Calibration and temperature measurement accuracy: why the thermostat shows differently than the thermometer
Why the thermostat shows a different temperature than the thermometer – and what to do about it
One of the most common questions we address with customers after installing a new programmable thermostat goes like this: "The thermostat shows 22 °C, but the wall thermometer says 20 °C. Which one is correct?" The answer is not simple or straightforward – and that is the core of the whole issue. The accuracy of temperature measurement in real conditions is a complex matter, where physics, building physics, electrical engineering, and pure practical experience from the field intertwine. This article will explain why such differences occur, what is behind them, how to minimize them, and when it is actually worth calibrating the thermostat at all.
What the thermostat actually measures – and how
Most common room thermostats – including programmable models in the mid-price range – use an NTC thermistor (Negative Temperature Coefficient) for temperature measurement. This is a semiconductor component whose electrical resistance changes with temperature. At higher temperatures, the resistance drops, and at lower temperatures, it rises. The microprocessor in the thermostat continuously samples this resistance change, recalculates it according to a pre-stored calibration curve, and displays the result on the screen.
NTC thermistors are cheap, reliable, and sufficiently accurate for standard heating applications. The typical deviation of a good thermistor is ±0.5 °C to ±1 °C in the range of normal room temperatures (15–30 °C). This by itself would not be a problem – but it is only one source of inaccuracy. In practice, other factors are added, which together can cause a total difference of 2–5 °C between what the thermostat displays and what you actually feel.
Thermostat position: the biggest source of differences
Of all the factors that cause a difference between the displayed and actual temperature, the position of the thermostat in the room is absolutely the most significant. The thermostat measures the air temperature in the immediate vicinity of its sensor – the air temperature near the wall, at the installation height, in a particular corner of the room. However, the air temperature is not the same everywhere in the room.
Physics is clear: warm air rises, cold air sinks. In a room with floor heating, the temperature near the floor is higher than near the ceiling, while in a room with a radiator, the situation is the opposite – it is warmer near the ceiling. In a room with a classic radiator under a window, the temperature difference between the floor and the ceiling can be 3–5 °C. A thermostat at a height of 1.5 m thus displays a temperature that corresponds to the measurement height, not the overall "perceived temperature" of the room.
From practice: I installed a programmable thermostat in a living room with a large northern window. The thermostat was mounted on the wall to the right of the window, at a height of 1.2 m. In winter, it showed 21 °C, but the customer complained that it was cold in the living room. Measurement showed that at sitting height (on the sofa, 0.6 m from the floor), it was actually only 18.5 °C – cold air flowing down from the window had accumulated near the floor. The thermostat was physically in good condition, it just measured the air temperature that did not correspond to the perceived temperature at sitting height.
Where the thermostat should not be
Based on experience from dozens of customer cases, these are the unsuitable positions:
- Behind or near a curtain – the curtain restricts air circulation, and the temperature in the "pocket" behind the curtain can be significantly lower than in the room
- Above a radiator or in the flow of warm air from it – the thermostat will measure the heat from the radiator, not the room temperature; the boiler will shut off before the room is actually warmed up
- On an external wall that is cold – the wall cools the air around the thermostat, the displayed temperature will be higher than the actual room temperature
- Near a window or door – drafts and cold air from outside disrupt the measurement
- Above a heat source (TV, computer, lamp) – locally heated air will cause the thermostat to shut off heating too early
- In direct sunlight – the most common mistake; direct sunlight can increase the displayed temperature by 3–8 °C compared to the actual room temperature
The correct installation height is 1.2–1.5 m from the floor, on an internal wall, away from direct sunlight, air currents, and heat sources. More about correct installation can be found in the article Installation of a programmable thermostat: step-by-step installation procedure.
Self-heating of electronics – thermostat self-heating
Every electrical device produces heat. A thermostat is no exception. Its microprocessor, power supply circuit, and display generate heat that warms the air inside the thermostat housing. This heat is then transferred to the thermistor sensor and affects its measurement.
This effect is called "self-heating" and is one of the reasons why lower-quality or older thermostats display a higher temperature than the actual one. The typical size of this error ranges from 0.5 °C to 2 °C, depending on the device's design.
Modern thermostats address this issue in several ways: by minimizing the power consumption of the electronics (with low-power chips, self-heating is minimal), by spatially separating the sensor from the power components, or by using an external sensor. For example, the Saswell 908 7 RF in the wireless version has the sensor physically separated from the main control unit – it measures the temperature directly where it is needed, not near the thermostat's electronics. That is why the problem of self-heating is almost non-existent in wireless thermostats.
With wired thermostats, where the sensor is integrated directly into the body of the device, it is important for designers to pay attention to this effect. High-quality models such as the SALUS 091FL are designed so that self-heating is minimized and does not affect the measurement accuracy more than the manufacturer declares in the product catalog.
Calibration of the thermometer: what are you actually measuring?
If you are comparing the temperature on the thermostat with a thermometer, the key question is: how accurate is your thermometer? Common cheap glass thermometers (mercury or alcohol-based) typically have an error of ±1 °C, and that is only if they are in good condition. Cheap digital thermometers from any electronics store may have an error of ±2 to ±3 °C – and no one will warn you about it, because no calibration is performed during their production.
This means that if the thermostat shows 21 °C and your "control" thermometer shows 19 °C, it does not necessarily mean that the thermostat is lying. It could easily be that the actual temperature is 20 °C, the thermostat has an error of +1 °C, and the thermometer has an error of -1 °C. Both are measuring incorrectly – but in opposite directions.
How to verify this? The laboratory method is to compare with a calibrated reference – which you don't have at home. A practical home method is the so-called ice water method: you place the sensors of both devices into a mixture of ice and water (0 °C at normal pressure) and compare the displayed values. If both show close to 0 °C (deviation within ±0.5 °C), both are fine. If the difference is greater, you know which one is lying. This method is accurate only at the freezing point of water, not at room temperatures, but it gives you at least a rough idea.
Thermal inertia and measurement dynamics
A thermostat does not measure temperature instantly and continuously. Most common models perform measurements at intervals of 30–60 seconds. Moreover, the sensor itself and its housing have a certain thermal inertia – they react to changes in air temperature with a delay of several seconds to minutes.
This causes an interesting practical problem: if the boiler just stopped heating and the room temperature is slowly dropping, the thermostat may still show a higher temperature for a while, because its sensor is cooling down more slowly. Conversely: when heating is turned on and the air starts to warm up, the sensor registers the change with a delay.
This effect is especially noticeable in programmable thermostats during the transition between temperature zones. For example, if you set a transition from 20 °C to 22 °C at 17:00, the thermostat will turn on the heating, but the displayed temperature will react more slowly than the actual air temperature in the room is changing. More about setting up temperature zones and their scheduling can be found in the article Weekly thermostat program: how to correctly set time zones and temperatures.
Calibration in the thermostat menu – what it is and how to use it
Most modern programmable thermostats include in the service or installation menu an option for manual correction (calibration) of temperature. This function is usually called "Offset", "Correction", or simply "Calibration" and allows you to shift the displayed temperature by a certain number of degrees (typically in the range of ±5 °C or ±3 °C, depending on the model).
For example, if you know (from a reliable measurement) that the actual temperature in the room is 20 °C, but the thermostat displays 22 °C, you set an offset of -2 °C. The thermostat will then display 20 °C and will control the heating so as to maintain the actual temperature at the set value.
This function is available, for example, in the Euroster Q7, where it is set via the service menu by pressing the appropriate buttons, or in the Saswell SAS 908 7, where temperature correction is available in the device settings in the range of ±5 °C with a step of 0.5 °C. A detailed procedure for a specific model is always found in the manual – and if you get lost in the menu, the article Resetting and restoring factory settings of a programmable thermostat explains how to safely navigate the settings without the risk of deleting the program.
When to calibrate and when to look for another problem
Calibration with an offset makes sense when the difference between the displayed and actual temperature is constant over time and at different temperatures. If the thermostat always shows 1.5 °C more than the thermometer, regardless of whether it is winter or transitional season outside, offset is the correct solution.
Calibration will not help in cases where the difference is variable – sometimes +2 °C, sometimes +0.5 °C, or even negative. In such cases, it is another issue: variable solar influence, variable air flow, unsuitable thermostat location, or a faulty sensor. In such cases, it is worth first checking the thermostat location and removing sources of interference, and only then considering possible calibration.
Thermostat hysteresis and its impact on perceived temperature
Another term related to thermostat accuracy is hysteresis. A thermostat does not switch heating on and off precisely at the set temperature – this would cause constant rapid switching (so-called "chattering"), which is harmful to the thermostat relay and the boiler. Instead, the thermostat operates with a hysteresis band.
Example: you set 21 °C, hysteresis is ±0.5 °C. The boiler turns off when the temperature reaches 21.5 °C, and turns on again when the temperature drops to 20.5 °C. Result: the actual room temperature oscillates between 20.5 and 21.5 °C, not exactly 21 °C.
The size of hysteresis varies among different models. Cheaper thermostats have hysteresis of ±1 °C or even ±2 °C, while higher quality ones have ±0.3–0.5 °C. Some models allow hysteresis to be adjusted in the service menu. In programmable thermostats for floor heating, where the system's thermal inertia is high, a larger hysteresis is sometimes intentionally set to avoid unnecessary switching.
Practical consequence: even if the thermostat is working perfectly accurately, the room temperature will always oscillate within the hysteresis band. This is normal and intentional – not a fault. If you want to minimize this oscillation, choose thermostats with adjustable or small hysteresis. More on how to choose the right model for your system can be found in the article How to choose a programmable thermostat: what to look for before buying.
External sensor vs. built-in sensor: impact on accuracy
Some programmable thermostats allow the use of an external floor or room sensor. This is especially important for floor heating, where it is common practice to regulate based on floor temperature (not air), or a combination of both.
When the thermostat regulates based on floor temperature, the sensor is placed directly in the floor structure, and air temperature measurement is not used for regulation. In such cases, the mutual difference between the thermostat and the room thermometer is natural and expected – both measure something different. The thermostat displays floor temperature (typically 26–30 °C in normal operation), not air temperature. This is not a measurement error.
Models such as Saswell T19 7 B support operation with an external floor sensor NTC 10kΩ and allow setting whether it regulates based on air, floor, or in a combined mode. This provides great flexibility for different types of floor structures. When choosing a thermostat for floor heating, compatibility with the boiler and heating system is key – this topic is covered in detail in the article Compatibility of programmable thermostats with boiler and heating system.
Difference between displayed temperature and actual thermal comfort
It is important to realize that even a perfectly accurate thermostat does not guarantee perfect thermal comfort. The perceived temperature – how warm or cold you feel – depends on several factors at once:
- Air temperature – what the thermostat measures
- Mean radiant temperature – the temperature of surrounding surfaces (walls, floor, ceiling, windows); if you have a large cold window, you will feel cold even with warm air
- Air humidity – dry air feels colder (perceived temperature can be 1–2 °C lower at low humidity)
- Air flow speed – wind causes a greater feeling of cold
- Physical activity and clothing – subjective factors
Operative temperature, which combines air temperature and mean radiant temperature, is a better indicator of thermal comfort from a physical perspective than air temperature alone. However, thermostats measure air temperature, not operative temperature. Therefore, it is absolutely normal that at the same displayed temperature, you feel different in different rooms – for example, in a room with large glazing you will feel colder than in a room without windows, even though the thermostat shows the same value.
Type of heating system and its impact on perceived thermostat accuracy
The heating system has a decisive impact on how much the thermostat measurements will differ from your thermal sensations. Systems with high thermal inertia (floor heating, brick stove) react to changes in setpoint much more slowly than radiator systems.
With floor heating, it can take 2–4 hours for a change in the setting to be reflected in the actual room temperature. The thermostat may show the set 22 °C, the heating is on, but the room is still 20 °C – and this is not a measurement error, but simply the physical reality of thermal inertia. The correct solution is preheating (advance heating), which many programmable thermostats allow to be set – they turn on the heating early enough to achieve the desired temperature at the desired time. More on this issue can be found in the related article Wired or wireless programmable thermostat: which is suitable for your system.
Practical recommendations: how to achieve the most accurate measurement
Based on experience from installations and service practice, there are several proven principles that significantly reduce measurement inaccuracies:
- Install the thermostat correctly: on an interior wall, at a height of 1.2–1.5 m, out of reach of sunlight, air currents, and heat sources
- Remove the causes before calibrating the offset: if the problem is the thermostat's location, calibration will not permanently solve it
- Use a reliable reference thermometer: perform calibration according to a laboratory-verified or at least a high-quality thermometer; common cheap digital thermometers from hypermarkets are not reliable references
- Let the system stabilize: perform measurements only after sufficient stabilization of the room temperature (at least 30 minutes after the last heating state change)
- Set the offset in 0.5 °C steps: do it gradually, not in one big jump; after each change, let the system run for 1–2 hours and then compare again
- Consider hysteresis: if the thermostat measures correctly but the temperature oscillates ±1 °C, try setting a smaller hysteresis (if the model allows it)
- For wireless thermostats, verify the sensor location: if you have a wireless system, make sure the sensing module (not the transmitter) is in the optimal position in the room
When measurement inaccuracy is really a fault
Most differences between the thermostat and the thermometer are caused by physical reasons, not faults. Real sensor faults are indicated by these symptoms:
- The thermostat displays an unrealistic temperature (e.g. 99 °C or –20 °C under normal conditions)
- The display is blinking or showing an error code (E1, E2, etc. – usually means a broken sensor circuit or short circuit)
- The difference between the displayed and actual temperature is greater than 5 °C and changes unsystematically
- The thermostat turns the heating on and off chaotically, without obvious relation to temperature
In these cases, a hardware fault needs to be addressed – either by replacing the sensor (if it is external and replaceable) or by replacing the entire thermostat. More about fault diagnosis can be found in the article Common faults of programmable thermostats and how to eliminate them.
Most frequently asked questions (FAQ)
The thermostat shows 2 °C more than the thermometer – is that normal?
Yes, a difference of 1–3 °C is common and has physical causes: the thermostat's position on the wall, self-heating of the electronics, or possible inaccuracy of the thermometer. Before calibrating the offset, check whether the thermostat is in direct sunlight or in a heat stream from a radiator. If the location is correct, calibrate the offset in the thermostat's menu. If your thermometer is not calibrated, the problem may also be with the thermometer itself.
Can I calibrate every programmable thermostat?
Most modern programmable thermostats have a temperature offset function in the service menu. For example, the Saswell SAS 908 7 offers correction of ±5 °C, and the Euroster Q7 also allows setting the offset. Older or very cheap models may not have this function – in such cases, the only option is to change the thermostat's location or replace it.
Why does the thermostat show the same temperature all the time, even when the heating is working?
If the displayed temperature does not change even when the heating is on, there may be several reasons: the thermostat is placed in an area with poor air circulation (behind a curtain, in a corner), the sensor is damaged, or the heating is not actually working and the perception is different. Try to verify with your hands whether the radiators are actually warm, and check the thermostat's location.
The reference thermometer shows 0.5 °C differently than the thermostat – should I correct it?
A difference of 0.5 °C is within the normal inaccuracy of both devices. Correcting such a difference has no practical sense – the probability is higher that the thermometer itself has an error of ±0.5 °C. Calibration of the offset is only meaningful for differences greater than 1–1.5 °C, provided you are sure that your reference thermometer is sufficiently accurate.
Why is the temperature at floor level 3 °C lower than the thermostat on the wall shows?
This is a normal physical phenomenon: warm air rises, and with radiator heating, the temperature at floor level is lower than at the height of the thermostat. A difference of 2–4 °C is common. A solution is floor heating (for a more uniform temperature profile), or a circulation fan that mixes the air. In this case, the thermostat is working correctly – it only measures the air temperature at its height, not at floor level.
Has the accuracy of the thermostat changed after several years of operation?
NTC thermistors are long-term stable and their properties hardly change under normal operation. If you have noticed a change in accuracy after several years, the cause is more likely: dust accumulation in the thermostat's ventilation holes (dust insulates the sensor), a change in the environment (new furniture, curtain, heat source), or mechanical damage to the sensor. Cleaning the thermostat with gentle air once a year is a good preventive maintenance.
Conclusion: accuracy is important, but not at any cost
The difference between the thermometer and the thermostat is not always a problem that needs to be eliminated at any cost. The key is to understand what causes the difference and to know whether it has a real impact on the comfort and energy efficiency of your household. If the thermostat is working so that the room is comfortably warm and the boiler is not switching unnecessarily, a possible difference of 1–2 °C in the displayed values is purely an academic matter.
If you are really bothered by accuracy, always start with the thermostat's location – that is the most important factor. Then consider the quality of the reference thermometer and only then proceed to calibrate the offset in the thermostat's menu. The right approach and patience during setup always pay off more than impulsively replacing thermostats or blaming devices for faults that are caused by physical laws.
If you are considering replacing the thermostat with a more accurate model, or looking for a device with an external sensor for more accurate measurement in a specific area of the room, also read the overview in the article Common questions about programmable thermostats, where the most important parameters for selection are summarized, or visit the programmable thermostats category directly on atria.sk, where you will find verified models from manufacturers Salus, Euroster, and Saswell with detailed specifications of measurement accuracy for each device.
Do you have a question about this topic?
Not sure or dealing with a specific situation in your household? Write to us – we are happy to help.