 | Thermally active slabs on steel deck
In observing the principles of sustainable development, the choices associated with all aspects of building design are made in accordance with an integrated design process. This process brings all the parties that are involved (professionals, builder and customer) around the table. The objective is to get each party’s point of view on the weaknesses and strengths of each aspect of the design in order to maximize the synergies between the options chosen for the project under discussion.
All of the criteria associated with LEED® certification—energy efficiency, thermal comfort, the intrinsic energy associated with the fabrication of materials, dismantling versus demolition at the end of a building’s useful life, the possibility of reusing or recycling materials and the durability of built-in systems—exert as much, if not more, influence on the decision-making process than construction costs or delivery dates.
This article refers to the head office building project completed for pet products manufacturer and distributor Rolf C. Hagen Inc. located in Baie d’Urfé, Quebec. The customer opted for a structural steel frame and concrete slabs on steel deck, used in conjunction with an infloor radiant heating system. The structural steel frame will eventually permit the “dismantling” (rather than “demolition”) of the building. The joists will therefore be reusable, undergoing minor or no modifications, and therefore will not have to be recycled into new products. The infloor radiant heating system will save money on heating bills, while providing a higher level of comfort than ordinary heating systems (air-based or wall convector heating).
General information on radiant heating
Total area heating through an infloor system that produces thermal radiation is not a new technology. The ancient Greeks and Romans devised such a system for heating their public baths (see “Hypocaust”). In the modern era, this technology emerged in the early 1900s but was limited to the residential sector with the advent of heating, ventilation and air conditioning (HVAC) systems in the 1950s. Within the last several years, infloor radiant heating has been gaining popularity in the commercial, institutional and industrial sectors. At this time, many cities possess a number of green buildings (including Montreal’s Mountain Equipment Co-op and Normand-Maurice Building) that have infloor radiant heating systems.
The new Rolf C. Hagen head office building enjoys all the advantages of an infloor radiant heating system in a concrete slab on steel deck, including a comfortable living space due to the absence of heat stratification and energy savings due to a lower room temperature set point.
Typical application
Typically, a heating element, such as an electric conduit or tubing that allows the circulation of hot water, is embedded in the concrete floor structure. Cross-linked polyethylene (PEX) tubing is generally utilized. (Note: For the sake of clarity, it will be assumed, from this point on, that water tubing is used.) The objective of the system is to heat the concrete until the surface of the floor covering reaches a temperature equal to the heating capacity desired. For example, if a floor heating capacity of ±60 W/m² (20 BTU/h·sq. ft.) is required for a room temperature of 21°C (70°F), combined with “average” window exposures, then the surface of the floor covering must be 26.7°C (80°F). It is important to note that installation standards for infloor radiant heating systems recommend a maximum surface temperature of approximately 29.4°C (85°F).
Figure 1: Diagram of a typical installation
Figure 2: Example of a typical installation with the water supplied from below the steel deck
Once the required heating capacity and floor surface temperature are known, the types of installation and floor covering determine the required temperature of the water circulating in the tubing. The tubing is attached to wire mesh placed on the steel deck (see Figures 1, 2 and 3) at approximately 90 mm (3.5 in.) from the top of the concrete slab.
Despite its relatively high thermal conductivity, concrete used in this application acts as insulation. Therefore, the greater the distance between the tubing and the top of the slab, the higher the temperature of the water must be to reach the desired temperature at the surface of the floor covering. However, since the thickness of the concrete slab is relatively small, it is possible to keep the water at a lower temperature range, while maintaining the heating capacity required.
Regarding the above example, the average temperature of the water in the slab must be 31.7°C (89°F), assuming there is a center-to-center distance of 300 mm (12 in.) between the tubing and a bare (i.e. uncarpeted or untiled) slab. Since this temperature is substantially lower than that of conventional heating systems, which are generally around 82°C (180°F), it is possible to use condensing boilers or even geothermal heat pumps. Infloor radiant heating system manufacturers recommend a maximum water temperature of approximately 60°C (140°F) for systems utilized with a concrete slab.
Such manufacturers and distributors provide design manuals that allow the calculation of these parameters (i.e. water temperature and distance between the tubing and concrete) in the majority of cases. It is also possible to find theoretical discussions and more basic equations on this issue in the ASHRAE publications.
Increased energy efficiency
Lower water temperatures in concrete slabs translate into increased energy efficiencies of heating systems. For example, the HVAC system installed in the Rolf C. Hagen head office building uses geothermal heat pumps. The lower the temperature of the water, the greater the efficiency of the heat pumps will be since they use less electric energy to produce thermal energy. COP (coefficient of performance) is the parameter used to quantify the efficiency output of heat pumps. The COP is equal to the quantity of heat produced divided by the amount of energy consumed by the heat pump. Assuming the use of average-quality heat pumps with a water heating system operating at a temperature difference of 5°C (10°F) and a geothermal exchanger return temperature of 4.4°C (40°F), the COP would be approximately 4.0 (equivalent to an efficiency output of 400%).
Energy efficiency, albeit to a lesser extent, is also observed for heating systems that use natural gas boilers. The heating temperature difference indicated above allows for the use of condensing boilers since the average temperature of the water in the slab would be lower than 60°C (140°F). The technical data provided by certain boiler manufacturers suggests that an infloor radiant heating system in a concrete slab on steel deck could translate into a maximum thermal efficiency output of approximately 95% when used in conjunction with a condensing boiler, compared to 87% using conventional boilers operating at a higher temperature.
Thermal inertia
The main disadvantage of installing an infloor radiant heating system in a concrete slab is thermal inertia. Under normal operating conditions, it requires approximately 25 minutes per centimetre of slab thickness (one hour per inch) to change the temperature of the concrete by 0.5°C (1°F). Therefore, it takes far more time to control the heating capacity with infloor radiant heating than with more conventional heating systems. Nevertheless, given its relatively small thickness, a thermally active slab will have a reaction time of a few hours only, allowing it to react to interior or exterior temperature variations within a reasonable amount of time.
Watch the tubing!
Precautions must be taken while boring or drilling holes in the underside of the steel deck due to the water-filled tubing in the concrete slab. Any boring or connecting required to install other building services and systems on the lower storey must be done before pouring the concrete (see Figure 3). It is also important to coordinate the location of the tubing with that of each bored hole and connector in order to identify the “safe” zones. It is recommended to limit the depth and location of the connectors so as not to interfere with the tubing.
Figure 3: Coordinating the location of bored holes while installing the tubing
Conclusion
From a sustainable development perspective, it is time to review and validate conventional construction practices according to a broader range of assessment criteria. With such a process, current practices could be validated, strengthened or changed significantly.
Also from this perspective, structural steel frames and concrete slabs on steel deck are well suited to the installation of infloor radiant heating systems. Increased comfort levels in heated areas and lower energy consumption are just two of the benefits of this sustainable development system.
The Rolf C. Hagen head office building, which is an example of this system, has garnered the following awards to date:
Honourable Mention in the Green Buildings Category within the scope of the 2006 CISC-Quebec Steel Design Awards;
Finalist in the Innovative Practices - Sustainable Category and Special Mention in the Best Practices - Sustainable Development Category within the scope of the 2006 Contech Awards.
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