An NTC sensor is the component inside a thermostat that actually measures temperature. Everything else the thermostat does, from displaying a reading to deciding whether to call for heat, depends on what this small resistor reports.
NTC stands for Negative Temperature Coefficient, meaning its electrical resistance falls as temperature rises. The thermostat measures that resistance and converts it into a temperature reading. This guide covers how it works, the specification that determines whether a sensor will work with your thermostat, where to place it, and how to tell when one has failed.
What an NTC Sensor Does, and What It Does Not
Being precise here matters, because the term is often used loosely.
An NTC thermistor is a passive resistor made from semiconductor material. It has no electronics inside it, no power supply of its own, and no ability to communicate. It does exactly one thing: its resistance changes predictably with temperature.
The thermostat passes a small current through it, measures the resulting resistance, and converts that value into a temperature using a calibration curve. All of the intelligence sits in the thermostat, not in the sensor.
Three clarifications worth making. An NTC sensor measures temperature only, not pressure or humidity, which require entirely different sensing devices. It does not transmit data wirelessly, since it is a passive component with two wires; where a thermostat sends readings to an app, the thermostat’s own electronics do that. And it does not cut off current to prevent overheating, which is a function of a different component type sometimes confused with it. The NTC sensor reports a temperature, and the control circuit decides what to do about it.
The material and why resistance falls
The sensing element is made from metal oxide semiconductors such as nickel oxide, manganese oxide, copper oxide, and iron oxide, formed into a dense, tightly packed body. As temperature rises, electrons gain enough energy to move into the material’s conductive band, which increases conductivity and therefore reduces electrical resistance.
This relationship is steep and repeatable, which is why NTC thermistors are used where accuracy matters. They typically cover a range from around -50°C to 120°C, well beyond anything a building control application requires.
The Specification That Actually Matters: 10K or 100K
This is the most practically important thing to understand about NTC sensors, and it is the detail most often overlooked when ordering a replacement.
NTC thermistors are specified by their resistance at 25°C. A 10K sensor measures 10,000 ohms at 25°C. A 100K sensor measures 100,000 ohms at the same temperature. Both are NTC sensors, both look identical, and both will physically connect to most thermostats.
They are not interchangeable. A thermostat calibrated for a 10K sensor reading a 100K sensor will display a temperature substantially different from reality, and it will control the heating to that wrong figure. The system appears to work while holding the room at the wrong temperature.
Confirm before ordering a replacement. Different thermostat models use different sensor values, including within a single manufacturer’s range. The Legom RT-X1 Ascent uses NTC 10K at 1% tolerance, while other models in the range use 100K. Check the thermostat’s specification rather than assuming any NTC probe will work, and note the tolerance figure as well, since a 1% sensor holds accuracy more closely than a 5% one.
Other specifications to check
| Specification | Why it matters |
|---|---|
| Resistance at 25°C | 10K or 100K must match what the thermostat expects |
| Tolerance | 1% holds accuracy more closely than 5% |
| Beta value | Defines the resistance curve shape; must match the calibration |
| Cable length | Must reach from thermostat to sensing position |
| Probe encapsulation | Epoxy or similar protects the element in a screed |
| Operating range | Must cover the temperatures it will encounter |
Internal and External Sensors
Most room thermostats contain an internal NTC sensor measuring air temperature at the wall. Many also accept an external probe, and understanding the difference matters particularly in underfloor heating.
Internal air sensor
Measures room air temperature at the thermostat position. This is what controls comfort, since it reflects what occupants actually feel. Its accuracy depends heavily on placement, discussed below.
External floor probe
A sensor on a cable, embedded in the floor construction, measuring the floor temperature directly. This serves a purpose the internal sensor cannot: it allows the thermostat to cap the floor surface temperature independently of room air temperature.
That protection matters because most floor coverings have a maximum permitted surface temperature, commonly around 27°C for timber. On a cold day with high heat demand, a thermostat measuring only air temperature will keep calling for heat, and the floor can exceed its limit while the room is still below target. The consequence appears months later as cupping, gapping, or delamination of the floor covering.
Both together
Better thermostats run both sensors simultaneously, controlling room air temperature while independently limiting the floor. The Legom Volcano RB-2305HT supports internal, external, or both modes, with the internal sensor covering 5 to 35°C and the external probe covering 5 to 60°C. The RT-X1 Ascent offers the same with a configurable floor temperature limit.
For any installation with a timber, engineered wood, or vinyl covering, dual sensing is not an optional refinement. It is the component that prevents the most expensive failure mode. Our guide to underfloor heating sensors covers this in more detail.
Where to Place the Sensor
Placement affects accuracy more than most owners expect, and a well-specified sensor in a poor position will control badly.
Mounting height. Install the thermostat on an interior wall at approximately 1.5 metres from the floor, which corresponds to roughly adult chest height. This is the standard for room thermostats because it represents the air temperature at the level where people actually are, rather than the cooler air near the floor or the warmer air near the ceiling.
Avoid direct sunlight. A sensor in sunlight reads warm and shuts the heating off while the room is still cold. This is one of the most common causes of complaints about a thermostat that “does not work”.
Avoid draughts. Positions near exterior doors, windows, or ventilation openings read cold and cause the system to overheat the room.
Keep away from heat sources. Stoves, ovens, microwaves, televisions, lamps, and radiators all warm the air around them. A thermostat near any of these reads the appliance rather than the room.
Avoid exterior walls where possible. An uninsulated exterior wall is cooler than the room, and a sensor mounted on it reads low.
For a floor probe, position it midway between two heating loops rather than directly over a pipe, so it reads the average floor temperature rather than the hottest point. Install it in a conduit where possible, which allows replacement without breaking into the floor.
How to Tell When a Sensor Has Failed
An NTC sensor fails in ways that are easy to misread, because the thermostat continues operating throughout.
| Symptom | Likely cause |
|---|---|
| Displayed temperature clearly wrong | Sensor drift, or wrong sensor value fitted |
| Error code on the display | Open circuit or short circuit in sensor or cable |
| Reads a fixed extreme value | Broken cable or disconnected probe |
| Room consistently over or under heated | Placement problem rather than sensor fault |
| Floor limit never reached | Floor probe disconnected or wrong mode selected |
The check that settles it. Place a separate thermometer next to the thermostat, allow both to stabilise for an hour, and compare. A deviation of more than a degree or two indicates a sensor problem rather than a control problem.
Before replacing anything, rule out placement. A thermostat reading three degrees high because it sits above a lamp is not faulty, and replacing the sensor will change nothing.
Do this comparison annually. A sensor that has drifted continues working in every visible respect while holding the room at the wrong temperature, so measurement is the only way to detect it.
Why NTC Sensors Are Used So Widely
High sensitivity. Resistance changes sharply with temperature, which allows precise readings across the range building controls require.
Fast response. The element is small with low thermal mass, so it tracks temperature changes quickly rather than lagging behind them.
Small size. The sensing element can be built into a thermostat body or encapsulated in a slim probe that fits inside a floor conduit.
Mechanical robustness. Resistant to shock and vibration, which is why they appear in automotive and industrial applications as well as buildings.
Low cost. Inexpensive to manufacture and to replace, which matters when the sensor is a consumable component in a system expected to last decades.
Easy integration. Two wires and a known resistance curve, which any control circuit can read without specialised interfacing.
NTC Sensors Beyond Thermostats
The same component appears throughout HVAC and industrial equipment wherever a temperature reading is needed.
In heat pumps, NTC sensors monitor refrigerant temperature at several points in the circuit, water flow and return temperature, and outdoor ambient temperature, and the control board uses those readings to manage compressor speed, defrost cycles, and protection functions.
In industrial machinery, they monitor operating temperature so control systems can respond before overheating causes damage. In domestic appliances, they control water temperature and cycle timing.
What connects these applications is that each needs a temperature reading delivered to an electronic control system, which is precisely what an NTC thermistor provides.
The Control Chain Around the Sensor
A sensor is the first link in a sequence, and understanding the rest explains why an accurate reading matters.
The NTC sensor reports temperature to the room thermostat, which compares it against the set point and produces a demand signal. That signal reaches a base station, which coordinates demand across the building and powers the thermal actuator on the correct manifold branch, opening that heating circuit.
Every decision in that chain rests on the sensor reading. A sensor reading two degrees high means the whole system heats the room two degrees cooler than intended, with no fault visible anywhere.
“The thing worth knowing about NTC sensors is that ten K and one hundred K look exactly the same and both will plug into your thermostat. Fit the wrong one and nothing appears broken. The display shows a number, the heating runs, and the room sits at a temperature nobody chose. We see this most often when someone orders a replacement floor probe without checking what the thermostat expects. Check the specification before you order, and once a year put an ordinary thermometer next to the thermostat and see whether the two agree. That takes five minutes and it is the only way to catch a sensor that has quietly drifted.”
— Maggie Shen, Director of Legom
Legom Room Thermostats and Sensors
Legom manufactures room thermostats at its facility in Jiaxing, Zhejiang Province, with four models covering water-based underfloor heating, electric heating, and radiant cooling systems.
All models support NTC sensors and accept an external probe alongside the internal sensor. The Volcano RB-2305HT runs both simultaneously with an internal range of 5 to 35°C and an external probe range of 5 to 60°C. The RT-X1 Ascent uses NTC 10K at 1% tolerance with a configurable floor temperature limit. All carry CE and RoHS certification and are compatible with Legom thermal actuators.
When ordering replacement probes, confirm the resistance value and tolerance your thermostat expects rather than assuming compatibility. OEM and ODM services cover firmware parameter defaults, housing colour, display language, and packaging for private-label supply.
Frequently Asked Questions
What is an NTC sensor?
A Negative Temperature Coefficient thermistor, a passive resistor whose electrical resistance falls as temperature rises. A thermostat passes a small current through it, measures the resistance, and converts that into a temperature reading using a calibration curve. It contains no electronics and no power supply of its own. All of the intelligence sits in the thermostat; the sensor simply provides a resistance that corresponds predictably to temperature.
Does an NTC sensor measure humidity or pressure?
No. An NTC thermistor responds to temperature only. Measuring humidity requires a humidity sensor, and measuring pressure requires a pressure transducer, both entirely different devices. This is worth clarifying because the three are often discussed together in the context of comfort, but a thermostat with an NTC sensor cannot report or control humidity, and adding one will not address a humidity problem.
What is the difference between a 10K and a 100K NTC sensor?
The resistance at 25°C: 10,000 ohms against 100,000 ohms. Both are NTC sensors, both look identical, and both will physically connect to most thermostats, but they are not interchangeable. A thermostat calibrated for 10K reading a 100K sensor will display a temperature substantially different from reality and control the heating to that wrong figure, with nothing appearing broken. Always check what your specific thermostat model expects before ordering a replacement.
Where should a room thermostat be installed?
On an interior wall at approximately 1.5 metres from the floor, which is roughly adult chest height and represents the air temperature at the level where people actually are. Keep it away from direct sunlight, which makes it read warm and shut the heating off prematurely, and away from draughts near doors or windows, which make it read cold and overheat the room. Also avoid proximity to stoves, ovens, televisions, lamps, and radiators, and where possible avoid mounting on an uninsulated exterior wall.
What is a floor sensor and do I need one?
An external NTC probe embedded in the floor construction, measuring floor temperature directly rather than room air. It allows the thermostat to cap the floor surface temperature independently of room air temperature, which matters because most floor coverings have a maximum permitted temperature, commonly around 27°C for timber. Without it, a thermostat controlling only air temperature can allow the floor to exceed its limit on a cold day, causing cupping or gapping months later. For timber, engineered wood, or vinyl coverings it is essential.
How do I know if my NTC sensor has failed?
Place a separate thermometer next to the thermostat, allow both to stabilise for an hour, and compare readings. A deviation of more than a degree or two indicates a sensor problem. Other signs include an error code on the display, a reading stuck at an extreme value which usually means a broken cable, or a displayed temperature that is clearly wrong. Before replacing anything, rule out placement, since a thermostat reading high because it sits above a lamp is not faulty.
Can I replace an NTC sensor myself?
An external floor probe connected to a thermostat terminal is generally straightforward to swap, provided the replacement matches the resistance value and tolerance the thermostat expects and the thermostat is isolated before working on the wiring. A probe embedded in a screed without a conduit is a different matter, since accessing it means breaking into the floor. This is why installing floor probes inside a conduit at construction is worth the small additional effort.
Where else are NTC sensors used?
Throughout HVAC and industrial equipment wherever a temperature reading feeds an electronic control system. In heat pumps they monitor refrigerant temperature at several points, water flow and return temperature, and outdoor ambient temperature, and the control board uses those readings to manage compressor speed, defrost cycles, and protection. They also appear in industrial machinery for overheat monitoring and in domestic appliances for water temperature and cycle control.
Reviewed by Maggie Shen, Director at Legom, on August 25, 2026. This guide to NTC sensors was reviewed for technical accuracy, including what the component does and does not measure, the practical difference between 10K and 100K sensors, and correct thermostat mounting height.