Switching power of thermostat for underfloor heating: how to determine it correctly
Switching power of a thermostat for underfloor heating: why it matters more than you think
When a customer chooses a thermostat for underfloor heating, most of the attention goes to the display, programming, WiFi, or price. The switching power – that one value expressed in amps or watts – remains unnoticed somewhere in the technical specifications. Yet it is precisely this value that determines whether the thermostat will stop switching after a year of operation, whether it will burn out its own relay contact, or – in the worst case – whether it will cause a fire in the electrical installation. This is not scaremongering: from practice I know that an undersized thermostat is one of the most common causes of premature failure of control equipment in electric underfloor heating systems.
In this article, we will explain what switching power actually is, how to calculate it for your specific situation, what reserves you need to observe, and what happens if you neglect it. By the end, you will not only know how to choose the right thermostat, but also how to check whether the existing one is operating on the edge of its capabilities.
What is the switching power of a thermostat and how is it expressed
A thermostat for underfloor heating works as a switch. When the temperature drops below the set value, it closes the electrical circuit and current begins to flow through the heating cable or mat. When the temperature reaches the desired level, the circuit is interrupted again. This switching on and off is ensured by a relay contact – mechanical or electronic – and this contact has its physical limits.
Switching power is expressed either in amps (A) – the maximum current that the contact can switch – or in watts or kilowatts (W / kW) at a specific voltage. In the Slovak market, we work with a network voltage of 230 V (AC), so the calculation is simple:
P (W) = U (V) × I (A)
For example, a thermostat with a switching current of 16 A at 230 V can handle: 230 × 16 = 3 680 W, i.e. 3.68 kW. A thermostat with a current of 10 A can handle 2 300 W. These are maximum values for resistive loads, and electric underfloor heating (resistive cables or mats) is precisely a resistive load – which is favorable from the perspective of thermostats compared to inductive loads such as motors or transformers.
Most standard thermostats for households have a switching current of 10 A or 16 A. There are also models with higher current, but for standard apartment and family installations, 16 A is more than sufficient – as long as you know what power your floor area actually consumes.
How to determine the power of your underfloor heating
This is the step where most people make a mistake. Either they do not verify the power at all, or they take the first number from the cable's packaging without checking how much cable length is actually laid. The procedure is as follows:
1. Power from the technical data sheet of the cable or mat
Every heating cable has its power consumption in watts (W) or specific power in W/m stated on the packaging or in the documentation. If you have a cable with a total power of, for example, 1 500 W, that is your input data. With heating mats, power is given in W/m² – here you multiply the value from the documentation by the area on which the mat is laid.
2. Calculation from the cable resistance
If you do not have documentation, you can measure it. Measure the resistance of the cable (Ω) with a standard multimeter at the terminals – just be careful to always measure on a disconnected cable, not on a live network. The power is then calculated as follows:
P = U² / R, i.e. P = 230² / R = 52 900 / R
For example: a cable with a resistance of 35.3 Ω → P = 52 900 / 35.3 ≈ 1 498 W ≈ 1 500 W.
3. Sum of multiple circuits on one thermostat
This is the most common problem. People add a second cable to the thermostat because the area of the first one is not enough, and no one tells them that the thermostat has a limit. If you have, for example, three cables of 800 W each on one thermostat, the total is 2 400 W – and that is already over the limit for a thermostat with a rating of 10 A (max. 2 300 W). Always sum the power of all cables or circuits connected to one thermostat.
Why Going Exactly to the Limit Isn't Enough: The 80% Rule and Why to Follow It
Electrical standards and manufacturers recommend that continuous load should not exceed 80% of the rated power of the switching component. In practice, this means: a thermostat with a switching current of 16 A (= 3 680 W) should not continuously switch a load greater than 2 944 W. A thermostat with 10 A (= 2 300 W) should not go above 1 840 W.
The reasons are physical and quite strict:
- Thermal load on the contact: A relay contact heats up with every switching. The higher the current, the greater the thermal shock. Continuous loading at the limit shortens the relay's lifespan from tens of thousands of operations to just a few thousand.
- Inrush current: When turning on a cold resistive cable, the resistance is slightly lower (the cable is cold, resistance increases with temperature) – the inrush current thus briefly exceeds the rated value. In a properly dimensioned system with a reserve, this is not a problem; in a system fully utilizing the limit, it means repeated short-term overloading of the contact.
- Influence of ambient temperature: A thermostat mounted on a warmer wall (near a window, on the sunny side) operates in worse cooling conditions. The contact heats up more, even if the current is the same.
- Aging of the installation: Terminals, cables, contacts – everything ages and the connection resistance increases. A system that worked for 10 years without problems may have higher resistance heating of the connections after a decade, which at power close to the limit causes problems.
From practice: I've seen a thermostat with a burned relay contact, where the customer had a 2 200 W cable connected to a thermostat with a limit of 2 300 W. Formally within the limit. However, the thermostat was in a bathroom with an elevated ambient temperature and the cable was a bit longer than originally planned, so the actual power was closer to 2 350 W. After 14 months of operation, the contact was burned out.
Practical Example: 6 m² bathroom, 25 m² living room, terrace
Case 1 – 6 m² bathroom
A small bathroom, with an electric floor mat of 600 W power, as the primary heating. Practically any standard thermostat will suffice – even a basic model with a current of 10 A (limit 2 300 W, respectively recommended 1 840 W). 600 W represents only 26% of the capacity of a 10 A thermostat. In this case, the selection of a thermostat in terms of power is problem-free; other parameters are more important – resistance to humidity (IP44 for a bathroom), type of sensor, etc. You will learn more about these parameters in the article Thermostat with floor sensor vs. air sensor: difference and when to use which.
Case 2 – 25 m² living room with two circuits
A living room, where 180 m of resistive cable is laid, divided into two circuits (90 m + 90 m), each circuit has a power of 1 350 W. Together 2 700 W. Here, a thermostat with 10 A (limit 2 300 W) is not sufficient even formally. You need either:
- a thermostat with a limit of 16 A (2 700 W is 73% of 3 680 W – a nice reserve), or
- divide the circuits into two separate thermostats (each switches 1 350 W – comfortably).
For such situations, for example, the Jablotron AC-83 is suitable, which has a switching current of 16 A and is intended precisely for more powerful installations with full support for an external floor sensor.
Case 3 – outdoor terrace / driveway with snow melting
Snow melting systems usually have a significantly higher specific power (200–300 W/m²) compared to standard floor heating in the interior (80–150 W/m²). For a terrace of 10 m² with a power of 250 W/m², this is 2 500 W. Here, a 16 A thermostat is needed with 100% certainty, or – for larger areas – an external contactor (contactor), through which the thermostat only switches the contactor coil, while the actual power goes through the contactor. This is a topic we will also cover further.
When Even 16 A Isn't Enough and What to Do: External Contactor
If you have a system with a total power exceeding 3 000 W (for example, a large living room with a long hallway, or an outdoor snow melting system), the thermostat alone is not sufficient as a switching component. The solution is an external electromagnetic contactor (contactor). In this wiring, the thermostat does not switch the power circuit directly – it only switches the contactor coil (usually 230 V, consumption about 5–15 W), and the contactor then switches the power circuit with any current for which it is dimensioned (25 A, 32 A, 40 A…).
From the perspective of the thermostat, this is a minimal load (only the coil), so the thermostat contact will function tens of thousands of times without problems. From the perspective of the power part of the installation – the contactor is designed for continuous switching of large currents. This solution is commonly used in commercial floors (fitness centers, showrooms), in floors on terraces and driveways, and anywhere the power exceeds 4–5 kW.
Installation and wiring of a contactor solution is beyond the scope of this article, but for standard apartment and single-family house installations with floor areas up to about 20–25 m² (at a standard specific power of 100–150 W/m²), a 16 A thermostat with an 80% reserve comfortably suffices.
Specific thermostats and their switching powers: what you find in practice
Let's take a look at a few specific products that we commonly encounter in practice:
SALUS RT10 – simple thermostat for smaller areas
SALUS RT10-230V is a basic digital thermostat with manual temperature setting. The switching current is 10 A (2 300 W), which makes it suitable for smaller areas – typically bathrooms, entrance halls, or supplementary heating in rooms. According to the 80% rule, the maximum recommended load is 1 840 W. A bathroom mat of 600 W, or a cable up to 10–12 m² at a specific power of 150 W/m² – that is its comfort zone.
SALUS ERT20 and ERT50 – electronic and programmable solutions
SALUS ERT20 - 230V and SALUS ERT50 - 230V are electronic, respectively programmable thermostats with higher comfort of regulation. Both usually operate with a switching current of 16 A (3 680 W), which places them in the category of universal thermostats suitable for larger areas. ERT50 also offers weekly programming – which significantly reduces electricity consumption for floor heating as the main heat source in a room by not heating the space at full capacity when no one is at home.
Jablotron AC-82 and AC-83 – more powerful regulation
Jablotron AC-82 is a digital thermostat with a switching current of 16 A, combined regulation (air + floor sensor) and a modern display. For standard installations up to 25 m² (at 150 W/m²), dimensioning is straightforward. Jablotron AC-83 adds more advanced functions and also operates with a current of 16 A. Both are suitable for situations where you want reliable regulation with sufficient power reserve and at the same time full protection of the floor from overheating via the floor sensor.
More about the comparison of these products can be found in the article Thermostat SALUS vs. Jablotron for floor heating: model comparison.
Mistakes from practice that cost money (and sometimes safety)
Over the years of working with heating technology, I have seen several recurring mistakes that are directly related to the switching power:
- A customer bought a "cheap" thermostat from an e-shop with unverified parameters. The box stated "16 A", but the real contacts were dimensioned only for 10 A. At a load of 1 800 W it worked fine. At 2 200 W the contact burned out after half a year. Always buy from verified manufacturers with proper documentation.
- Adding a cable without calculation. The customer had a 10 A thermostat (2 300 W) with one 900 W cable. Renovation – they added another 700 W cable and a third 800 W cable. Together 2 400 W – formally over the limit, actually according to the 80% rule 560 W more than it should be. The thermostat lasted another two years, then the relay contact failed.
- Mysterious "outages" of heating in winter. A contact loaded at the limit was briefly and repeatedly overloaded (cold floor = lower resistance = higher current). The result was repeated outages. The customer spent three months solving the "thermostat failure", which in reality was not a failure – it was a consequence of poor dimensioning.
- Neglected floor overheating protection. This is not directly about switching power, but it is related: if the thermostat controls only the air sensor (without the floor sensor), and the load is high, the floor can overheat beyond the allowed limit (usually 27–35 °C depending on the type of floor). More about the correct setting of the floor sensor can be read in the article Setting the floor temperature sensor: correct depth and placement.
Rated vs. actual power: what can surprise you when measuring
Not always does a cable labeled "1 500 W" actually have a power of exactly 1 500 W. Manufacturers state rated values at standard temperature (usually 20 °C). Resistive materials change resistance with temperature – most heating cables have a positive temperature coefficient of resistance, which means a cold cable has lower resistance → higher current → higher power compared to the rated value. This difference is usually small (a few percent), but when dimensioning close to the limit, it is another reason to follow the 80% rule.
In addition, with long cables and lower cross-section of the supply line, there is a voltage drop in the line. If the voltage at the cable terminals is not 230 V but 220 V, the power drops to 220²/R – which is a bit less. This is more of an interesting fact than a practical problem in standard apartment installations, but with long supply lines (20+ meters) it is worth checking.
What to look for in the technical documentation of a thermostat: what is (not) written
When you look at the technical datasheet of a thermostat, look for these parameters:
- Switching current: stated in amperes, usually at 230 V AC. This is the key data.
- Type of load: thermostats for floor heating are designed for resistive load. Make sure the parameter values are for resistive load, not inductive.
- Contact type: NO (normally open) or NC (normally closed). For heating, NO is standard – the contact closes when heating is needed.
- Protection class (IP rating): at least IP44 for bathrooms, IP65 and more for outdoor use. It does not directly relate to switching power, but it is part of the correct dimensioning of the entire system.
- Supply voltage: all the products mentioned here operate at 230 V AC – the Slovak standard.
What is sometimes not written: some cheap thermostats do not distinguish between "total" and "recommended" power. They only state the maximum current, but do not take the 80 % rule into account. You have to apply this rule yourself.
Summary table: approximate sizing by area and power
| Area (m²) | Specific power (W/m²) | Total power (W) | Recommended thermostat |
|---|---|---|---|
| up to 4 m² | 100–150 | up to 600 W | 10 A (any standard one) |
| 4–10 m² | 100–150 | 400–1 500 W | 10 A (with reserve) |
| 10–15 m² | 120–160 | 1 200–2 400 W | 16 A (recommended) |
| 15–20 m² | 130–160 | 1 950–3 200 W | 16 A (necessary, with reserve) |
| over 20 m² | 130–200 | over 2 600 W | 16 A + relay, or divide into multiple thermostats |
| Terrace/snow (exterior) | 200–300 | high | Always use a relay for areas over 8–10 m² |
The values in the table are approximate. Always refer to the specific technical data sheet of the heating system, not just the area.
What happens if you ignore the switching power: real consequences
I will outline three scenarios ranging from mild to severe:
Scenario 1 – mild overload (105–115 % of the limit): The thermostat works for months, maybe a year. The relay contact gradually degrades. This is manifested by a gradual slowdown in response (the contact cannot firmly "engage"), micro-arcs during switching, and eventually failure. Result: replacement of the thermostat and the feeling of a "defective product," although the problem was sizing.
Scenario 2 – significant overload (125–150 % of the limit): The contact heats up significantly and may start to short (remains permanently closed – the floor overheats) or conversely remains permanently open (floor cold). This leads to the risk of floor damage due to overheating, deformation of wooden floors, or adhesive cracking in laminate floors.
Scenario 3 – severe overloading (over 150 %): Risk of fire due to thermal stress on the terminals or the relay itself. This is not a common outcome, but with a combination of poor connections, an unsuitable thermostat, and power significantly exceeding the limit, it is real. Electrical inspection technicians are familiar with such situations.
Can a thermostat protect itself?
Some modern thermostats have built-in thermal protection for their electronic components – if the internal temperature of the thermostat rises above a certain threshold (e.g. 70 °C), the thermostat shuts down or displays an error code. This is good news, but not a substitute for proper sizing. Thermal protection is the last safety net, not the primary strategy.
Information on wiring, installation, and possible error codes can be found in the articles Thermostat installation for underfloor heating: step-by-step guide and Thermostat wiring for underfloor heating: diagram and most common errors.
Most frequently asked questions (FAQ)
Can I connect a cable with a power of 2 000 W to a thermostat with a switching current of 10 A?
Formally, 10 A at 230 V = 2 300 W, so a load of 2 000 W is below the nominal limit. However, according to the 80 % rule, the recommended maximum is 1 840 W – so 2 000 W is above this limit. The thermostat will work, but the contact life will be reduced. We recommend choosing a 16 A thermostat, where 2 000 W represents only 54 % of capacity – an ideal reserve.
What if I have two cables and I don't know their exact power?
Measure the resistance of each cable with a multimeter (on a disconnected cable!). Calculate the power using the formula P = 230² / R. Add the powers together and compare them with the thermostat capacity. If you don't have a multimeter, look for a label on the cable or check the documentation from the installer – the power is always recorded somewhere.
Does the 80 % rule apply to more expensive/higher quality thermostats as well?
Yes. The 80 % rule is not about the quality of the thermostat, but about the physical limits of electrical contacts in general. A more expensive thermostat will have better dimensioned contacts and a longer life at the same current, but the principle of continuous loading at the maximum limit shortens the life of even the best relay.
The thermostat "flashes" or turns off, although the load is within the limit – what is happening?
If the load is formally within the limit but close to it, and the thermostat behaves unstably, check the terminals – a loose terminal increases contact resistance and generates heat exactly where it is worst. Also check the circuit breaker – if it is undersized for the load, it may trip before the thermostat. More about faults and their causes can be found in the article Common thermostat faults for underfloor heating and how to eliminate them.
Is the switching power of the thermostat the same for all types of load?
No. Technical data sheets list different values for resistive load, capacitive load, and inductive load (e.g. motor loads). For electric underfloor heating (resistive cables, mats), the value for resistive load applies – this is usually the highest. If you connect a thermostat actuator (e.g. for a combined system water + electric) to the thermostat, the load may have an inductive component and you need to verify whether the thermostat can handle this load.
How can I tell that the thermostat is at the limit of its capacity?
Direct signs: the thermostat is warm to the touch (normally it should be slightly warm, not hot), you smell a faint smell of overheated plastic or contact, or switching is imprecise (the device turns on and off for shorter periods than it should). Indirect signs: increasing electricity consumption without changing the settings (a contact with higher resistance generates extra heat), and a shorter interval between faults.
Conclusion: dimensioning the switching power is a simple calculation with a large impact
Correctly determining the switching power of a thermostat for underfloor heating is not a complicated science – it is a three-step process: determine the total load power, compare it with 80 % of the thermostat capacity, and choose the right model. Despite the simplicity of this procedure, neglecting dimensioning is one of the most common and unnecessarily costly mistakes in the installation of underfloor heating.
If you are unsure, always choose a thermostat with a higher switching current – switching from 10 A to 16 A usually means a minimal price difference, but significantly higher reliability and longer system life. An overview of specific products and their comparison can be found directly in the category thermostats for underfloor heating, where you can filter by switching current and other parameters.
Do you have a question about this topic?
Having trouble making a decision or dealing with a specific situation in your household? Write to us - we'll be happy to help.