes, and the symptom is a zone that never quite reaches temperature or never quite shuts off.
3. Force
The actuator must overcome the valve’s return spring. Standard Legom models deliver 100 ± 10N, with higher-force options at 110N and 120N for valves with greater spring resistance.
An underpowered actuator on a stiff valve produces intermittent operation that is difficult to diagnose, because it works sometimes and not others depending on system pressure.
And the control signal
Beyond the mechanical fit, the actuator must match the controller output. Confirm the voltage your room thermostat or building management system provides, and whether the application requires on/off or proportional control.
Does 230V or 24V Affect Reliability?
A question that comes up regularly in retrofit discussions, and the answer is more practical than technical.
Both voltage variants use the same wax motor principle, the same PTC heating element approach, the same PA66 + 30% glass fibre housing, and the same 2W power draw. Both comply with EN 60730 and carry CE and RoHS certification. There is no inherent reliability difference between them.
What determines the choice is the control system rather than durability.
230V suits systems where the room thermostat switches mains directly to the actuator, which is common in simpler installations and much European residential practice.
24V suits systems using a central controller or building management system, which is more typical in commercial fan coil installations, and it simplifies low-voltage wiring routing in a large building.
What does affect service life is cycle count and build quality. An actuator in a fan coil unit responding to a thermostat throughout the day accumulates cycles considerably faster than one on an underfloor heating manifold, so endurance testing at manufacture matters more in this application than the voltage on the label.
Choosing the Fan Coil Valve
The actuator is only half the assembly. The valve it drives determines how the circuit behaves.
2-way valves
One inlet and one outlet. The valve either allows flow through the fan coil or blocks it entirely.
Simple and economical, but when it closes, flow through that circuit stops completely. In a system with a constant-speed pump and no differential pressure bypass, several zones closing simultaneously can cause pressure surge. 2-way valves therefore suit systems with variable-speed pumps that can respond to changing flow.
3-way valves
Three ports, redirecting flow rather than stopping it. When the valve closes the fan coil path, it simultaneously opens a by-pass, so total circuit flow stays constant and the pump is protected from surge.
The Legom 3-way design controls both the straight-through and by-pass paths simultaneously through a single plug, which means the same valve body works as either a diverting or mixing valve depending on how it is piped. In diverting service, flow enters the AB port and is directed to either the fan coil or the by-pass. In mixing service, flow enters from two ports and exits blended through AB.
That dual capability is practically useful: one valve body covers both functions, so stock and procurement are simplified.
3-way with 4-port built-in by-pass
The by-pass is integrated into the valve body rather than requiring separate pipework. Total circuit flow stays constant as the valve modulates between fan coil and by-pass, which suits constant-speed pump systems with no separate differential pressure bypass.
Sizing by Kvs
Valve sizing is determined by Kvs, the flow coefficient in m³/h at 1 bar pressure drop, and getting it wrong is a common source of poor control.
For adequate control authority, the pressure drop across the valve at design flow should be at least 30 to 50% of the total circuit pressure drop. A valve that is too large relative to the circuit has little authority: most of its travel produces almost no change in flow, and control becomes effectively on/off regardless of how precisely the actuator positions it.
| Size | 2-way Kvs (m³/h) | 3-way straight | 3-way by-pass |
|---|---|---|---|
| DN15 (1/2″) | 1.7 | 1.7 | 1.6 |
| DN20 (3/4″) | 2.8 | 2.8 | 2.0 |
| DN25 (1″) | 3.1 | 2.4 | 1.4 |
The Legom Fan Coil Range
Fan coil valves
| Parameter | Specification |
|---|---|
| Configurations | 2-way, 3-way, 3-way with 4-port built-in by-pass |
| Sizes | DN15, DN20, DN25 (1/2″, 3/4″, 1″) |
| Body material | Brass, with nickel-plated brass stem |
| Spring and plug | Stainless steel spring, EPDM plug rubber |
| Maximum pressure | 16 bar constant Kv, 10 bar variable Kv |
| Fluid temperature | 4°C to 110°C |
| Compatible liquids | Water, ethylene glycol up to 50% |
| Plug stroke | 2.5mm |
| By-pass leakage | <0.02% Kvs |
| Actuator connection | Threaded ring nut M30 × 1.5 |
| Control direction | NC two-wire actuators |
The 16 bar rating on constant Kv models matters in high-rise applications, where static column pressure at lower floors is a design consideration. And by-pass leakage below 0.02% Kvs means the closed path is genuinely closed, which improves zone control accuracy.
Full details are on the fan coil valves product page.
Thermal actuators
Legom manufactures an 11-model thermal actuator range at its facility in Jiaxing, Zhejiang Province. Several models suit fan coil applications.
| Model | Voltage | Control | Force | Key note |
|---|---|---|---|---|
| 920066PL | 12V to 230V | NC / NO | 100 ± 10N | Standard, dual adapter, IP54 |
| 920039PL | 12V to 230V | NC / NO | 100 ± 10N | Easy tool-free removal for maintenance |
| 920018PL / 920083PL | 230V / 24V / 12V | NC / NO | 100 ± 10N | Manual override lever and position indicator |
| 920062PL | 12V to 230V | NC | 100 ± 10N | Optional fast 60 to 90 second actuation |
| 920080PL | 24V DC | 0 to 10V modulating | 120 ± 10N | Proportional control, highest force in range |
All models share EN 60730 compliance, CE and RoHS certification, PA66 + 30% glass fibre housing, and a 2W power draw, with M30×1.5mm and M28×1.5mm adapter options.
Two are worth highlighting for fan coil work. The 920018PL and 920083PL include a manual opening lever and visual position indicator, which allows a technician to open a valve by hand during commissioning or an outage and to confirm valve state at a glance during inspection. In a hotel with hundreds of rooms, being able to verify state visually rather than by measurement saves considerable time.
The 920062PL offers optional 60 to 90 second actuation where the standard three to five minutes is too slow for the application.
Where proportional control is required, the 920080PL accepts a 0 to 10V signal with energy-saving, comfort, heating, and off modes for building management system integration. Confirm valve compatibility for modulating service, since the standard fan coil valve range is specified for NC two-wire actuators.
“The question I would put back to anyone specifying NC for a hotel is what happens on a long outage in winter. NC is right for fan coil zone control, and I would specify it every time, but people sometimes assume it protects the building and it does not. Every valve closes, nothing circulates, and if the outage runs long enough in a cold climate you have a pipework problem rather than a comfort problem. Freeze protection needs to be its own strategy: glycol, primary circulation that does not depend on zone valves, low-temperature alarms. Choose NC for the reasons NC is good, and solve freezing separately.”
— Maggie Shen, Director of Legom
Specifying the Assembly
Five parameters, confirmed together, produce a working pairing.
1. Valve configuration. 2-way for variable-speed pump systems, 3-way where constant flow must be maintained, 4-port where a built-in by-pass avoids separate pipework.
2. Valve size by Kvs. Sized for adequate control authority, meaning 30 to 50% of circuit pressure drop across the valve at design flow.
3. Control direction. NC for standard fan coil zone control, NO only where continuous flow is the safer default.
4. Actuator voltage and signal. Matched to the thermostat or building management system output, on/off or 0 to 10V.
5. Thread, stroke, and force. M30×1.5 for the Legom fan coil range, actuator stroke exceeding valve travel, and force sufficient for the valve spring.
Legom manufactures both the actuators and the fan coil valves at the same facility, which removes compatibility uncertainty from the specification. Our HVAC valve range and room thermostats complete the control loop, and OEM and ODM services cover voltage, control direction, adapter thread, cable length, force output, and housing colour.
Contact the technical team with your valve configuration, controller output, and thread specification to confirm the pairing.
Frequently Asked Questions
How does a thermal actuator work on a fan coil unit?
A PTC heating element inside the actuator warms a sealed wax charge. As the wax expands it drives a piston that operates the valve controlling water flow through the fan coil. When the thermostat de-energises it, the wax cools, contracts, and a return spring resets the valve. The action is silent and gradual, taking three to five minutes on standard models, which protects the valve seat and avoids the hydraulic shock a fast solenoid would create.
Should I use Normally Closed or Normally Open for fan coil units?
Normally Closed for standard zone control, and it is what most hotel and office installations use. On power loss the valve closes, stopping flow so the room does not drift from uncontrolled circulation, and energy is not consumed by default. Normally Open holds the valve open at rest, which suits positions where maintaining flow is the safer default such as some district heating connections. Legom fan coil valves are specified for NC two-wire actuators.
Does NC protect the building during a long power outage?
Not against freezing, and this is worth understanding. During a normal outage NC is clearly correct. But in an extended outage in cold conditions, every NC valve in the building closes and nothing circulates, which in a cold climate can eventually allow pipework to freeze. Freeze protection should be a separate strategy rather than something actuator selection provides: glycol in the circuit, primary circulation independent of zone valves, low-temperature alarms, or backup power to critical equipment.
How do I know if an actuator fits my existing fan coil valve?
Three parameters must match. The thread, with M30×1.5mm the most common standard and M28×1.5mm also widely used. The stroke, which must exceed the valve’s travel, since an actuator with insufficient stroke never fully opens or closes the valve. And the force, which must overcome the valve’s return spring, with 100N standard and higher-force options available for stiffer valves. Confirm all three from the valve datasheet before ordering.
Is there a reliability difference between 230V and 24V actuators?
No inherent difference. Both use the same wax motor principle, the same housing material, the same 2W power draw, and both comply with EN 60730 with CE and RoHS certification. The choice is determined by your control system: 230V where the thermostat switches mains directly, 24V where a central controller or building management system is used, which is more typical in commercial fan coil installations. What does affect service life is cycle count and build quality rather than voltage.
What is the difference between 2-way and 3-way fan coil valves?
A 2-way valve either allows flow through the fan coil or blocks it, so when it closes, flow through that circuit stops entirely. This can cause pump pressure surge in systems with constant-speed pumps and no differential pressure bypass. A 3-way valve redirects rather than stops: closing the fan coil path simultaneously opens a by-pass, keeping total flow constant and protecting the pump. 2-way suits variable-speed pump systems; 3-way suits constant-speed installations.
How do I size a fan coil valve?
By Kvs, the flow coefficient in m³/h at 1 bar pressure drop. For adequate control authority, the pressure drop across the valve at design flow should be 30 to 50% of the total circuit pressure drop. A valve that is too large has poor authority: most of its travel produces almost no change in flow, so control becomes effectively on/off however precisely the actuator positions it. Use the manufacturer’s flow curves to verify the selection at your design flow rate.
Why do thermal actuators take several minutes to operate?
Because they work by heating wax rather than driving a motor. The PTC element must warm the wax charge before it expands enough to move the piston, which takes three to five minutes on standard models. This is a design feature rather than a limitation: the gradual movement protects the valve seat and avoids water hammer and pressure spikes, which matters in a building where many zones open and close throughout the day. Faster 60 to 90 second models exist where quicker response is needed.
Reviewed by Maggie Shen, Director at Legom, on September 25, 2026. This guide to thermal actuators for fan coil units was reviewed for technical accuracy, including actuator and valve compatibility parameters and the limits of NC configuration as freeze protection.