Frequently Asked Questions About Electric Underfloor Heating
Electric Underfloor Heating: Answers to Everything You Really Want to Know
After years of practical experience in heating technology, we keep encountering the same questions – and that's fine, because electric underfloor heating is a topic where it's easy to get lost in technical details, conflicting advice from the internet, and contradictory information from various installers. This article is an attempt to give you, in one place, honest and practical answers to the questions our customers ask most often – from choosing a system, through installation, to operation and troubleshooting.
If you're looking for a deeper look at specific subtopics, we refer in the text to other articles in our Knowledge Center. Here we'll focus mainly on the questions that are hard to find a clear answer to at first glance.
Basic Questions: What Electric Underfloor Heating Actually Is and How It Works
How does electric underfloor heating physically work?
The principle is simple: a resistive conductor (cable, mat, or foil) converts electrical energy into heat. The heat spreads upward into the room through the floor structure by radiation and conduction. Unlike forced-air heating, where warm air circulates and rises to the ceiling, underfloor heating creates an even thermal field from the floor upward – meaning your feet get warm first, which is physiologically more pleasant.
Regulation is done via a thermostat, which measures either the floor temperature (sensor in the floor), the room air temperature, or a combination of both. Based on the set value, the thermostat switches the circuit on and off, maintaining the desired temperature.
What's the difference between primary and supplementary heating?
This is one of the most important questions to answer before any purchase. Electric underfloor heating can serve as a primary heat source (i.e., the only source in the room) or as supplementary heating (in addition to a boiler, heat pump, etc.).
You can use it as a primary source if the room is well insulated and heat loss is not too high. As a rough guide, primary use requires an output of at least 100–150 W/m² of heated area (you'll find a more detailed calculation in the article What output of electric underfloor heating do I need for my space?). As supplementary heating, even 80–100 W/m² is sufficient, since the system doesn't need to cover the entire heat loss of the room on its own.
We cover this topic in more detail in the article Electric underfloor heating as a primary vs. supplementary heat source.
Choosing a System: Cable, Mat, or Foil?
When should you choose a heating cable?
The heating cable is the most flexible system – you or an installer lay it directly into the screed or adhesive and define the loop spacing yourself. This means you can cover even irregularly shaped rooms, avoid areas under fixed furniture (where heating is unnecessary and even undesirable), and precisely set the output per square meter by changing the spacing.
Typical use: bathrooms with a more complex shape, larger rooms, renovations where a new screed is being poured. An example of a cable from our range is the HAKL TCX 10/1230W heating cable – a twin-core cable with a total output of 1,230 W, suitable for an area of roughly 8–12 m² depending on the chosen spacing.
When should you use a mat?
A heating mat is a cable pre-fixed to a mesh at fixed spacing (usually 65–100 mm). The advantage is quick and easy installation – you unroll it, cut the mesh (not the cable!), turn it as needed, and cover it with a thin layer of adhesive. The total added layer thickness is only 3–5 mm, which is a huge advantage in renovations where you don't want to raise the floor level.
Mats are best suited for regular areas under tiles or ceramics. If you have, for example, a typical 5–8 m² bathroom without too many recesses, a mat is a faster and more convenient choice than a cable.
What about foil?
Foil (infrared heating foil) is a thin, flexible sheet with a heating element that is not embedded in cement – it is laid dry under a floating floor (laminate, vinyl, wooden boards). It's therefore the fastest solution for dry installation, but it also has limitations: it must not be embedded in adhesive or screed, requires a special type of underlay board, and is not suitable under tiles in wet areas.
You'll find a detailed comparison of all three systems in the article How to choose electric underfloor heating: cable, mat, or foil? and in Heating cable vs. heating mat: which solution is right for you?
Questions About Calculations and Sizing
How many watts per square meter do I need?
This is question number one, and also the one where mistakes are most often made. The basic guidance is as follows:
- Supplementary heating (comfortable floor temperature): 80–100 W/m²
- Primary heating in a standardly insulated room: 120–150 W/m²
- Bathroom (humid room, cold walls, tiles): 150–180 W/m²
- Conservatory, space with a large glazed area: 180–200 W/m²
Note: these values apply to the heated area, i.e., the area where the cable is laid, not the entire room area. If 30% of the room is under furniture and you don't lay cable there, you calculate the output for the remaining 70% of the area, but you still need to account for the heat loss of the entire room. That's why it's important not to underestimate the output.
For example, if a bathroom has a total area of 6 m², the mat/cable will cover 4.5 m² (the rest is toilet, sink, shower area without heating). To cover primary heating in the bathroom, you need an output of at least 4.5 × 150 = 675 W, ideally 700–800 W.
How do I calculate cable spacing?
Cable spacing (in mm) is calculated using the formula:
Spacing = (heated area in m² × 1,000) / cable length in m
Example: you have a 63 m cable and a 7 m² area. Spacing = (7 × 1,000) / 63 = 111 mm. That's a reasonable spacing for supplementary heating. If you want higher output, choose a shorter cable or a smaller coverage area.
The minimum spacing for most twin-core resistive cables is 80 mm (below this, there's a risk of local overheating). The maximum recommended spacing for comfort heating is 150 mm – at wider spacing, "warm stripes" and "cold stripes" on the floor become clearly noticeable.
Installation: The Questions That Concern People Most
Can I install it myself, or do I need an electrician?
The technical installation of the cable or mat (laying, fixing, covering) can be done even by an experienced layperson – no professional qualification is required for this. However, the electrical connection to the mains must be carried out by a qualified person (an electrician). In practice, this means you can lay out and cover the cable yourself, but leave the thermostat and connection to the circuit to an electrician. Otherwise, there is not only a risk of injury but also potential problems with insurance in case of damage.
What should I do if I damage the cable while pouring the screed or laying tiles?
This is one of the most stressful moments of the entire installation. If the cable gets damaged, do not turn on the system – this could cause a short circuit and damage the thermostat or circuit breaker. Before covering it, always measure the cable resistance (with a multimeter) and compare it to the value stated in the documentation. If the resistance differs significantly (by more than 10%), the cable is damaged.
Repairing a damaged cable embedded in the screed is done by a specialist using a special connection kit – it's not a standard procedure, but it can be done. The best prevention is to protect the cable during pouring and tiling work with a protective hose or by being careful when working with tools.
How deep should the cable be embedded?
The recommended minimum thickness of screed or adhesive above the cable is 30–35 mm for screed and 10–15 mm when installing a mat in adhesive. If the layer is too thin, the cable can overheat locally (the thermal energy doesn't have time to spread evenly), shortening its lifespan. If the layer is too thick, the system has greater thermal inertia and responds more slowly, which increases the demands on regulation.
Where should the floor temperature sensor be placed?
The floor temperature sensor is placed in a protective conduit (flexible tube), usually in the middle between two cable loops, i.e., at the coldest spot on the floor. The conduit must lead to the thermostat box and must be replaceable – meaning it must not be permanently embedded in concrete without the possibility of replacement. Seal the ends of the tube with tape before pouring to prevent screed from getting in.
The sensor should be at least 50 cm from the wall, not above an expansion joint, and not in a spot with another heat source (for example, above a hot water pipe).
Thermostats: Questions About Regulation
Which thermostat is right for me?
This is a topic where people most often decide based on price rather than need – which is a mistake. The choice of thermostat has a direct impact on energy consumption and comfort. The basic types are:
- Manual thermostat: a simple dial, no programming. Cheap, but energy inefficient – heating runs whenever you don't manually turn it off or lower it.
- Digital programmable thermostat: allows you to set a weekly program. The temperature automatically drops at night or when you're not home, saving 20–40% of costs compared to a manual thermostat. An example is the HAKL TH 900 digital thermostat – compact, with a clear display and simple daily and weekly cycle programming.
- WiFi thermostat: everything a digital thermostat offers, plus control via smartphone from anywhere. If your plans change (coming home later, unexpected time off), a WiFi thermostat lets you turn the heating on earlier or later without being physically at the thermostat. We offer, for example, the HAKL TH 951wifi or the more advanced HAKL TH 952wifi with a touchscreen display and more precise regulation.
You'll find a detailed guide to choosing a thermostat in the article How to choose a thermostat for electric underfloor heating: manual, digital, or wifi?
Should the thermostat measure floor or air temperature?
It depends on what you want to achieve. For comfort heating (for example, a warm bathroom floor where you stand barefoot), set the thermostat to floor temperature – typically 28–30 °C for tiles, 27 °C for wood and laminate. The system ensures the floor reaches the set temperature and doesn't exceed it.
For primary heating, it's better to measure air temperature, or use a combination of both sensors. The thermostat then responds to the actual room temperature, not just the floor, and better maintains overall thermal comfort in the space.
Modern thermostats like the HAKL TH 952wifi allow you to set both sensors at once with priority – for example, air as the controlling sensor and floor as a protective one (to prevent the maximum floor temperature from being exceeded).
Questions About Consumption and Costs
How much will I pay for electricity with underfloor heating?
Consumption depends on three main factors: installed output, hours of operation, and the price per kilowatt-hour. Consider actual operation – a properly set programmable thermostat keeps the system active only 30–60% of the time (the rest is thermostatic downtime). So a system with an installed output of 1,000 W actually consumes roughly 400–600 Wh/h on average.
Example: a 6 m² bathroom, installed output of 900 W, 8 hours of operation per day, actual consumption 50% = 450 Wh/h × 8 h = 3.6 kWh/day. At a price of €0.20/kWh, that's €0.72/day, or around €22/month during the season. For most households, this is an acceptable amount for the comfort of a warm bathroom floor.
The insulation of the house is decisive. In a well-insulated new build, actual consumption can be up to 40% lower compared to an older renovation with worse insulation.
Is a dual-rate electricity tariff worth it?
For electric underfloor heating used as a primary system on a larger scale (for example, an entire 80 m² apartment), a dual-rate tariff (low rate at night and during the day, high rate at peak times) is usually recommended. The condition is sufficient installed output at the electricity meter and adequate thermal storage capacity of the floor – which acts as a heat reservoir if the screed is sufficiently massive.
For smaller systems (a single bathroom, a single room), a dual-rate tariff usually isn't worth it – the tariff savings are smaller than the fixed difference in monthly fees.
Compatibility with Different Types of Flooring
Can I install underfloor heating under laminate or vinyl?
Yes, but with conditions. Laminate and vinyl flooring must be certified as suitable for underfloor heating (usually indicated by a bathroom or heat symbol on the manufacturer's packaging). This is because many laminate boards can change dimensions, crack, or release unhealthy emissions from adhesives at temperatures above 27 °C.
The maximum floor temperature under laminate is usually 27 °C (some types 29 °C), under vinyl 28–30 °C. The thermostat must have active floor protection – a floor sensor and a set temperature limit.
You'll find details in the article Electric underfloor heating under different types of flooring: tiles, vinyl, laminate.
Can I lay cable under wood or parquet flooring?
Solid wood and electric underfloor heating are not an ideal combination. Wood is naturally a poor thermal conductor, so a lot of heat stays under the wood and the floor overheats, while the room gets little heat. In addition, wood reacts to heat by drying out and cracking.
If you want wood flooring, choose multi-layer (engineered) boards with special certification for underfloor heating, a maximum thickness of 15 mm, and set the system output lower (max. 80 W/m²). Always consult the flooring manufacturer.
Frequently Asked Questions and Common Mistakes
Why doesn't the heating work right after installation?
We see this regularly. A customer installs the cable, turns on the thermostat – and the heating seems not to work. There can be several reasons:
- The screed isn't dry and fully cured yet – a new cement screed needs at least 7–14 days before the first startup, anhydrite up to 28 days. Premature startup can cause the screed to crack.
- The thermostat is set to air temperature and the room is currently warm – the system thinks heating isn't needed.
- The floor temperature sensor is in the wrong position or not properly connected.
- The circuit breaker in the distribution board is switched off or undersized.
We describe how to troubleshoot in more detail in the article Common faults in electric underfloor heating and how to fix them.
Can I place a carpet or rug on the heating?
Yes, but only temporarily and with care. A carpet acts as thermal insulation and prevents heat from escaping into the room. If the carpet stays in place for a long time (all winter), the cable underneath can overheat. The thermostat will detect this via the floor sensor and shut the system off sooner – so the room will never reach a comfortable temperature.
For permanent carpet coverage, it's better to use a lower output (80–100 W/m²) or avoid underfloor heating under carpets altogether and use a different heat source.
Frequently Asked Questions (FAQ)
How long does it take for the floor to warm up after the heating is switched on?
It depends on the thickness and mass of the floor construction. Tiles on a 60 mm anhydrite screed typically warm up to the desired temperature in 2–4 hours from a cold start. Tiles with a mat in a thin adhesive bed heat up considerably faster – 20–40 minutes. Foil under laminate reaches operating temperature fastest, usually within 15–20 minutes. That's exactly why, for thicker assemblies, it's crucial to have a programmable thermostat that pre-heats the room before you need it.
What happens if I forget about the cable and accidentally cut it during a renovation?
Cutting the cable causes a complete circuit failure – the cable stops working because the resistive circuit is broken. It's not dangerous for your home (the circuit breaker will trip), but the cable needs to be repaired using a special connection kit installed by a professional. This repair is possible, but not always 100% reliable. The best prevention: before any demolition or drilling, mark the cable's position, and a wiring map (photo documentation from installation) is invaluable.
Is electric underfloor heating suitable for allergy sufferers?
Yes, and this is one of its main advantages over forced-air systems. Underfloor radiant heating doesn't create air circulation, so it doesn't blow dust and mites around the room. The floor surface temperature (28–30 °C) also isn't high enough on its own to kill dust mites, but the calm air combined with regular cleaning significantly reduces the burden for allergy sufferers. In humid rooms (bathrooms), the higher floor temperature also suppresses mold growth in the grout between tiles.
What is the lifespan of a heating cable? Can it be replaced after years of use?
Quality resistive cables have a lifespan of 25–50 years with proper installation and operation. This is because there are no moving parts in the cable – it simply "just" conducts current. The most common cause of premature failure is mechanical damage (cutting, puncturing) or overheating due to insufficient covering. The thermostat has a shorter lifespan (8–15 years) and is easily replaceable without any demolition. The cable itself, once embedded in the screed, is practically not replaceable without breaking up the floor – which is why it's important to choose a quality product from the start.
Can I use electric underfloor heating only in the summer months as a "floor dehumidifier" in the bathroom?
Not only can you – it's actually recommended. Briefly turning on the heating (for example, for 1 hour in the morning) during spring and summer helps keep the floor dry, which prevents mold from forming in the grout. A temperature of 28–30 °C is enough to evaporate surface moisture and is significantly lower than the critical temperature for any type of common bathroom tile. In terms of consumption, summer operation is negligible (minutes to hours per day), and the benefit to the bathroom's hygienic condition is real.
What are the legal requirements for installing electric underfloor heating in Slovakia?
In Slovakia, electrical installations are governed by STN standards (the applicable one is STN 33 2000 and a series of related standards). Electric underfloor heating, as a fixed installation, must be connected by an electrician with the appropriate qualification. In practice, this means: connecting the thermostat to the distribution circuit, and possibly installing a new circuit breaker or residual current device (a 30 mA RCD is mandatory in bathrooms and wet areas). The law doesn't reserve the laying of the cable or mat itself exclusively for professionals, but the electrical connection is reserved for them. After completion, the electrician should issue a certificate of electrical installation inspection.
Conclusion: A Few Final Practical Tips
Electric underfloor heating is a technically reliable, comfortable, and – when properly sized – economically attractive way to heat your home. Most of the problems we see with customers stem from three sources: insufficient output (the calculation was underestimated), poor installation (cable damage, poorly placed sensor), or an unsuitable thermostat (manual where digital would have sufficed and saved money).
Invest time in correctly calculating the output and choosing the system before you buy. During installation, take photos of every step – when you're drilling into the wall in five years, you'll be grateful for it. And never treat the thermostat as an unimportant detail – it's the brain of the entire system, and it's what determines how much you'll pay for electricity.
If you're unsure about a specific choice, take a look at our electric underfloor heating category or read more articles in our Knowledge Center – topics like Installing a heating mat under tiles: step-by-step guide or Installing a heating cable: spacing, covering, and connection will give you concrete instructions for every step of the installation.
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