A comparative study on the applicability of six radiant floor, wall, and ceiling heating systems based on thermal performance analysis

Highlights

• Applicability of six floor, wall, and ceiling radiant heating systems was studied.

• Output, controllability, heat storage, building retrofit and costs were considered.

• Wall with pipes in plaster performed best, had fast response but limited storage.

• Classical floor heating performed consistently in all criteria.

• Ceiling TABS was feasible only when long-term heat storage was needed.

Abstract

Holistic comparisons of radiant heating systems that would help make an informed decision on the selection of the most convenient system for the specific application are lacking. The applicability of six representative radiant floor, wall, and ceiling heating systems was therefore compared in terms of thermal output and surface area required, controllability, short-term and long-term heat storage, suitability for building retrofit, and investments. Temperature and heat flux distribution in the structure, time constant τ₆₃, response time τ₉₀, and the number of operating cycles were computed by a custom-made and verified software tool using the finite volume method. Thermal energy stored was used to determine the ability of energy storage, whereas investment costs indicated affordability. Wall heating with pipes attached to a thermally insulating core had the highest thermal output, was easy to control, suitable for building retrofit, and most affordable while providing limited thermal storage. The performance of the wall system was retained when locating the pipes in plasterboard separated from the core by an air gap. Floor heating performed consistently in all the aspects evaluated. It was demonstrated that inserting a metal fin between pipes and the concrete spread layer improved thermal output, controllability, and storage capacity of the floor system with minor effect on investments. Ceiling with pipes insulated from the core performed well when thermal storage was not required. Ceiling with pipes embedded in the core was only feasible when long-term heat storage was needed.

Introduction

Water-based radiant systems present a potentially viable solution for space heating because they are suitable for integration with renewable energy sources [[1], [2], [3]] and have the ability to create a comfortable thermal environment [[4], [5], [6], [7]]. The applicability of the individual system types depends on their location (floor, wall, or ceiling), the configuration of material layers, and the level of thermal mass. These characteristics are crucial for the selection of the most suitable system for the specific situation such as the construction of new building vs. retrofitting of an existing building, thermal storage vs. fast thermal response, and traditional vs. low-temperature renewable heat source.

Several studies have compared the radiant systems assuming various locations and configurations of the material layers. The first category of studies involves calculations performed for a representative fragment of the heating or cooling element. Oxizidis and Papadopoulos [8] compared the energy performance and thermal comfort created by a floor cooling system, ceiling with pipes embedded in plaster, wall with pipes located in the plaster, and a generic thermally active building system (TABS). Floor cooling consumed the least energy, but it could not provide enough cooling output to attain thermal comfort. Ning et al. [9] calculated the response times of typical radiant systems as defined in ISO 11855 [10]. The systems were classified into three categories according to their thermal response: fast, medium, and slow. The difference in response times of ceiling/floor heating/cooling systems was explained by the different heat transfer coefficients between the radiant surface and indoor air. Krajčík and Šikula [11] compared four wall cooling systems. The system's suitability depended on the requirements such as exploiting thermal storage, avoiding interventions on the interior surfaces, or attaining a rapid thermal response. The cooling output was sensitive to insulation thickness for the systems with pipes located in the thermal core and to pipe spacing for the systems having pipes underneath the surface.

The second category of studies involves experiments and computer simulations on a whole-room level. Mustakallio et al. [12] experimentally compared a chilled beam, chilled beam combined with a radiant panel, chilled ceiling combined with mixing ventilation, and four localized cooling panels combined with mixing ventilation. The differences in the thermal environment created by the systems were small. Le Dréau and Heiselberg [13] performed computer simulations of an active chilled beam, radiant floor, radiant ceiling, and radiant wall in an occupied room. Using radiant floor resulted in the lowest cooling demand but created least uniform thermal conditions. The most homogeneous thermal environment was attained by the radiant ceiling. Bojić et al. [14] numerically investigated the performance of ceiling, wall, floor, and floor-ceiling heating systems. The floor-ceiling system performed best in terms of energy and exergy saving, exergy destruction, CO₂ emissions, and operation costs whereas a single ceiling system was the least preferable option. Karabay et al. [15] recommended using wall over floor heating because it provided more favourable thermal conditions with lower water temperature, thus reducing energy consumption. On the other hand, the computer simulations by Myhren and Holmberg [16] showed that the vertical temperature gradient occuring in the centre of the room was smaller for floor heating than for wall heating.

Thermal performance of radiant systems represented by, e.g., heating capacity, thermal resistance, and thermal response, is more relevant for design, testing and control of radiant systems compared to geometry and structure [9]. The thermal response is a decisive factor to determine the control strategy that is appropriate for the specific application. It is especially important for the design and operation of radiant systems with larger amounts of thermal mass. The related literature describes several approaches to assess the dynamic thermal performance of radiant systems. The literature survey has shown that response time τ₆₃, also referred to as the time constant, is frequently used as an indicator of thermal response. Time constant represents the time to reach 63% of the final value of the surface temperature, thermal output, or room temperature [[17], [18], [19]]. Alternatively, other percentages of the final value, e.g. 80% [20], 95% [9], may be used for the calculation. Depending on the radiant system used, the response time ranges from several minutes for suspended ceiling panels up to several tens of hours for TABS [9]. A system with such a long response time requires using precise control strategy to provide comfortable conditions [[21], [22], [23]].

As reported by Ning et al. [9], a single number like τ₆₃ or τ₉₅ may not be fully representative of the thermal response of radiant systems. In such a case, calculating several response times, e.g., τ₂₅, τ₅₀, τ₆₃, or τ₈₀ may be needed [9,24]. Other indicators of thermal response found in the related literature include the peak value of surface temperature [25], thermal admittance and transmittance [26,27], visual comparison of the step-up and decay curves of surface temperature [20,[28], [29], [30]], the required number of operation cycles to maintain thermal output between defined boundaries [11], and the heat storage efficiency that takes into account the evolution of surface temperature until reaching steady-state [31].

The literature review has shown that the existing studies usually compare the systems only from thermal comfort or energy efficiency point of view. Most of the research focuses on systems with massive thermal layers, whereas less attention is given to lightweight systems and their comparison with the massive systems. Moreover, the potential use of radiant systems for building retrofit has rarely been considered. It follows that holistic comparisons of radiant heating systems that would help make an informed decision on the selection of the most convenient system are lacking. The present study aims to facilitate the selection of the most suitable radiant heating system for both newly constructed and renovated buildings. The emphasize is on the thermal performance of the systems while also considering their price and suitability for building retrofit.

To accomplish this, numerical investigations of six representative radiant floor, wall, and ceiling heating systems (Fig. 1) have been performed in terms of thermal output, area of the heating surface required, controllability, short-term and long-term energy storage, suitability for building retrofit, and investment costs. Thermal fields and heat flux distribution in the structure were computed to evaluate the thermal output. Time constant (τ₆₃), response time (τ₉₀), and the number of operating cycles were used to assess the thermal response and controllability. Thermal energy stored in the structure and investment costs were also determined to help evaluate the systems in a broader context.