Joachim Morhenne 1121 Concept

By heating spaces with large surfaces - that is, a wall or a floor - large quantities of heat can be transferred by radiation at comfortable low temperatures. Houses with very small heat demand are ideal candidates for such heat delivery. To ensure that the heat is primarily transferred by radiation, the heating surface temperature has to be close to the room air temperature, otherwise convection quickly becomes the main path of heat transfer for normal room temperatures. Because the radiant heat flux of surfaces, with a temperature of 22°C to 32°C, is low, the surface area has to be large to cover the heat demand. In this case, the part of the energy transferred by radiation is up to 60 per cent.

Warm surfaces increase indoor comfort. Human beings are very sensitive to infrared radiation because they emit heat in this way, mainly by their uncovered skin. The heat loss of the body then depends on the radiation exchange with colder surfaces. In winter, warm room surfaces prevent such discomfort.

Increased surface temperature can save energy by making it possible to provide the same comfort at lower temperatures than possible for houses with cold surfaces. The low operating temperatures of radiating surfaces increase the efficiency and annual useful output of active solar combi-systems (combined water heating and space heating). The low supply temperature allows the solar system to operate at cooler temperatures, increasing system efficiency and extending the hours of solar coverage. Condensing gas furnaces and heat pumps are also able to operate more efficiently when coupled to surface heating.

The most commonly used surface for such heating is the floor, although radiant walls provide better comfort. The latter is used less because it limits freedom in furnishing a space. Systems differ by mass (light or heavy constructions) and by building integration (being inside the wall or installed as a separate surface, such as radiators).

The heating power of radiant surfaces depends on the surface temperature; up to 80 W/m2 can be achieved. In the case of solar heated systems, 30 W/m2 to 50 W/m2 are realistic due to the reduced temperature level.

In high-performance houses heated by ventilation air, the surface temperature of the walls is quite uniform. As an alternative, a radiant heating system with its higher temperatures has the advantage of being able to compensate for the lower surface temperature of windows. A second advantage is the opportunity to tie in an additional heat source. This allows the creation of different indoor temperature zones and provides more power to heat up the building more quickly - that is, after an unoccupied period.

An additional benefit of building integrated radiant heating is that it allows the capacity of the house to be activated as a thermal storage. However, when the mass is used as storage, the risk of overheating increases because of the lag in the heat release.

Heavy radiant heating systems therefore require accurate planning. Houses with minimal passive solar gains and high-performance houses with active solar heating are ideal. In the summer, the thermal mass of a house helps to keep indoor temperatures low and enhances nocturnal cooling in the northern or mid European climate.

11.2.2 Radiant heating system based on water

Floor and wall heating is usually done with tubes embedded in the plaster. This increases the required thickness of the finish slightly. In addition, 1 cm to 2 cm of insulation is necessary. The details are well known and proven. New materials for the tubes (compounds of aluminium and polyethylene) allow endless runs without joints inside the construction. Oxygen diffusion as well as potential leakage is no longer a problem. Metal tubes are still common, too, but have the disadvantage of requiring joints inside the construction.

The design parameters are tube distances, the tube diameter, resistance of the embedding material and inlet water temperature. The achieved heating power varies typically from 40 to 80 W/m2 for floor heating systems (>0.05 K/W (35 mm screed, 10 mm parquet), tube separation ranging between 33 cm and 5.5 cm, with a tube diameter of 14 mm). Wall heating systems can reach higher values because the tubes are right behind the surface and have a higher heat transfer; up to 150 W/m2 can be achieved (temperature difference between wall and room air temperature: 15K).

Wall and floor heating systems, in most cases, have an adjacent massive construction. The system needs time to heat up and time to cool down when no more heat is required, making control more difficult. In rooms with high passive solar gains, only a fraction of the heat demand should be covered by wall or floor heating systems to avoid overheating.

Dos and don'ts

• In high-performance houses, wall heating systems should not be installed in exterior walls to avoid higher transmission losses. In case of floor heating, the floor should not be an exterior surface - that is, an overhang.

• Rooms with high passive gains need fast responding heat delivery, best achieved by air or small radiators.

• For solar heated systems using the thermal mass of the building as storage, see the following section on air-heated systems.

11.2.3 Radiant heating systems based on air

The advantage of air is that air does not drip or freeze, so no antifreeze is needed and tightness is not a serious problem. Disadvantages are the poor heat capacity and the low density of air; therefore, large volumes of air are needed to transfer heat. In this section, wall, floor or ceiling heating systems operating in a temperature range of below 35°C are described. This is an ideal condition for solar air collectors. Such systems are also well suited for high-performance houses because such houses need very low heating power (< 4.2 W/m2).

11.2.4 Solar-supplied radiant heating systems

Direct supply of solar air heat to the rooms of a high-performance house is not advisable. The heat demand is low or non-existent during sunshine hours because windows' solar gains can cover most or all of the heating demand. As a result, solar air heat can only lead to overheating. Thermal storage is therefore essential to delay the radiant heat release into the evening. This can be achieved with either hypocaust (floor heating) or murocaust (wall heating) systems. Such systems can serve many functions: transporting heat, storing heat, radiating heat and acting as a structural part of the building. Furthermore, these systems are partially self-regulating. As the room temperature rises, the temperature difference between the radiating surface and other room surfaces and the room air decreases, so less heat is released (see Figure 11.2.1).

E nergy flow by convection

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Source: Morhenne Ingenieure GbR, Wuppertal

Source: Morhenne Ingenieure GbR, Wuppertal

Figure 11.2.1 The self-regulation effect of a radiant heating system

The time delay (t) between solar heat input and heat release to the room can be engineered by changing the amount of available thermal mass and the dimension of the tubing or air channels (see Figure 11.2.2).

If passive gains are minimal (due to an unfavourable orientation or site shading), active solar gains can compensate the missing passive gains. Figure 11.2.3 illustrates such a configuration.

The dimensions and maximum solar air coverage of the heating demand are limited by the risk of overheating. Due to the time shift, the heat is stored when it is not known whether there will be a future demand for it. To reduce this risk and to increase the storage capacity for solar heat, it is advis-

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Source: Morhenne Ingenieure GbR, Wuppertal

Figure 11.2.2 Phase shift and delay time of a massive radiant heating system able to provide separate storage areas for solar heat and for the backup heat. Indeed, in high-performance houses, during most of the heating season the backup heating can be delivered using the ventilation air without the need for storage. Thus, radiant storage for the backup heat is economically questionable.

Source: Morhenne Ingenieure GbR, Wuppertal

Figure 11.2.3 A building scheme with hypocaust and murocaust heated from a solar air collector

Source: Morhenne Ingenieure GbR, Wuppertal

Figure 11.2.3 A building scheme with hypocaust and murocaust heated from a solar air collector

11.2.5 Description of typical solutions

Hypocaust

A hypocaust consists of a massive structure with air channels to heat or cool the element, as shown in Figure 11.2.4.

Source: Morhenne Ingenieure GbR, Wuppertal

Figure 11.2.4 Scheme of a hypocaust

The material and its thickness affect the heating delay or time shift. The core, through which the warm or hot air is blown, can be built in different ways: prefabricated hollow core floor elements (see Figure 11.2.5) or metal or plastic tubes embedded in cast concrete (see Figure 11.2.6). Systems working by gravity based on the differences in density between cool and warm air have been used in Sicily, but are not known in Central Europe.

Source: Morhenne Ingenieure GbR, Wuppertal

Figure 11.2.5 Examples of hypocaust systems: (a) prefabricated; (b) poured in place

Source: Morhenne Ingenieure GbR, Wuppertal

Figure 11.2.5 Examples of hypocaust systems: (a) prefabricated; (b) poured in place

To avoid increased heat losses, exterior facing surfaces should not be used. For construction details, see Hastings and Mork (2000, Chapter IV6) and Morhenne (1995).

Hypocaust or murocaust heating systems for summer cooling

High-performance houses with hypocaust or murocaust systems can be more comfortable in summer due to their heavier construction. The effect of the mass can be increased by fan-forcing cool air through the channels in the evening, when temperatures can be K to 4.2K cooler than the structure during clear weather periods. When air for the channels in the mass is cooled in an earth heat exchanger, a temperature reduction of at least 5K of the ambient air can be achieved.

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