Electric underfloor heating

Electric underfloor heating is a local heating system. Electrical energy is converted into heat, and that heat is transferred and released into the space, in a single compact unit located directly in the heated room. The conversion efficiency is almost 100%, with no unnecessary losses in transporting energy to the point of use. The heating is environmentally friendly, as it causes no pollution at the place of use. No chimney, fuel store or space for a boiler room or boiler is needed. Compared with other heat-transfer media, distributing electricity is considerably simpler and cheaper to install, with less disruption to the building structure and lower space requirements. As far as the spread of heat flow from the heated floor into the interior is concerned, the same principles apply as for the water-based type. A great advantage of heating with electricity is that it can be metered precisely for each individual consumer, which encourages them to reduce their consumption wherever possible.

The range of uses for electric underfloor heating systems is very wide. The system can provide thermal comfort as the main heat source, or it can serve as supplementary heating that keeps the floor at a pleasant temperature. Because the heating can be run in different operating modes, which determine the floor construction and the output of the heat source, the system is suitable not only for newly built properties but is also one of the options for renovation. It can be installed across the whole floor area, or only certain parts can be heated. As there is no risk of the system freezing, it is also suitable for rooms or buildings that are heated only occasionally. The range of applications for electric heating elements can be summed up in the following areas:

• large-area heating and background heating,

• protective systems,

• special applications.

The history of electric heating

Heating with electricity does not have a long tradition in Slovakia. Its beginnings date back to the 1930s, when only a few dozen family houses were heated electrically. After the Second World War this method of heating was practically banned because of the acute shortage of electricity and distribution networks. Electricity was used first and foremost to meet needs where it was irreplaceable (lighting, drives), and also for cooking, ironing and heating water.

The development of electric heating began in the 1960s, when the power industry started to support mainly storage-type consumption, which was intended to even out the load on the electricity grid. A reduced tariff was introduced for this type of consumption. The first regulation on permitting electric heating in buildings was also issued, setting strict requirements for the thermal performance of the building envelope. Electric heating developed slowly, because there was a lack of electric heating appliances and control equipment, as well as of design experience. The price of electricity was also considerably higher than that of other fuels.

After 1969 electricity became one of the media that could help solve the problem of air quality. It was permitted mainly in new buildings and as a replacement for solid fuels in the non-manufacturing sector. Suitable conditions for the development of storage heating began to emerge through the reconstruction of secondary networks, the setting of a suitable tariff for households with storage consumption, and promotional and advisory activities. In this period approximately 10 000 households in Slovakia were heated electrically, either as supplementary heating or as heating in the transitional seasons.

Pressure to develop electric heating came after the oil crisis of 1973, which led to a rise in the price of liquid fuels. The share of electricity generated in nuclear power stations increased, and the technical means by which the power industry managed electricity consumption were gradually developed as well. A wider range of electric heating appliances and more modern automation and control equipment began to appear on the market. In the 1980s about 25 000 households in Slovakia were heated by storage heaters, and the range was extended to include electric hybrid heaters.

The rapid growth of electric heating began in the 1990s. The range of heating appliances from domestic and foreign manufacturers was enriched with new types. The oil crisis of 1991 resulted in a rise in the price of gaseous fuels, which brought an unexpectedly favourable change in the price of electricity. A new tariff was created for direct-acting electric heating. At that time about 40 000 flats were heated by electricity alone, of which about 90% used storage heaters. Electricity consumption changed not only in volume but also in structure. Consumption by households and the tertiary sector took a larger share.

From 1993 there was a rapid and almost uncontrolled growth in direct-acting electric heating. This situation arose as a result of a very favourable tariff for direct consumption, the low investment cost of the heating system and a wide range of direct-acting appliances. This boom brought a number of problems with it. The capacity of the electricity networks, especially at low and high voltage level, was insufficient. The differences in the daily load curve deepened; storage heating was supposed to stabilise it. The requirements of the regulations in force, which set the conditions for installing electric heating in buildings, were not respected. Heating was installed in buildings with high heat losses, which increased running costs by 100 to 250 %.

Since the end of the 1990s there has been a slight decline in interest in electric heating, caused by the constant rise in electricity prices. Today electric heating in Slovakia accounts for approximately 4% compared with other types of heating. By comparison, the figure is approximately 8% in the Czech Republic, 10% in Austria and 10% in Germany.

Calculating electric underfloor heating

The thermal design calculation for electric underfloor heating covers:

• the design of the floor build-up and the materials of the individual layers;

• determining the temperature conditions in the floor while the system is running;

• determining the power input of the heat source.

Basic types of electric underfloor heating

The construction of the heated floor depends on the chosen operating mode of the heating system, which can be storage, semi-storage or direct-acting. The operating mode determines the requirements for the thickness of the storage layer, the depth at which the heating elements are laid, the quality of the thermal insulation and the type of floor covering, and possibly the type of heating element used. The floor construction and the depth of the heating cable plane also determine the size and ratio of the heat flows from the plane of the heating cable axis in both vertical directions.

Storage heating mode

Because of the limited charging time and the need to store heat over a long period, this heating system has the most massive floor construction. The storage slab is a concrete layer whose thickness ranges from 90 to 150 mm, depending on how much heat needs to be stored. The heating elements are laid roughly in its lower third, so the concrete is poured in two stages. To keep downward heat loss to a minimum, good-quality thermal insulation (50 to 60 mm) is needed beneath the heated surface. The installed power input of the heat source ranges from 160 to 200 W/m2, depending on the purpose of the heated space.

Semi-storage heating mode

A charging time spread over the day, with the option of topping up when needed, allows a thinner storage layer. A thickness of 60 to 90 mm is sufficient for the required storage, and the heating cable plane is placed roughly halfway through it. The thermal insulation layer is of similar construction and quality to that used in storage mode. The installed power input of the heat source ranges from 120 to 160 W/m2.

Direct heating mode

As electricity is drawn from the grid throughout the working cycle, there is no need to store heat in reserve. The concrete layer serves only to embed the heating elements and is no more than 50 mm thick; alternatively, the heating elements can be laid just beneath the floor finish, which makes the system more responsive. The heating cable plane is placed as close as possible to the floor surface. The installed power input of the heat source is one third to one half lower than in storage mode, ranging from 80 to 140 W/m2. The insulation beneath can be thinner.

Thermal design calculation for electric underfloor heating

The following boundary conditions must be respected in the design:

• the total heat loss of the room;

• the nature of the heated space (living room, kitchen, hallway, etc.);

• the way the room is used (permanent, irregular or other occupancy);

• the operating mode of the heating system (storage, semi-storage, direct);

• the floor area available for laying the heating cables;

• the requirements for the floor covering.

Designing the floor build-up

For an electric underfloor heating system to work at its best, it is important to design correctly the individual layers above and below the heating cable plane, together with their thickness and thermal conductivity. The thermal resistance of these layers affects the surface temperature of the heating elements, so care must be taken not to exceed the maximum value set by the manufacturer. Otherwise the insulating sheaths overheat and then degrade, which shortens the service life of the whole element. Heat must therefore be able to dissipate freely from the heating cable plane, which is ensured by surrounding the heating element with a material whose thermal conductivity is greater than 0.1 W/m.K.

Resistance core

This is where electrical energy is converted directly into heat. The right choice of conductor material and its geometric dimensions can significantly influence the resulting heat output of the cable, as well as the thermal load on the cable materials. By construction, they can be divided into three groups:

• resistance wires,

• stranded resistance conductors,

• semi-conductive polymer compounds between two stranded copper conductors.

Resistance cores can have a circular cross-section or be made in the form of a flat strip. They are made from copper alloys with added nickel, manganese or chromium. Their characteristic properties are a low coefficient of thermal expansion and a low temperature coefficient of resistance. They are manufactured in diameters of 0.1 to 4 mm, with a heat output of 5 to 70 W/m, which depends largely on the cross-section of the cable and the number of cores. Self-regulating conductors are a special group of heating cables. They consist of two parallel stranded copper conductors with a bridge of temperature-dependent resistive material between them. The output of the cable falls along its entire length as the temperature rises (and vice versa), so it cannot overheat under any circumstances, even where several elements cross.

Insulating sheath

Depending on the intended use of the cable, this may be single or double, and is usually made of polymer-based materials. Its basic function is to insulate the conductor and at the same time dissipate the heat generated. It should therefore have good electrical insulating properties, high long-term heat resistance, high thermal conductivity, suitable mechanical properties and good workability. The materials of the insulating sheaths and any outer jacket of the heating cables may differ, in which case they can affect one another, sometimes quite markedly, especially at higher temperatures. Compatible combinations of polymer materials are therefore recommended.

Protective elements

These consist of a metal braid, an outer jacket or a combination of the two. They increase the mechanical strength of the heating conductor or provide additional electrical protection in conditions where this is necessary (for example, in potentially explosive atmospheres).

Heating cables can be used widely in the following areas:

• large-area heating and background heating,

• protection against frost and ice,

• process applications in industry,

• applications in agriculture,

• special uses.

Cables for large-area heating and background heating

The most commonly used conductors have one or two cores and a double insulating sheath, sometimes with a protective braid. They are used for large-area heating and background heating of interiors, or of outdoor surfaces to protect pavements, access ramps, bridges, roads and the like from ice. They are laid in concrete or screed floor layers or under tiles, and are also suitable for rooms with increased humidity. Cables with an output of 10 to 20 W/m are used for heating interiors; higher loads are permitted for outdoor heating, but these depend on the specific cooling conditions. The cables form heating circuits with cold leads at the ends, which allow connection to a 230 V supply, or less commonly 400 V. They heat up to a maximum core temperature of 80 °C. The overall thickness of cables used indoors ranges from 4.5 to 8.5 mm, depending on the specific type used.

Electric heating mat

This consists of a carrier mesh to which a heating cable with a maximum diameter of 3 mm is attached in a serpentine pattern, with cold copper leads at both ends. The overall thickness of the mat is approximately 2.5 to 3.5 mm. Mats are made in a standard width of 0.5 m and in lengths from 1 to 20 m, with an output per unit area of 100 to 160 W/m2 (depending on the manufacturer). The required total heating output is achieved by a suitable combination of individual pieces.

Mats are intended primarily for heating interiors, as main or supplementary heating, but they can also be used for other applications. Being so thin, they are well suited to floor renovation. Their advantages are a much shorter installation time and less scope for installation errors and damage to the heating cable. Whether a mat is suitable for a particular purpose depends on the type of cable used.

Electric heating film

The heating element consists of two laminated polyester films with a layer of homogenised graphite between them. Supply conductors run along the edges of the film. The active width of the film is 0.5 m, and on both sides there are passive strips 0.05 m wide, which are used to fix the film to the floor structure. The overall width of the film is 0.6 m. It is made with outputs of 150 and 200 W/m2 for electric underfloor heating installed in concrete, 60 W/m2 for wooden floors and 80 W/m2 for laminate floors. The overall thickness of the film is approximately 0.4 mm. It is used for electric underfloor heating and background heating of interiors in new buildings, but thanks to its low thickness it is particularly suitable for renovations, because it can be placed directly on the existing floor beneath the new floor finish.

Installing underfloor heating

The installation of electric underfloor heating proceeds in the following stages:

• creating the heated floor,

• installation work,

• testing the heated floor.

Creating the heated surface for electric underfloor heating

The resulting floor build-up may differ from that of water-based heating because of certain features specific to the electric type, such as a different heating mode or the heating element used.

Ways of constructing the heated floor

In terms of construction technique, the heated floor can be built in three basic ways:

• installation in concrete,

• installation directly beneath the floor finish,

• installation in timber joist floors.

Installation in concrete

The heating elements are embedded in a layer of concrete consisting of two parts:

• a levelling layer, on which the heating elements are laid and which makes up 1/3 to 1/2 of the total thickness of the concrete slab, depending on the mode;

• a covering layer, which is poured over the heating elements.

With direct heating, the heating elements can also be laid directly on the thermal insulation and then covered with a concrete or cement screed. When heating cables are used, wire mesh is placed between them and the insulation; a heating mat or film is fixed directly to the insulation. Heat must be able to dissipate freely from the surface of the heating elements, otherwise they overheat and their service life is reduced. They must therefore not be pressed into the insulating material or surrounded by it.

Installation beneath the floor finish

This method is used in direct mode to make the heating system as responsive as possible, when renovating old floors, or where there is not enough room height available. The heating elements are laid directly on the thermal insulation layer of a new or existing floor structure in line with the principles described above. The heated surface is covered with a layer of levelling compound or adhesive and the floor finish is laid, most commonly tiles or laminate flooring.

Installation in timber joist floors

When the heating system is installed in timber floors, the heating elements are laid on the thermal insulation, and the same principle applies as for the wet method. An air gap of at least 25 mm must be left between the heated surface and the underside of the timber floor.

Ways of laying heating elements for electric underfloor heating

The heating elements for electric underfloor heating are heating cables, heating mats and heating films.

Heating cables

When the system is running, the heating elements warm up evenly along their entire length or area, which gives an even surface temperature across the heated floor. As regards the horizontal temperature gradient, temperatures fall from the external wall towards the internal wall in much the same way as with the water-based type. This effect can be partly eliminated by creating edge zones with a higher output per unit area. The heating circuit is formed by laying the heating cables in a serpentine pattern, with the spacing between individual runs depending on the cable loading and the required output of the system per unit area.

Technique and installation of electric underfloor heating

In this section we describe how to build a heated floor with electric heating elements embedded in concrete or placed directly beneath the floor finish; the sequence of steps is the same as for the water-based type installed by the wet method. We shall focus on the differences, which lie in certain points of the technique and installation itself.

Preparing the building

The heated floor is laid only after all plastering work has been completed and the insulation has been laid. Before the internal plastering, the electrical wiring and connections for the heating system must be installed. The supply conductors are sized according to the designed output of the relevant heating circuits and must meet the following parameters:

• supply for heating rooms < 10 m2: copper conductor 3 x 1.5 mm2;

• supply for heating rooms > 10 m2: copper conductor 3 x 2.5 mm2;

• supply for heating edge zones and for other supplementary heat sources < 2 kW: copper conductor 3 x 1.5 mm2;

• where a central time switch is used, a 1 x 1.5 mm2 copper control conductor must be installed to each thermostat

Back boxes are fitted where the controllers will be located. The position of the controllers depends on the function of the room and the client's requirements. Thermostats with room temperature sensors should be positioned so that their operation is not affected by unwanted factors (direct sunlight, radiant heat from a heat emitter, draughts, etc.). The thermostats are connected to the electricity supply. The heating elements must be permanently connected via the controller; a flexible lead is not permitted. The supply points are laid out in accordance with the technical documentation. Two flexible conduits must run from the thermostat down to the floor, one for connecting the cold lead of the heating cable and one for the floor temperature sensor. The entire electrical installation must be carried out in accordance with the regulations in force.

The principles for preparing the floor, laying perimeter insulation strips, thermal and acoustic insulation and damp-proof membrane, forming expansion joints, and laying the concrete screed and floor finish are the same as for the water-based type.

Laying electric heating elements

Electric heating elements are laid according to plans prepared in advance for each room, showing the layout of the heating loops or the strips of mat or film, and where the cold leads run to the back boxes. Always bear in mind that heating elements are laid only in free, unobstructed areas, so you need to know at least the approximate position of fixtures and fittings in the room. If furniture stands above the heating cable plane, air must be able to circulate freely.

Installation in concrete

Heating cables and heating mats are the heating elements most commonly used with this method. In storage and semi-storage mode, a levelling concrete layer is first laid on the damp-proof insulation (1/3 to 1/2 of the thickness of the finished concrete slab, as determined by the designer's calculation). The concrete mix must be thoroughly compacted and free of sharp fragments, and the surface must be smooth, without protrusions or unevenness, and swept clean. It is primed before the heating elements are laid.

To prevent damage to the elements, to maintain the prescribed cable spacing and to stop mats overlapping, the cables must be firmly fixed to the substrate during installation. Heating cables are usually secured with fixing strips, which are anchored to the concrete slab at right angles to the intended direction of the cables. The strips should be no more than 1 m apart; with wider spacing, the even spacing of the cables could be disturbed when the concrete is poured. The strips have cable clips at standard intervals, for example 25 mm.

Another option is to fix the cables to steel mesh. This method is chosen where the concrete slab is thin, in direct mode, or where reinforcement is prescribed for the floor for structural reasons. The drawbacks are that installing the elements is more laborious and there is a higher risk of mechanical damage during laying. On suitable surfaces, the heating loop can be unrolled loosely on the substrate and fixed with adhesive tape. The cables are fixed at the spacing given by the calculation, and the minimum bending radius is specified by the manufacturer. The cables must be at least 50 to 100 mm from vertical walls and from fixtures and fittings. If the heating element is stiff, it is unrolled and connected to the supply for a short time until it becomes sufficiently flexible. The heating elements are laid only once the concrete slab has hardened sufficiently (2 to 4 days), usually working from the windows towards the door. The following principles must be observed when laying electric cables:

• Take care not to damage the cables, for example with tools. If a cable is damaged nevertheless, it must not be repaired provisionally; the damaged loop is replaced with a new one.

• The active parts of the cables must not cross, touch or pass freely through the air, and must not be in contact with water pipes, the bath, etc.

• The cables must not be shortened. Only their cold leads may be shortened.

• The joint between the cold lead and the heating cable must not be installed in a bend.

• The cables must never pass across expansion joints.

• Where cold leads pass across expansion joints, they must be laid loosely in two concentric protective conduits

At the same time as the heating elements, the thermostat's floor sensor is placed in the heating cable plane in accordance with the following principles:

• The sensor and its lead are placed in a protective conduit midway between two adjacent cables.

• The end of the conduit is sealed to prevent the screed mix from getting in.

• The protective conduit is fitted before the cables are laid.

Heating elements are installed at temperatures above 5 °C. Once the elements have been fixed, the resistance of the conductor and of its insulation must be measured. If everything is in order, the covering layer of concrete mix is applied. The mix should completely encase the heating element so that no air pockets form, as these would reduce the efficiency of heat dissipation from the cable surface. When spreading it, care must again be taken not to damage the elements. Finally, the resistance of the heating circuits is measured once more. Both the active parts of the elements and the cold leads must be embedded in concrete and must not come into contact with air anywhere. The floor finish is laid only once the concrete mix has dried out sufficiently (28 to 30 days).

Once all building work is complete, the thermostats and floor temperature sensors are fitted and the whole system is commissioned.

Installation directly beneath the floor finish

With this method the finished floor structure should be as thin as possible, so thin heating mats and heating films are usually used as the heating elements. The substrate on which they are laid must be perfectly clean, smooth and free of dust and sharp objects. It is treated with a suitable primer, and the heating elements are laid only once it has dried. In a renovation, the heated floor can also be laid on the existing floor finish. If this is tiled, it must be sufficiently level, and minor defects must be repaired. If the existing layer is made of wood products (parquet, chipboard), its load-bearing capacity must not be impaired. The substrate must be primed and covered with a levelling layer to separate it from the heating elements. For fire safety reasons, this layer must be at least 5 mm thick.

A groove is cut in the floor for the protective conduit containing the floor temperature sensor, which will run to the back box. The end of the conduit is sealed and its position fixed. Recesses are then formed in the substrate to accommodate the joints between the active parts and the cold leads.

The heating elements are unrolled according to the laying plan and prepared for laying. To prevent them from shifting during installation, they must be firmly fixed to the substrate. This can be done in several ways. Heating elements may be self-adhesive, with a special adhesive on the element that allows it to be stuck down and lifted repeatedly. They can also be fixed with adhesive tape or pressed into a layer of flexible adhesive.

During laying, care must again be taken not to damage the heating element anywhere, and its resistance values must be measured. The strips of heating element are laid one at a time, and strips laid in parallel must not overlap. If the direction of a strip needs to change, in the case of a mat the carrier mesh is cut right up to the heating conductor. The strip can be turned through an angle of up to 180°, but the conductor must not be left under tension. Overlapping sections of mesh must be removed. If heating film is used, after fixing it is covered with a protective vapour barrier, which is taken far enough up the wall and secured. The cold leads of the elements must be routed to the back box clear of the laid strips.

Levelling compound or tile adhesive is applied over the whole heated surface so that the heating elements are evenly covered and no air pockets form anywhere. The power input of the heating elements must be measured again. If everything is in order, the floor finish can be laid.

Testing electric underfloor heating

Underfloor heating is put into operation only after the concrete has cured properly, after 4 to 6 weeks. The floor temperature must be raised gradually, because switching on at full output could cause water to evaporate rapidly from the concrete near the heating elements and air pockets to form. The system is switched on for the first time at a reduced temperature of no more than 20 °C. The following procedure must be observed:

• commissioning is carried out with the consent of the heating system supplier;

• on the first day the floor temperature is 5 °C higher than the temperature of the heated interior;

• on the second and subsequent days the temperature is raised by 5 °C each day until the floor surface temperature reaches 30 °C;

• the floor temperature must be kept at 30 °C for at least three days;

• the surface temperature is then lowered by 10 °C each day until it reaches the value it had at the start of the tests;

• the control and regulating devices must be set to their standard values before the system is switched on.

A record of the test is drawn up. Once all work is complete, the installing company hands over to the customer valid technical documentation and drawings, the test record and the user operating instructions.

References:

Dušan Petráš, Miroslav Kotrbatý: Heating of Large-Space and Hall Buildings