Heat output control
You will find our range in the electric underfloor heating category – heating mats, cables and thermostats.
The output of electric heating systems, such as Fenix underfloor heating, is controlled by two-position (on/off) control, based on the principle of interrupting or restoring the electricity supply to the heating circuit when the limit value set on the controller is reached.
In most cases the heated area is divided into occupied zones and perimeter zones, which have different floor surface temperatures. Each heating circuit must then be controlled separately. The devices used for this control are generally far more reliable on the power side than the final control elements of a wet heating system (actuators with control valves).
When choosing a suitable type of control equipment, you need to take into account:
• whether the heating system, be it Fenix underfloor heating or another, works as a supplementary or as the main heat source;
• the operating mode of the system (storage, direct);
• the type of building;
• the user's requirements.
The most accurate control is achieved by fitting separate controllers in each room. Electronic thermostats are used, usually with a night setback function, the size of which is either fixed or can be set by the user. Thermostats can be supplemented with a time switch with a time programme; more modern products have the timer built in. The choice of control components is wide – from simple analogue units to complex digital control units with a variety of intelligent functions.
Control according to room temperature
This is the basic way of controlling the output of any electric underfloor heating, for example Fenix underfloor heating, designed as the main heat source. A thermostat fitted with a floor temperature sensor and an indoor air temperature sensor is used.
The floor temperature sensor is built into the plane in which the heating cable is laid, ideally in its centre at the intersection of the diagonals, but at least 0.5 m from its edge, parallel to the heating conductors so that they do not cross one another. So that it can be replaced, it is placed in a metal tube whose end is sealed.
The air temperature sensor is either built directly into the housing of the thermostat installed in the heated room, or it can be external, mounted on the wall. It is positioned approximately 1,200 to 1,500 mm above the floor so that it gives the most objective information possible about the actual temperature. It must not be screened by furniture or exposed to draughts or to heat gains from internal or external sources. Thermostats with an external temperature sensor are well suited to buildings where tampering by unauthorised persons has to be prevented (for example in schools, hotels, public administration buildings, etc.). They are installed in the distribution board.
Every thermostat that controls floor temperature must be fitted with a floor temperature limiter, which protects the heating elements against an undesirable rise in temperature in the plane in which the heating cable is laid. It is located in the connection box in the wall. It works by switching the system off when the maximum temperature in the heating plane is reached .
The required air temperature in the heated room and the maximum floor surface temperature are set on the thermostat. The control device works by keeping the air temperature at the set value by regulating the floor temperature. When the set air temperature or floor temperature is reached, it interrupts the electricity supply to the circuit.
Control according to outdoor air temperature
This method is suitable for rooms with large glazed areas, where a room thermostat reacts late to heat gains from sunshine and, because of the thermal inertia of the system, overheating occurs. In this case the outdoor air temperature sensor is placed on the south façade; the control equipment detects the sunshine in advance and lowers the floor temperature ahead of time.
Control to a constant value
For rooms in which there are heat gains, or where the basic heat loss is covered by another heating system, underfloor heating, whether Fenix underfloor heating or another, can be designed as a supplementary source to provide a pleasant floor temperature (kitchens, corridors, halls, bathrooms). The output is controlled by a thermostat with a temperature sensor built into the plane in which the heating cable is laid, and the floor temperature is kept at a constant value. The temperature sensor is placed in a metal tube at the level of the heating elements, parallel to them so that they do not cross one another.
Heat output control of storage underfloor heating
In storage mode there is a time lag between the phase in which energy is drawn and converted into heat and the phase in which the heat is released into the room; the system discharges of its own accord and can hardly be influenced by control equipment. Output control is therefore more demanding than in direct mode, and the quality of thermal comfort is not always optimal either. A suitable solution is to supplement the basic underfloor heating, for example Fenix underfloor heating, with a dynamic system (electric convectors, radiators, direct heating in the perimeter zone).
Output is regulated by weather-compensated control, which manages the charging of the system according to the outdoor temperature and the residual heat in the floor. The controllers are also fitted with a floor temperature limiter to protect the heating elements against exceeding the permitted temperature in the plane in which the heating cable is laid. The same principles apply to positioning the floor temperature sensor as for a direct system. The outdoor air temperature sensor is installed on the north side of the building, in a spot protected from sunlight, approximately 2 to 3 m above ground level. The sensor is fitted in an installation box. So that the heating system adapts better to the thermal inertia of the building, it can be recessed into the external masonry (render).
Charging can be controlled manually or automatically. With manual control, various parameters have to be set by hand according to the weather and the time at which energy is drawn. With automatic control, the controllers have a built-in function that adjusts the charging characteristics and adapts the start of charging, the setback time and the discharge time independently, according to the current weather. This may be forward control or backward control. With forward control, the system is instructed to charge from the start of the low-tariff period, for the time required. With backward control, the required charging time within the low-tariff period is counted back from the time the low tariff ends. Forward control is suitable where the charging times are the same or where the starts and ends of the charging periods are irregular. Backward control can be chosen in the other cases.
New-generation controllers work on the principle of fuzzy logic. The temperature set for switching the system on/off can vary within a certain interval. The controller can, for example, recognise a temporary drop in temperature caused by an open window and assess whether the heat stored in the floor will compensate for it. Older-generation controllers control the heating output by switching the electricity supply to the circuit on/off when the set temperature is reached, so short-term fluctuations can lead to intermittent overheating.
Control by ripple control signal (HDO)
Electricity consumption from the grid is not even throughout the day; there are peak intervals and intervals of surplus energy. It depends on a number of factors arising from people's habits and from industrial production processes, and is influenced by the weather (outdoor temperature, cloud cover, etc.). From the point of view of economy, conditions must be created in which the distribution equipment and power stations are used optimally, i.e. energy consumption is evened out as far as possible, while at the same time avoiding losses in production and restrictions on consumers.
Energy companies influence electricity consumption firstly by means of price tariffs, which form part of the conditions of energy supply and create pressure to use energy at off-peak times, and secondly operationally, by means of ripple control. Ripple control makes it possible to release or block electrical appliances remotely from a control centre, depending on the actual conditions in the distribution network (Figs. 5.39 and 5.40) . In the past only storage appliances were controlled by ripple control; today the range has widened to include the appliances of both small and large consumers, such as storage water heaters in households and in industry, storage and direct electric heating, heat pumps, feed preparation equipment in agriculture, air-conditioning equipment, electric furnaces and ovens in industrial and food production, pumping equipment in waterworks, spas, swimming pools, etc.
Controlling consumption has a favourable effect on:
• reducing the cost of purchasing energy,
• better utilisation of power stations and minimising the cost of generating electricity,
• making fuller use of investment in the equipment of distribution networks, since a larger number of storage and other appliances can be connected.
Operation of electric underfloor heating
The operation of electric underfloor heating (EUH) – Fenix underfloor heating and others – depends on:
• the operating mode,
• the energy demand,
• the building.
Operating modes of electric underfloor heating
Depending on the time at which electricity is drawn from the grid, we distinguish three basic operating modes of electric underfloor heating:
• storage,
• semi-storage,
• direct.
Fenix underfloor heating offers floor build-ups for each and every operating mode.
EUH storage mode
The working cycle of this method of heating is divided into two stages. In the first stage (charging time), electricity is drawn continuously from the grid, converted into heat and stored in the thermal mass of the floor. This takes place when the load on the grid is reduced, most often at night, for 8 to 10 hours (the period in which the low tariff applies). In the second stage (discharge time) the system is blocked. The stored energy is released into the room of its own accord, accompanied by a gradual fall in the floor surface temperature.
The limited charging time results in a higher installed power input, which in turn calls for a larger electrical service connection. In areas where the electricity networks have a low capacity, a large connected load causes problems with connecting a larger number of consumers.
Storage heating places heavy demands on the floor construction, reduces the clear height of the room and imposes increased requirements on the load-bearing capacity of the floor slab.
The controllability of the system is problematic. The system cannot respond flexibly to sudden changes in operating conditions (a change in temperature as a result of a change in the weather or of heat gains). There are also problems with ensuring thermal comfort in the afternoon hours when the outdoor air temperature drops, and in the transitional seasons. Because of its high thermal inertia, it needs to be designed for solid, heavyweight buildings with a uniform thermal regime and without large glazed areas.
Although this description is not very favourable to the storage system, this method of heating plays an irreplaceable role as a regulating element in balancing demand on the electricity networks, and until quite recently storage consumption enjoyed preferential prices and was given priority when applications for electric heating were approved.
EUH semi-storage mode
This heating mode could also be described as storage mode with a split charging time. As in storage mode, the working cycle of the system consists of two stages – a charging stage and a discharge stage. The charging stage can be spread over several time bands, with the main consumption of electricity taking place at night; if needed, which is not always the case, the system can also be topped up during the day, mainly in the afternoon hours. This also means lower demands on the quality of the thermal insulation beneath the plane of the heating cables and on the thickness of the storage layer of the floor construction. Given the shorter time during which energy is drawn and the option of topping up, the required installed power input of the heat source is reduced.
Compared with storage consumption, the capital costs of the floor construction, the heat source and the size of the service connection are lower. The grid is not burdened with such sudden loads, and the number of consumers that can be connected to a given network capacity in the area increases. The system is easier to control, which also brings energy savings in operation compared with the previous type. It is suitable for new buildings in use all day. Because its thermal inertia is still relatively high, it is difficult to ensure thermal comfort with it in the transitional seasons.
EUH direct mode
The operating mode of this method of heating is diametrically different from the previous two types. The heat source draws electricity from the grid as needed throughout operation, except during the time the system is blocked, which is at least 2 hours a day.
The system is characterised by a minimal thickness of the heated floor, as there is no requirement for heat storage. On the contrary, it is desirable to make the system more responsive, which is achieved by laying the heating cable as close as possible beneath the floor finish. If the heating elements are installed in a layer of concrete, it serves only as a covering. The installed output is almost half that of storage mode, which also means a smaller electrical service connection and the possibility of connecting half as many consumers again to a given grid capacity.
Thanks to the minimal thickness of the floor construction and the possibility of laying the heating surface on an existing floor without the need for demolition work, the system can be used when refurbishing a heating system. Compared with the storage and semi-storage modes, it features a short warm-up time (30 to 120 minutes depending on the floor covering used and the thickness of the concrete layer), better controllability and a flexible response to changes in operating conditions, which means energy savings. It is not a typical direct system with an immediate response to the controller. A certain thermal inertia, determined by the mass per unit area of the floor finish, is useful for bridging the time the system is blocked without reducing the quality of thermal comfort.
The capital costs are also lower than for the other modes, thanks to the thinner floor, the smaller electrical service connection, the lower installed power input of the heat source and the lower static load on the floor structures.
Operating electric underfloor heating in buildings
Thanks to the variety of operating modes, the simple control of heat output (on/off) and the possibility of using familiar control strategies, electric underfloor heating is widely used in buildings of various kinds for heating indoor or outdoor areas. It can be operated in buildings in permanent use, in occasional use, or as supplementary heating.
Buildings in permanent use
Electric underfloor heating as a permanent heat source is used in residential buildings (family houses, housing developments), hospitals, crèches and nursery schools, recreational and spa facilities, and mountain hotels and chalets. The heating system is designed as the sole heat source and covers the total heat loss of the heated room. It can be applied in direct, storage or semi-storage mode, depending on the user's requirements, the conditions of the building and the capacity of the electricity distribution system in terms of the power input needed in the given location. The customer has many systems to choose from; for example, Fenix underfloor heating is available.
For the heating system to work optimally, its thermal regime needs to be matched to the thermal regime of the building. The storage and semi-storage versions are best designed for solid, heavyweight buildings with a minimal area of transparent structures. The direct version, thanks to its dynamic response to the control equipment depending on the time constant, can be used in buildings with any type of external envelope.
Because of the limited time during which energy is drawn, a storage system may have problems ensuring thermal comfort towards the end of the discharge time, for example if the weather suddenly deteriorates. The rooms are best used immediately after the system has finished charging. This method is therefore suitable for buildings used mainly in the morning, or in combination with a supplementary convection system. The semi-storage and direct systems provide thermal comfort throughout all-day use of the building. When a heat source is being refurbished and an electric underfloor system is under consideration, the direct version appears to be the most acceptable, and often the only, alternative.
Thermal comfort is ensured by individual output control separately for each heated interior. In direct mode, the air temperature is controlled to a constant value or according to the outdoor air temperature; in storage and semi-storage mode, exclusively according to the outdoor air temperature. At the same time the controller ensures that the floor surface temperature (in the plane of the heating cable) does not exceed the set values laid down by hygiene regulations in line with the nature of the room. Control is provided by thermostats located either directly in the room or in the distribution board. Older-generation thermostats work on the principle of interrupting/restoring the electricity supply to the system when one of the set values is reached. To increase living comfort, they are often supplemented with a time switch that allows the heating regime to be programmed several days ahead (working days, weekends, holidays). New-generation thermostats work as intelligent programmable devices with a timer built into their design. They control the air temperature and the floor temperature at optimum running costs. They ensure thermal comfort by assessing the temperature gradient in the room and can determine the time needed to reach the desired temperature.
Buildings in occasional use
Electric underfloor heating as an occasional heat source has a wide range of applications:
• In residential spaces that are not used regularly. Such spaces include, for example, weekend cottages, garages, hobby workshop rooms in residential buildings, etc . During the period when the building or space is not in use, the heating system must not freeze, and when it is brought into operation the interior must heat up quickly. These requirements are met by an underfloor system in the direct version with a minimal time constant (elements installed directly beneath the floor finish). With a higher specification, where the system is run by a programmable control device that can start it up so as to ensure thermal comfort by the time the building is used, the other heating modes can also be used. This option is less common and, for economic reasons, is worth considering only if the space will be used for a longer period once the system has been started (weekend cottages). Another option is to switch the heating on by telephone, so that the interior is already warm when you arrive. Heat output is controlled in the same way as for buildings in permanent use.
• In non-residential spaces used occasionally. Spaces of this type include places of worship, churches, stadiums and stands, where people stay for a short time (1 to 2 h). It is not necessary to heat the whole interior, only part of it. Thanks to their low build height and simple installation, electric mats and foils are the heating elements to consider. They are placed on the lower part of the seating (heat radiates to the legs) or on the backrest (heat radiates to the back), under carpets or floor tiles, under work tables (warming the hands), etc. The system is usually controlled by a thermostat fitted with a temperature sensor in the plane of the heating cable.
• In non-residential spaces which, although used regularly several days a week, would be uneconomical or unsuitable to heat in any other way. This group mainly includes industrial production halls, unheated warehouses, workshops, etc. People are often present here as much as 24 hours a day, but heating such extensive spaces would be both very demanding (great ceiling height) and uneconomical (control is almost impossible – a great deal of extraneous heat from machinery). In such cases heating is provided by creating heat islands, i.e. the workstations, the places where people stay and the handling areas are heated directly. Once again heating mats and foils are well suited, or alternatively an underfloor system in direct mode. Control is provided by thermostats fitted with a temperature sensor in the plane of the heating cable.
Several underfloor heating systems are available on the market, including Fenix underfloor heating.
Supplementary heating
Electric underfloor heating, for example Fenix underfloor heating, in direct mode as a supplementary heat source is designed:
• for spaces where there are heat gains (halls, corridors, WCs), to provide a pleasant floor surface temperature;
• for spaces where the basic heat loss is covered by another heating system and a cold floor is not wanted (kitchens, bathrooms);
• to ensure thermal comfort in the transitional seasons, when the main heating system is out of operation.
The system is run by a thermostat fitted with a floor temperature sensor. In these cases the heating output is determined on the basis of the purpose of the room, the floor build-up and the requirements for the responsiveness of the system.
References:
Dušan Petráš, Miroslav Kotrbatý: Heating of large-space and hall-type buildings
