Radiant heating and cooling systems (Part 2)

By Kwang Woo Kim and Bjarne W Olesen

Control of the heating and cooling system needs to be able to maintain the indoor temperatures within the comfort range under varying internal loads and external climates.

To maintain a stable thermal environment, the control system needs to maintain the balance between the heat gain/loss of the building and the supplied energy from the system. Several studies in the literature deal with control.

The control of a radiant heating and cooling system can be classified as central control, zone control and individual room control. Figure 1 is a diagram on the principles of control.

Figure1Credit: ASHRAE

The central control controls the supply water temperature for the radiant system based on the outside temperature. The room control then controls the water flow rate or water temperature for each room according to the room setpoint temperature.

Instead of controlling the supply water temperature, it is recommended to control the average water temperature (mean value of supply and return water temperature) according to outside and/or indoor temperatures. During the heating period, as the internal load increases, the heat output from the radiant system will decrease and the return temperature will rise. If the control system controls the average water temperature, the supply water temperature will automatically decrease due to the increased return water temperature. This will result in a faster and more accurate control of the thermal output to the space and will give better energy performance than controlling the supply water temperature.

Radiant surface cooling systems need controls to avoid condensation. This can be done by a central control of the supply water temperature limiting the minimum water temperature based on the zone with the highest dew-point temperature. If the supply water temperature is limited, the temperature of the rest of the system will be higher than the dew point, and there is no risk of condensation on the pipes and on the surface of the radiant system. Limiting the supply water temperature will lower the cooling power of a radiant system at high indoor humidity levels. Dehumidifying the ventilation air will result in lower dew-point temperature and will allow higher cooling capacity of a radiant system.

Larger buildings should be divided into several different thermal zones to optimise energy and control performance. Each zone can be controlled with reference to a temperature sensor in a representative space of the zone.

For improved comfort and further energy savings, use an individual room temperature control. Each valve on the manifold is controlled by each room’s thermostat. An apartment or one-family house normally was regarded as one zone, but installing thermostats for each room is becoming popular. For better thermal comfort, it is preferable to control the room temperature as a function of the operative temperature.

The heat capacity of surfaces with embedded pipes plays a significant role in the thermodynamic properties of the heating system and hence for the control strategy. An obvious consequence of the response time of a conventional floor structure is that the instant control of the heating power is not necessary. The temperature of heat transfer medium, the time response, and the thermal capacity of systems depend on the thickness of the surface layer where the pipes are embedded.

For a low-temperature heating and high-temperature cooling systems, a significant effect is the "self-regulating" control. This "self-regulating" depends partly on the temperature difference between room and heated/cooled surface, and partly on the difference between room- and average water temperature in the embedded pipes. This impact is bigger for systems with surface temperatures close to room temperature, because the small temperature change represents a higher percentage compared to the same temperature change at a high temperature difference. The self-regulating effect supports the control equipment in maintaining a stable thermal environment and providing comfort to the persons in the room.

For thermally active building systems (TABS), the concrete slab can be controlled at a near constant core (water) temperature year-round. Therefore, zone control (south-north), rather than individual room control is more appropriate, because zone level supply water temperature, average water temperature, or flow rate control would be possible. Relatively small temperature differences between the heated or cooled surface and the space would result in a significant degree of self-control.

Figure2Credit: ASHRAE

As a TABS does not remove the room load immediately, the control cannot keep a constant room temperature during the day. Instead, a small room temperature drift will result. An example from a simulation is shown in Figure 2. The figure is comparable with Figure 8 of Part 1 (RACA Journal February 2017 edition) showing the energy flows. Water of 20°C is circulating in the concrete slab from 18:00 to 08:00 the next morning. It can be seen that the room is kept within the comfort range of -0.5<PMV<+0.5. The operative temperature runs between the line for air temperature and mean radiant temperature, and is within the comfort range of 23–26°C. This is an example on how a dynamic building simulation may be used to verify that by the used water temperatures or given chiller capacity the room will be kept within the comfort range.

Figure8Credit: ASHRAE

The objectives for the most economic operation and operation strategies of the building system are the minimum total energy costs and the minimum peak electricity demand. As the radiant system can only take care of sensible heat load, the system needs to be operated in combination with air systems for ventilation, dehumidification and additional thermal requirements. Therefore, the system designer should consider more energy-efficient HVAC systems and use of renewable energy sources.

Another possible strategy is to reduce "room side" energy demand. The room temperature control strategy, allowing a little fluctuation, may bring significant energy savings in comparison with keeping constant room temperatures. Temperature fluctuations of up to 3–4K per hour will not cause any additional comfort problems as long as the room temperature is within the specified comfort zone.

Installation
For the installation of systems embedded in floors, walls or ceilings, the manufacturer’s instructions must be followed. To limit the heat flow towards the outside (not exceed 10% of total heat flow) or to adjacent spaces, a minimum thermal resistance of the insulating layer shall be specified in the design. The effective thickness of the insulating layer depends on the construction of the radiant system. The thermal conditions under the floor structure should be considered for an embedded floor heating system insulation.

The dimensions of pipes must comply with the requirements of the standards. Minimal pipe thickness should comply with the requirements for service conditions, operation pressure (higher than 4 bar) and durability (more than 50 years). The use of plastic pipes with an oxygen-barrier layer is recommended to reduce corrosion problems. However, the risk for oxygen penetration is highest at very high water temperatures, which you do not find in modern buildings. For pipes embedded in concrete such as TABS, the temperature variation of the water is very small and the lifetime of the pipes are more than 100 years.

All couplings within the embedded construction should be exactly located and designated on the record drawing. The bending radius shall not be less than the minimum bending radius defined in the relevant product standards.

The thickness of the screed layer should be calculated according to carrying capacity specified in national codes. The screed thickness above pipes must be at least 30mm. The temperature of the liquid screed and the room should not be lower than 5°C for at least three days. Hardening screed should be protected from draft, fast drying and harmful effects.


Floor heating of high spaces (large industrial building, churches, and so forth) ensures uniform thermal conditioning and temperature profiles in the occupied space.


Initial heating should be carried out in accordance with the manufacturer’s instructions, but should be maintained for at least seven days for systems with anhydrite screeds. This operation commences at a supply temperature of between 20°C and 25°C, which should be maintained for at least three days. Subsequently, the maximum design temperature should be imposed. The process of heating up must be documented.

The thickness of the concrete for TABS should be calculated according to load-bearing capacity specified in national codes, and the position of pipes should be considered in the gravity load calculation of the slab. Pipes are commonly installed in the centre of the concrete slab between the reinforcements. If the system is constructed on site, the pipes are supplied in modules, which include a pipe coil attached to the metal grid and equipped with fittings.

Prior to embedding in screed or concrete, the pipe circuits should be checked for leaks by means of a water pressure test. The test pressure should be twice the working pressure with a minimum of 6 bar. Where danger of freezing water occurs during winter installation, air can be used instead. During the laying of the screed, this pressure should be applied to the pipes.

For TABS, the pipes are installed during the main construction of the building. This requires that the decision on which heating-cooling system to use must be made at an early stage. It is also important that the installation of the pipes does not prolong the building construction and costs. Therefore, it is today possible to use prefabricated concrete slabs with pipes embedded from the producer’s side. For in situ casting of the concrete, it is recommended to supply the pipes pre-mounted on mesh-like modules.

Applications
Embedded surface systems are used for heating and cooling in various types of buildings. Principally, ceiling systems are used as supplementary air-conditioning systems in non-residential office buildings. The system can work with a quite high cooling capacity of 50–100W/m2, limited by the risk of condensation. Ceiling heating is limited by standard requirements for radiant asymmetry to a capacity of 40–50W/m2 depending on ceiling height. Floor and wall heating systems are popular for residential buildings, mostly in single-family houses and apartments because of extra space created by embedding the pipes in structure. The system is suitable for spaces with heating loads of 10–100W/m2 and cooling loads of 10–40W/m2. The warm surface is comfortable for children playing on the floor. The absence of radiators avoids injury in rooms occupied by the elderly and children.

Wall systems may limit furnishing possibilities and the mounting of wall pictures. Floor cooling systems in spaces that are influenced by direct sunlight may rarely reach a short-time cooling capacity of more than 100W if unshaded and directly exposed.

Floor heating of high spaces (large industrial building, churches, and so forth) ensures uniform thermal conditioning and temperature profiles in the occupied space. The accumulated heat in the floor of an aircraft hangar can warm it up again in a half hour after the aircraft moves out and the doors closed. The system with a heat conductive device or with micro pipes requires a thin floor construction, and can be used for the renovation of buildings, as well as for lightweight structure (for example wooden) buildings. In lightweight building structures with a lack of thermal mass, the installation of TABS with PCM can be a solution. A PCM panel of 50mm thickness is able to store the same amount of energy as a 250mm thick concrete slab. TABS are usually installed into the ceiling concrete slabs of multi-storey non-residential buildings. There are also known application for hospitals, museums, show rooms, schools and libraries. TABS is suitable for buildings with cooling loads up to 40–60W/m2. In buildings with loads over 60W/m2, the installation of a complementary convective system for cooling is recommended in case of fast load changes.

TABS is not fully suitable for installation as the only thermal conditioning system in family houses, as the user may want to reduce the temperature level in sleeping rooms during the daytime, when the room is unoccupied. In that case, an additional system for individual control is required.

Examples of applications

Examples of buildings in Canada with embedded radiant heating and cooling systems include:

  • ICT Building at the University of Calgary
  • Gleneagles Community Centre, West Vancouver, BC
  • Kortwright Conservation Centre (Earth Rangers)
  • Simon Fraser University Residences
  • MacLeod II ECE Building (Fred Kaiser Building) at the University of British Columbia
  • Manitoba Hydro Headquarters, Winnipeg Manitoba
  • Vancouver General Hospital Cancer Research Centre
  • Irving K. Barber Learning Centre, UBC
  • City of North Vancouver Main Library.

Examples of buildings with a combination of TABS and ground source heat pumps have been collected in a European project (GEOTABS, http://www.geotabs.eu/Database).

Office building in Germany
A 6 500m2 and five-storey annex building of an office in Stuttgart, Germany is installed with TABS and a mechanical ventilation system. The construction of the slab is shown in Figure 3. The heat carrier circulates in meandering pipe circuits (VPE pipes, 20mm diameter) embedded into the load-bearing 300mm concrete ceiling. Piping material (VPE instead of common PE-X) was chosen in accordance with higher load strain of the slab due to the 15m distance between the columns. Total length of piping is about 49 000m at 9 750m2 of active ceiling area.

The trapped air layer of 180mm significantly influences the heat conduction in the slab upwards and the radiated heat released from the floor surface (Figure 3). The air gap space is used for installation of IT and electricity cables, water distribution and air duct systems.

Figure3Credit: ASHRAE

Figure 4 shows the summer conditions of heat exchange and room airflows. TABS is associated with the air-conditioning system using 100% fresh air supplied by plinth units (close to the facade) and floor inlet units (building core area). The system controls the individual room humidity and covers a percentage of the peak loads (cooling ~10%, heating ~18%). The fresh air inlet units provide 80–100m3/h per person corresponding to about 45m3/h and 1.57ach. The air velocity is less than 0.11m/s in 0.6m distance from the inlet units and the relative humidity varies between 45 and 60% (for closed windows).

Figure4Credit: ASHRAE

The mean water temperature in the activated slabs is controlled according to outdoor temperature year round. The actual supply water temperature varies between 19°C to 23°C. Using this strategy, in conjunction with the ventilation system, the room temperatures are maintained between 22°C and 26°C in summer and 21–24°C in winter.

As the heat carrier in summer circulates only during the night, power demand is greater during night-time and takes advantage of the cheaper electricity night tariff. The field measurements results showed that operative temperatures during working hours were kept between 22°C and 25°C in summer and 21–23°C in winter (Figure 5).

Figure5Credit: ASHRAE

High-rise apartment building in Korea
In Korea, 100% of residential buildings are heated with radiant floor heating systems. Even the 300m high, 80-storey high-rise residential apartment building "We've the Zenith" (Figure 6), is heated with a radiant floor heating system. Hot water is generated by boilers in rooftop mechanical room, and serves seven vertical heating zones with proper hot water temperature for radiant floor heating after heat exchange.

Figure6Credit: ASHRAE

University building in Korea
Ewha Woman’s University Campus Centre (ECC) in Seoul, Korea, is a good example of non-residential building application (Figure 7). It is a university complex including lecture rooms, offices and public spaces. ECC is designed by Dominique Perrault and local Baum Architects.

Figure7Credit: ASHRAE

Accumulated energy in massive ceilings is used by means of concrete core activation to reduce energy demand for thermal well-being. The cooling with the concrete core activation is done by chilled water temperatures of 17°C supply and 20°C return, and heating by hot water temperatures of 29°C supply and 26°C return.

The first step of the cooling is done by the remaining cool energy of the returning water to an absorption chiller. The second step is to use stored energy of the ground-water storage tanks. The third step is to use earth energy through the pipes under the basement floor. The cooling energy for the concrete core activation is supplied continuously over 24 hours, therefore, there is no peak load and the sizes of all necessary equipment could be reduced to a minimum.

Airport in Bangkok
The international airport Bangkok, which opened in September 2006, is thermally conditioned by a floor surface cooling system in combination with a displacement ventilation system (Figure 8). Its 150 000m2 of cooled floor area, comparable to 20 football fields, is recognised as the world’s largest application. With a length of 440m, a width of 110m, and an area of almost 500 000m2, the terminal became the largest combined building complex of its kind in the world. The H-shaped concourses have a total length of 3.5km.

Figure8Credit: ASHRAE

To provide both cooling and ventilation, two separate systems are combined. The under-floor cooling system directly absorbs the solar gains while maintaining a comfortable floor surface temperature (minimum 21°C). The displacement ventilation system with a variable flow volume provides dehumidified fresh and re-circulated air at floor level via an approximately 2m high air diffuser. Due to the warm climate, the temperature in winter may achieve an average of 21°C during the night (summer 25°C and 31°C during the day (summer 34°C). The solar radiation usually incidents perpendicularly to the earth’s surface and reaches a level up to 1 000W/m2. Due to the narrow range of the operative temperature at 24°C and a relative humidity of between 50 and 60% during the 24 opening period the airport requires constant cooling and dehumidification.

Acknowledgements
This article was supported by Velux guest professorship, and a grant from the National Research Foundation of Korea (NRF) funded by the Korean government (MEST) (No. 2014-050381).

References

  1. Olesen, B.W. 2001. "Control of floor heating and cooling systems." Clima 2000/Napoli 2001 World Congress.
  2. Rhee, K.N., Yeo, Myoung S., Kim, K.W. 2011. "Evaluation of the control performance of hydronic radiant heating systems based on the emulation using hardware-in-the-loop simulation", Building and Environment 46(10).
  3. Kang, D.H., et al. 2010. "Effect of MRT variation on the energy consumption in a PMV-controlled office." Building and Environment 45:1914-1922. 2010.9.
  4. Rhee, K.N., Ryu, S.R., Yeo, M.S., Kim, K.W. 2010. "Simulation study on hydronic balancing to improve individual room control for radiant floor heating system." Building Services Engineering Research and Technology 31(1):57-73.
  5.  ISO 11855-6: 2012. Building environment design - Design, dimensioning, installation and control of the embedded radiant heating and cooling systems - Part 6: Control.
  6. Olesen, B.W. 1997. "Possibilities and limitations of radiant floor cooling." ASHRAE Transactions 103(1):42-48.
  7. Wiercioch, H. 2001. Betriebserfahrung mit Betonkernaktivier-ung, BV M+W Zander Stuttgart, In proc: 23.Velta kongress 2001, Wirsbo-Velta, Nordestedt, Germany.
  8. De Carli, M., Olesen, B.W. 2001. "Field measurement of thermal comfort conditions in building with radiant surface cooling sys-tern." Clima 2000.

About the authors
Kwang Woo Kim, Arch.D., is a professor of architecture at Seoul National University, Seoul, South Korea, and president of the Architectural Institute of Korea. Bjarne W Olesen, Ph.D., is director, professor at the International Centre for Indoor Environment and Energy, Technical University of Denmark in Lyngby, Denmark, and vice president of ASHRAE.



Credit
This article was published in the February 2015 edition of ASHRAE Journal (copyright 2016 ASHRAE). Posted at www.ashrae.org and reprinted with permission from ASHRAE. This article may not be copied nor distributed in paper or digital form by other parties without ASHRAE’s permission. For more information about ASHRAE, visit www.ashrae.org. This article was supported by VELUX guest professorship, and a grant from the National Research Foundation of Korea (NRF) funded by the Korean government (MEST) (No. 2014-050381).


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