Electric Underfloor Heating as a Primary vs. Supplementary Heat Source
Electric Underfloor Heating as a Primary vs. Supplementary Heat Source: A Complete Decision-Making Guide
When a customer asks whether electric underfloor heating will be enough as the sole heat source in an apartment or house, there is rarely a clear yes or no answer. It depends on a dozen factors – from the building's heat losses, through the type of space, to the comfort level the customer actually expects. Over years of practice, I've seen projects where underfloor heating worked as an excellent primary source, others where it served only as a pleasant supplement to a gas boiler, and equally projects where the customer was disappointed because they chose the wrong strategy. This article will help you understand when it makes sense to invest in electric underfloor heating as the main heating system and when it's wiser to rely on it only as a secondary source.
Basic Principle: What Does "Primary Heat Source" Actually Mean
A primary heat source is one capable of maintaining the designed indoor temperature on its own during the coldest winter period – usually 20 to 22 °C at an outdoor temperature of around −15 °C (for most of Slovakia, the design outdoor temperature is −12 to −15 °C). This means it must be sized so that its installed output covers the room's heat losses with a sufficient reserve.
A supplementary (secondary) heat source, on the other hand, serves to improve comfort, heat up a specific zone, or act as a backup. It doesn't need to cover the entire heat loss – it's enough if it adds 30 to 50% of the output that the main source can't deliver quickly or comfortably enough.
Electric underfloor heating can play both roles, but the conditions for successfully fulfilling the primary role are considerably stricter. Understanding this difference is key before you start planning an installation.
When Electric Underfloor Heating Can Truly Handle the Role of a Primary Source
For electric underfloor heating to serve as a primary source, several conditions must be met simultaneously. It's not impossible – in fact, it's quite common in correctly chosen scenarios – but you need to be realistic.
New Construction with a Low-Energy or Passive Standard
This is ideal ground for electric underfloor heating as a primary source. A new building with a wall thermal resistance of U ≤ 0.15 W/(m²·K), triple glazing, heat recovery ventilation, and an airtight building envelope has such low heat losses that an installed electric underfloor heating output of 80–100 W/m² easily covers what the building needs. In practice, we see houses with consumption below 15 kWh/(m²·a) where electric underfloor heating works comfortably as the sole heat source, and the monthly heating costs don't exceed what they would otherwise pay just for servicing a gas boiler.
For such houses, the HAKL TCX 10/1230W heating cable is an excellent fit, offering high power density and reliability proven in practice.
Smaller Enclosed Spaces with Low Heat-Exchange Surface
Bathrooms, hallways, corridors, small offices – spaces up to 20–25 m² where heat losses don't exceed 1.5–2 kW, are an ideal environment for electric underfloor heating as a primary source. In a bathroom, moreover, you don't need to reach 20 °C but rather 24–26 °C, which sounds paradoxically more demanding, but the bathroom is small and the heat loss is low. An installed output of 150–180 W/m² in a bathroom will comfortably ensure a pleasant temperature even during severe frosts.
Renovations of Apartments in Well-Insulated Panel Buildings
After a panel building has been insulated and windows replaced, the heat losses of the apartments drop dramatically. A 70 m² apartment in an insulated panel building may have a heat loss of only 3–4 kW. In such a case, electric underfloor heating with an installed output of around 100 W/m² (7 kW total) has a sufficient reserve and can function as a primary source without problems – no chimney, no gas connection, no boiler.
When Electric Underfloor Heating Is Not Sufficient as a Primary Source
It's equally important to know when you shouldn't go into such a project without a secondary heat source.
Old, Poorly Insulated Houses
A house built before 1980 without additional insulation may have heat losses of 150–200 W/m² of floor area, and sometimes even more. Installing electric underfloor heating with an output of 100 W/m² in such a house as a primary source is like trying to heat a room with a poorly insulated wall using a small electric heater. Physics simply won't allow it. In this case, you either need to insulate the house first, or plan for electric underfloor heating only as a supplementary source.
Spaces with Large Glazed Areas
Conservatories, spaces with glazed façades, or panoramic windows have extremely high heat losses. A glazed area with standard double glazing (U = 1.1 W/(m²·K)) at an outdoor temperature of −15 °C and an indoor temperature of 21 °C loses 36 W per square meter of glass. For conservatories with 30–40 m² of glazing, this means heat losses through the glass alone of 1,000–1,500 W. Electric underfloor heating can work here as a supplement, but the primary source must be something else.
Production Halls, Warehouses, and Industrial Spaces
In industrial spaces with high ceilings, frequent door openings, and insufficient insulation, heat losses are enormous, and electric underfloor heating has no primary role there. In addition, the area available for laying cable or mats is often limited.
Power Sizing: Specific Figures for Various Scenarios
The question of output is crucial, and this is exactly where it's decided whether electric underfloor heating will function as a primary or only a supplementary source. This topic is covered in more detail in a separate article in our Knowledge Center: What Output of Electric Underfloor Heating Do I Need for My Space? Here we'll summarize the most important values.
Recommended Outputs by Space Type and Function
| Type of Space | Supplementary Source (W/m²) | Primary Source – New Building (W/m²) | Primary Source – Standard Building (W/m²) |
|---|---|---|---|
| Living room, bedroom | 60–80 | 100–120 | 130–160 |
| Bathroom | 100–130 | 150–175 | 175–200 |
| Hallway, corridor | 60–80 | 100–130 | 130–150 |
| Kitchen | 60–80 | 80–110 | 110–140 |
| Basement, garage (tempering) | 30–50 | – | – |
These values apply to the actual heated area – that is, the area where the cable or mat is laid, not the total room area. In practice, you subtract the area occupied by furniture and cabinets standing on the floor. In a typical apartment, this means 60–80% of the total floor area.
Practical Example: 65 m² Apartment in an Insulated Panel Building
A customer had a 65 m² apartment in an apartment building after complete renovation – 12 cm of polystyrene insulation, new plastic windows with triple glazing. Heat loss of the apartment after renovation: approx. 3,200 W. He wanted electric underfloor heating as a primary source. Actual area available for heating installation: 48 m² (subtracting the kitchen equipment, wardrobes in the bedroom, sanitary fixtures). Total installed output at 100 W/m²: 4,800 W. This covers the heat loss with a 50% reserve, which is more than sufficient even for the coldest winter days. The customer has been satisfied with this solution for the fourth winter now.
Practical Example: Family House from 1975
Another customer had a 120 m² family house without insulation, with original windows. Heat loss: estimated at 14,000–16,000 W. With an installable area of 80 m², at 160 W/m² we would only get 12,800 W. That's not enough. Solution: the house was first insulated and the windows replaced – heat loss dropped to 7,500 W. Electric underfloor heating was then supplemented with a wood-burning fireplace insert as backup during extreme frosts. This works well, and electric underfloor heating covers 85–90% of the days in the year on its own.
Economic Perspective: Operating Costs of Primary vs. Supplementary Heating
Here comes the topic that interests customers the most: how much will it cost? And it's precisely with a primary source that the economic question becomes critical, because consumption will be several times higher than with supplementary heating.
Annual Electricity Consumption – An Indicative Calculation
Electric underfloor heating works in cycles – the thermostat switches it on and off. The actual operating time during the season is usually 6–8 hours a day in transitional periods and 10–14 hours a day in the coldest month. The average over the whole season (October–April) is roughly 8–10 hours a day.
For a 65 m² apartment with an installed output of 4,800 W and an average operating time of 9 hours a day over 180 days: 4.8 kW × 9 h × 180 days = 7,776 kWh per season. At an electricity price of €0.22/kWh, this comes to approximately €1,710 per year. Is that a lot, or a little? For comparison – gas heating at a gas price of €0.08/kWh and a condensing boiler efficiency of 95% would cost a similar apartment €1,000–1,200 per year. There is a difference, but you must deduct the cost of the boiler (€3,000–5,000), the chimney, and servicing.
A smart thermostat has a major impact on operating costs. A Wi-Fi thermostat such as the HAKL TH 951wifi or HAKL TH 952wifi allows you to set several temperature programs throughout the day – for example, lowering the temperature to 16 °C at night and while you're away from home. This alone can reduce consumption by 20–30%.
Advantages of Low Operating Costs of Supplementary Heating
If electric underfloor heating serves only as a supplement to a gas boiler or heat pump, its annual consumption is much lower – typically 500–1,500 kWh per season. In this role, it's mainly about comfort (a warm floor underfoot), not economics. The costs are negligible, and the customer is satisfied.
Thermal Inertia and Regulation: A Key Property for Primary Heating
When customers consider electric underfloor heating as a primary source, they rarely think through one important aspect: the thermal inertia of the system. Electric underfloor heating embedded in a concrete screed (the most common case with cable) has high thermal inertia – the screed heats up slowly (30–60 minutes to full output) and cools down equally slowly. This has consequences for control.
For primary heating, a quality programmable thermostat is therefore absolutely essential. You cannot switch the heating on and off manually – the system must control the temperature predictively, an hour or more in advance. The HAKL TH 900 digital thermostat with a weekly program handles this solidly, but for a large-scale installation as a primary source, you'll appreciate Wi-Fi thermostats with remote control and detailed programming – for example, the HAKL TH 952wifi, which handles control with daily and weekly profiles and communicates via smartphone.
Conversely, electric underfloor heating in a thin adhesive layer under tiles (mat or thin cable) has much lower thermal inertia and responds faster. This is advantageous for supplementary heating in a bathroom, where you want a warm floor within 10–15 minutes of switching on.
Combining Electric Underfloor Heating with Other Heat Sources
In practice, combining heat sources is very common and makes good sense. Here are the most frequently proven combinations:
Electric Underfloor Heating + Air-to-Air Heat Pump
The air-to-air heat pump (split unit) is very widespread today. It has an excellent COP (coefficient of performance) of 3–5 at temperatures above 0 °C, but at extreme frosts below −10 °C its output drops. Here, electric underfloor heating serves as a stable backup source for the coldest days and as a pleasant heat source for parts of the house that the split unit doesn't heat well (hallway, bathroom).
Electric Underfloor Heating + Wood-Burning Fireplace Insert
A popular combination in family house renovations. The fireplace insert covers peak heat loads and serves as backup during power outages. Electric underfloor heating, in turn, ensures even heat in all rooms, including those that heat from the fireplace doesn't reach. This combination works, but when sizing electric underfloor heating, you still need to proceed on the assumption that the heating must be able to handle the house on its own.
Electric Underfloor Heating as a 100% Supplement to Gas Heating
The most common case, where the customer has gas heating installed but wants a warm floor in the bathroom and possibly the hallway. Here, electric underfloor heating is pure comfort – an area of 4–8 m², output 600–1,200 W, and the customer can also afford a more expensive type of system, because the economics of supplementary heating are more forgiving.
Types of Electric Underfloor Heating and Their Impact on Use as a Primary Source
Choosing between a cable system and a mat or film is another factor that affects suitability for the primary role. This topic is covered in detail in an article in our Knowledge Center: Heating Cable vs. Heating Mat: Which Solution Is Right for You? Here we'll briefly summarize the key differences from the perspective of primary vs. supplementary heating.
Heating Cable – A Strong Candidate for Primary Heating
The heating cable is embedded in a concrete screed or anhydrite floor, creating a system with high thermal storage capacity. This storage capacity is advantageous for primary heating – the system "stores" heat in the mass of the floor and gradually releases it. The cable also allows precise regulation of spacing (typically 8–12 cm), which affects power density. For a primary source, we recommend a spacing of 8–10 cm; for a supplementary source, 10–15 cm.
The powerful HAKL TCX 10/1230W heating cable is an example of a product sized for areas where you need sufficient output for primary heating in a standard building.
Heating Mat – Suitable for Supplementary Heating and Lighter Floors
A mat with a cable spacing of 8–9 cm is fixed and laid directly into adhesive under tiles or under a floating floor. The layer thickness above the mat is only 10–15 mm, and thermal inertia is minimal. This means a fast response (the bathroom is warm within 15 minutes), but also fast cooling after switching off. For supplementary heating, this is ideal; for a primary source, it's acceptable only in well-insulated spaces where the thermostat runs almost continuously.
Practical Recommendations for Design: Step by Step
Based on practical experience, we recommend the following procedure when deciding whether electric underfloor heating will be a primary or supplementary source:
- Step 1: Calculate (or have calculated) the heat losses. Without this figure, you cannot decide. As a rough estimate: new low-energy building 30–50 W/m², insulated apartment 50–80 W/m², standard building 80–120 W/m², uninsulated old building 130–200 W/m² and more.
- Step 2: Determine the installable area. Subtract furniture, cabinets, and sanitary fixtures. In practice, this is 55–75% of the total floor area.
- Step 3: Calculate the maximum installable output. Area × chosen power density (e.g., 120 W/m² for a new building, 150 W/m² for an older building).
- Step 4: Compare with the heat loss. If the installable output covers the heat loss with at least a 20% reserve → electric underfloor heating can be the primary source. If not → supplementary heating or insulation first.
- Step 5: Choose a thermostat appropriate for the role. Primary source = programmable thermostat with a daily profile and both floor and air sensors. Supplementary heating = a simpler thermostat with a floor sensor is sufficient.
- Step 6: Check the electrical preparation. Primary heating in a 120 m² house may mean an installed output of 12–15 kW. This requires a correspondingly sized main circuit breaker, supply cables, and fuses in the distribution board.
Installation is covered in detail in further articles in the Knowledge Center: Installing a Heating Mat Under Tiles: Step-by-Step Procedure and Installing a Heating Cable: Spacing, Screeding, and Connection.
Electrical Installation and Wiring Requirements for Primary Heating
This is an aspect customers often think about too late. Electric underfloor heating as a primary source in a house or larger apartment means a substantially higher power draw than a typical household.
Example: a 100 m² house with an installed output of 110 W/m² over a 75 m² installable area = 8,250 W. That's a separate circuit with a current of about 37 A at 230 V (or 16 A per phase in a 3-phase system). A standard household 3×25 A circuit breaker may not be sufficient. Before design preparation, it's essential to consult an electrician and verify the agreed reserved power capacity with the electricity distributor.
Each room should have a separate circuit with its own 16 A breaker (or 20 A for larger areas) and a 30 mA RCD. This is not only a safety standard but also practical protection – if one circuit fails, the rest of the house keeps working.
Underfloor Heating Under Various Types of Floor Covering
Choosing the right covering directly affects the efficiency of underfloor heating, and therefore its ability to fulfill the role of a primary source. This topic is covered in a separate article, Electric Underfloor Heating Under Various Floor Types: Tiles, Vinyl, Laminate. Here's a brief summary:
- Ceramic tiles and stone – the best thermal conductor, ideal for both roles (primary and supplementary). Tiles with a thermal resistance of up to 0.10 m²·K/W are optimal.
- Laminate and wooden floors – a thermal resistance of 0.05–0.15 m²·K/W is acceptable, but the flooring must be specifically labeled as suitable for underfloor heating. For a primary source, choose a shorter cable spacing.
- Vinyl (LVT) – thin, a good thermal conductor, works well. Important: the surface temperature must not exceed 28–30 °C, which a thermostat with a floor sensor easily monitors.
- Carpet – the thermal resistance of carpet can be as high as 0.15–0.25 m²·K/W, which significantly reduces efficiency. Not recommended for a primary source.
Most Common Mistakes When Installing Electric Underfloor Heating as a Primary Source
Over the years, the same mistakes keep recurring. This topic is also covered in the article Common Faults of Electric Underfloor Heating and How to Fix Them; here are the design-related ones:
- Underestimating heat losses – the customer thinks the house is "well insulated," but the actual losses are twice as high as expected. Result: the system runs at full output continuously and still can't maintain the temperature.
- Incorrectly estimated installable area – the design uses the full room area, but after furnishing with cabinets and a kitchen unit, only 60% of the area remains. Result: the output is insufficient.
- Lack of thermal insulation under the cable – a cable without thermal insulation underneath loses 20–30% of its output downward into the concrete slab. For primary heating, insulation is mandatory.
- Thermostat with only a floor sensor and no maximum floor temperature limit – prolonged operation risks damaging sensitive floor coverings.
- One shared circuit for the whole apartment – if that one circuit fails, you lose heating everywhere.
Frequently Asked Questions (FAQ)
Can electric underfloor heating replace gas central heating in a family house?
Yes, but only under clearly defined conditions. The house must have low heat losses – ideally a new building of low-energy or passive standard, or a house after comprehensive insulation. Heat losses should not exceed 80–100 W/m² of floor area. If these conditions are met, electric underfloor heating with an output of 120–150 W/m² of installable area will cover the house's heat needs even on the coldest winter days – without gas, a chimney, or a boiler.
What is the average monthly consumption of electric underfloor heating as a primary source?
It depends on the size of the space, heat losses, outdoor temperature, and thermostat settings. As a rough guide, for a 60–70 m² apartment in an insulated panel building, expect monthly consumption of 300–600 kWh in January and February (the coldest months), and 150–300 kWh in transitional periods. Annual consumption is usually 2,500–5,000 kWh depending on location and comfort level. A smart programmable thermostat can reduce this consumption by 25–35%.
Is electric underfloor heating as a supplementary source economically worthwhile?
Yes, even despite the higher price of electricity. Supplementary heating in a bathroom with an area of 5–6 m² consumes only 200–400 kWh per year, which represents €44–88 at a price of €0.22/kWh. In return, you get a pleasantly warm floor every morning, faster drying of moisture after bathing, and greater comfort. This investment is clearly worthwhile.
Do I need to install thermal insulation under the cable for electric underfloor heating as a primary source?
Yes, thermal insulation under the heating cable (usually extruded polystyrene XPS 20–30 mm) is essential, not optional, for primary heating. Without it, 20–40% of the heat output escapes downward into the concrete slab and foundations, which increases consumption and reduces efficiency. On the first floor and above, insulation is less critical but still recommended.
Can electric underfloor heating maintain the temperature during extreme frosts of −20 °C?
It depends on the sizing. In a passive-standard new building: yes, without problems. In a standard insulated building: probably yes, but with minimal reserve and assuming continuous operation. In an old, uninsulated house: no – at such temperatures, the heat loss can be 2–3 times higher than the designed output. For extreme conditions, we recommend a backup source (fireplace, heat pump).
How long does it take for a floor with an embedded heating cable to reach operating temperature?
For a cable embedded in a concrete screed, expect 30–90 minutes from switching on to reaching full surface floor temperature. This is why a programmable thermostat is key for primary heating – the system must switch on the heating predictively, with sufficient lead time. Modern Wi-Fi thermostats such as the HAKL TH 951wifi have an adaptive start function that "learns" the system's thermal inertia and automatically sets the lead time.
Conclusion: How to Correctly Choose the Role of Electric Underfloor Heating
Electric underfloor heating is an exceptionally comfortable and hygienic form of heating – heat distribution from the bottom up, no drying out of the air, no dust carried by convection from radiators, no visible heating system elements. These properties apply to both roles – primary and supplementary.
For the primary role, the key prerequisite is low building heat loss. If the building meets this condition – a new energy-efficient building or a well-renovated older property – electric underfloor heating is a fully-fledged, reliable, and technically elegant primary heat source. No boiler, no chimney, no fuss. With smart control and minimal maintenance.
For the supplementary role, far fewer restrictions apply. Here, electric underfloor heating excels in all situations – it adds comfort, can heat up spaces that the main source can't reach, and also serves as a backup. The investment is lower, operation simpler, and the customer is almost always satisfied.
If you're hesitating between the primary and supplementary role, start with your building's heat losses. That's the one figure that will give you the right answer. Everything else – output, area, cable type, thermostat – follows from it. And if you're not sure about the calculation, check out our article What Output of Electric Underfloor Heating Do I Need for My Space? – you'll find a detailed procedure there, along with practical tables for various types of buildings.
You can find more about choosing the specific system – cable, mat, or film – in the article How to Choose Electric Underfloor Heating: Cable, Mat, or Film? and about choosing a thermostat in the article How to Choose a Thermostat for Electric Underfloor Heating: Manual, Digital, or Wi-Fi?
Do You Have a Question on This Topic?
Can't decide, or are you dealing with a specific situation in your household? Write to us - we're happy to help.