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Low-temperature heating combined with renewable energy sources can be used to supply heat to any space where wet radiant heating systems are used. These low-energy systems are particularly suitable for:
• houses and blocks of flats,
• special-purpose spaces in public buildings,
• hall-type buildings,
• agricultural premises.
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Houses and blocks of flats are the most common application for low-temperature heating, or for solar, geothermal and heat pump systems, because they give the owner a clean architectural solution for the heated interior together with energy-efficient operation and optimum thermal comfort.
However, a building with a low-temperature heating system must meet the following requirements:
• the heat loss of the heated building must be as low as possible (15 - 20 W. m- 3),
• the heat loss of the heated space must be one that the heating surface can cover with its output without exceeding the hygienically permissible surface temperature of that surface (floor, wall, ceiling),
• where the heat loss is greater than the achievable output of the heating surface, the thermal resistance of the building envelope must be increased with additional insulation, or the shortfall in output must be covered by a radiant heating surface elsewhere, or by a radiator.
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Heating in a low-energy house
Before discussing the use of low-temperature heating with renewable energy sources in a low-energy house, we first need to define the requirements and then analyse the energy balance and the thermal conditions in the heated interiors.
Requirements for a low-energy house
Reducing the energy consumed in running buildings by drawing up a comprehensive energy-efficiency programme leads to the design of the low-energy house. This can be achieved in two ways :
• by reducing the building's heat losses caused by heat transmission and ventilation, together with passive use of solar energy through ordinary building elements such as windows and glazed walls
• by using renewable energy sources, above all solar energy and the energy of geothermal water and the surroundings, by means of active systems with a low energy potential.
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Thermal conditions in a low-energy house
In very well insulated buildings, the temperature of the heating medium and the difference between flow and return temperatures are markedly lower than in traditional buildings, while the internal surface temperatures of the external walls are higher. A uniform thermal environment is created indoors, without unwanted vertical and horizontal temperature differences, radiant temperature asymmetry or excessive air movement, i.e. local thermal discomfort.
Measurements and theoretical calculations have confirmed that in a low-energy house the position of the radiator is not important for ensuring thermal comfort . The main problem is the excessive variation in indoor temperature in response to changes in the heat load, i.e. heat gains from people, lighting or solar radiation . Because the specific heat losses are small, changes in the heat load can have a marked effect on the resulting indoor temperature. This effect depends mainly on the thermal inertia of the building and on how well the heating system can be controlled.
The thermal inertia of a low-energy house is higher than that of a standard building. Changes in outdoor air temperature can be dealt with by weather-compensated control with a characteristic heating curve . The effectiveness of such control depends on choosing the heating curve to suit the type of building and the differing heat demand of the various interiors.
The strong influence of solar radiation through the windows on the resulting indoor temperature calls for further operating functions in the heating system that allow the heat output to be varied. An important feature of low-temperature heating systems is their self-regulating effect, caused by the small temperature difference between the interior and the heating surface. This self-regulating ability needs to be combined with a room thermostat which, by monitoring the indoor temperature, controls the mass flow rate in the low-temperature heating system.
Where a night-time interruption of heating is used, the saving in a low-energy house amounts to 1 to 3 % of total energy consumption.
Low-temperature heating systems with minimised heat losses in heat generation (e.g. condensing boilers) and distribution allow much greater use to be made of renewable energy sources.
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Examples of use in houses and blocks of flats
To illustrate the use of low-temperature systems in houses and blocks of flats, we will show installations of:
• low-temperature underfloor heating,
• underfloor heating with a heat pump and solar collectors in a low-energy house,
• low-temperature heating with a heat pump,
• domestic hot water (DHW) heating by a solar system in a block of flats,
• heat supply for heating and domestic hot water from geothermal energy.
Low-temperature heating with a heat pump
The heating system described is installed in a single-storey family house with a habitable loft. It is a central heating system with flow/return temperatures of 80/60 °C, and the original heat source was a solid fuel boiler. The source of low-temperature heat is waste water at a temperature of 40 to 46 °C.
Both a monovalent and a bivalent operating option were originally drawn up; the investor chose and implemented monovalent operation of the heat pump. For this reason the flow/return temperatures had to be adjusted to the maximum temperature of the water leaving the heat pump condenser, in this case to 55/43 °C. To achieve this it was necessary to:
• reduce the heat losses of the building,
• enlarge the heating surface to match the new flow/return temperatures.
The primary connection to the geothermal water main is about 100 m from the building. Two polyethylene connecting pipes are laid loose in the ground. Because the geothermal water is aggressive, an intermediate circuit had to be added on the evaporator side of the heat pump. This intermediate circuit, with its own circulation, contains a plate heat exchanger. The temperatures for the heat pump evaporator will then be 20/15 °C.
The heat pump, with a heat output of 20 kW, is fitted with two hermetic compressors. The electrical input is 5.5 kW and the achievable coefficient of performance of the heat pump is around 3.2. The values actually achievable by this heat pump running on R 22 range from 3.5 to 4.5. Downstream of the heat pump, on the condenser side, a buffer vessel with a volume of 0.3 m3 is fitted in the water circuit, from which the hot water is distributed to the heating system. The heat pump system includes the necessary controls, weather-compensated control with day and night setback, and so on. For emergencies, the connection to the existing heat source (the solid fuel boiler) has been retained. Domestic hot water for the building is provided separately, without using the heat pump.
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Domestic hot water heating in a block of flats with a solar system
The largest solar energy installation in residential construction in Slovakia was built in 1985 on a housing estate in Šurany. An experimental block of eleven flats with a total demand of 2000 l of water was chosen for using solar energy to heat domestic hot water.
The block is a 4-storey building with a basement housing the plant room and a flat roof on which the solar collectors are mounted.
The solar energy system is bivalent, with short-term (2-day) heat storage. In terms of available solar radiation, the experimental block with the solar installation for domestic hot water is located in the area with the longest annual sunshine duration and the highest amount of incident solar radiation.
Solar energy is converted into heat by 34 flat-plate liquid solar collectors mounted on the walkable flat roof and set in three rows in a steel supporting frame. The (thermally insulated) pipework of the collector circuit runs down to the basement of the block, where it is connected to a storage cylinder with a capacity of 4000 litres. A second cylinder with a capacity of 2000 l, connected in series, is used to top up the temperature of the water heated by solar energy. The cylinders are fitted with temperature sensors, and a differential controller governs their charging.
Public buildings
The scope for using wet underfloor heating in public buildings is exceptionally wide (office buildings, hotels, restaurants, cinemas, theatres, cultural and community buildings, etc.), but in most cases it is closely linked with conventional heating, i.e. as what is known as combined hot-water heating.
Requirements for the heated building
Combined hot-water heating offers all the advantages of both systems, i.e. large-area radiant heating and convective heating, and is particularly suitable for heated buildings that need:
• the ability to cover a large heat loss in cases where the installed output of the underfloor heating is not sufficient on its own,
• a flexible response to changes in heat demand caused by external heat sources (solar radiation, heat from cooking, heat given off by people, etc. ), with the underfloor heating covering the basic heat loss and the convective heating responding flexibly to swings in outdoor temperature, especially in the transitional seasons of spring and autumn),
• the option of changing the position and output of the radiator when the interior layout or the purpose and function of the room changes.
Combined hot-water heating
A wide range of materials, advanced technical solutions and new installation technologies mean that today a heating system can be chosen and designed “made to measure”. One such solution is combined hot-water heating systems based on plastics, which make it possible to combine convective and radiant heat emission into the room in a suitable way. In other words, to use traditional radiators and large-area radiant heating surfaces at the same time.
Combined heating brings together the advantages of two heating systems, namely large-area radiant heating and convective heating, and eliminates the drawbacks each has on its own, with both systems being low-temperature.
The heating system consists of pipework running to the individual manifolds for the underfloor and convective heating. The heating pipes from the manifold to the individual radiators are most often routed in one of two ways:
• In the first case, where the pipes are embedded in the floor, the shortest connection between the manifold and the radiator is achieved. This solution is used mainly in new buildings, where the floor layer is laid only after the pipes are in place.
• The second solution allows the pipes to be run to the radiators in the skirting around the perimeter of the room. In buildings where the heating system is being replaced, it would be laborious and above all uneconomical to run plastic pipes to the radiators in the floor. It would mean first removing the floor finish and then chasing out channels for the pipes, which would lengthen the time needed to replace the heating system and increase the cost as well. In existing buildings it is therefore advisable to run the plastic pipes from the manifold to the radiators along the wall, housed in the skirting.
The plastic pipes are connected to the radiator either directly from the floor or from the wall. Connection from the wall additionally requires a channel to be chased out for the pipes. Where skirting is used, the pipes are connected directly from it. Depending on how the pipes are connected to the radiators, we distinguish between one-pipe and two-pipe systems. The type of valve used on the radiator also depends on which of these two systems is chosen. Would you like underfloor heating, but how to do it is the question keeping you awake? Not any more!
Heat sources for combined heating
All commonly used hot-water heat sources can be employed (hot-water boilers running on solid fuel, gas or electricity, as well as renewable energy sources and heat transfer stations). Of these, solid fuel boilers are the least suitable, as the scope for controlling their output is limited.
With regard to providing the required temperature of the heating medium, the design, and the operating mode in relation to the heat source, combined hot-water systems based on plastic pipework can be arranged as follows:
a) a central hot-water heat source producing heating water directly for the convective circuit, with mixing (a three-way or four-way valve) for the low-temperature large-area radiant heating circuit;
b) a central low-temperature heat source producing heating water for the whole combined low-temperature system, but with the radiator surfaces enlarged accordingly;
c) a central hot-water heat source producing heating water directly for the convective heating circuit, but with a heat exchanger station for the low-temperature large-area radiant heating circuit.
These three basic options differ in their technical complexity and material requirements. At present option a) is used most often, even though in principle it involves two separate heating systems. Option b) can be used only for low-energy buildings with specific heat losses of 15 to 20 W. m-3, because of the reduced heat output of the radiators. Solution c) is theoretically the most perfect: it creates two heating circuits that are independent in pressure and temperature, and for the low-temperature large-area radiant heating in particular it forms a circuit that is fully protected from dirt in the boiler circuit and at the same time highly resistant to the effects of oxygen diffusion.
Wondering about underfloor heating and how to do it? Then you have come to the right place!
References:
Dušan Petráš: Low-temperature heating and renewable energy sources
