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Thermal comfort and acoustic quality in buildings using radiant systems

  • Author(s): Karmann, Caroline
  • Advisor(s): Schiavon, Stefano
  • et al.

In the US, people spend about 90% of their time indoors. This long exposure to indoor conditions affects our well-being, performance and health. Design and operation of these spaces also impacts energy use in building which, in the US, accounts for 40% of primary energy use. With these dual challenges, researchers and building professionals seek design strategies to simultaneously address the challenge of indoor environmental quality (IEQ) and energy use. Radiant heating and cooling systems have the potential to achieve significant energy savings primarily due to the use of lower temperature differences between the space and the heating or cooling source. Compared to buildings with all-air systems, buildings with radiant systems have been commonly associated with increased thermal comfort but decreased acoustic quality. The concern of reduced acoustics is particularly the case with regard to massive radiant systems and the need to preserve heat transfer, thereby keeping radiant surfaces uncovered. Achieving improved IEQ is fundamental for the successful adoption of radiant technologies in buildings. This dissertation proposed to address thermal comfort and acoustic quality in spaces using radiant systems through two major questions:

- How do spaces with radiant systems compare to spaces with all-air systems in terms of thermal comfort and acoustic quality?

- Can the combination of free-hanging acoustical clouds and fans below a massive radiant ceiling address simultaneously thermal comfort, cooling capacity and acoustical performance issues?

As part of a larger research team I utilized literature review, occupant surveys, statistical analysis, and laboratory experiments of a market-ready solution to address these questions.

We performed a literature review to assess if there was existing evidence that radiant systems provide better, equal or lower thermal comfort than all-air systems. This review identified five studies that could not establish a preference between the two systems and three studies showing a preference for radiant systems. These studies used multiple methods to demonstrate their findings and, in addition, several types of all-air and radiant systems were tested. This limited number of available studies did not allow us to draw a conclusion about the effectiveness of radiant systems for thermal comfort.

Following this review, we conducted occupant surveys in buildings using radiant systems. We gathered responses from 1284 occupants (20 buildings) that we complemented with responses form 361 occupants (6 buildings) previously surveyed. We used an existing database to extract a subset of occupant responses from all-air buildings whose key characteristics match those radiant buildings. This comparison involved 3892 responses from 60 buildings total. All-air and radiant buildings have equal indoor environmental quality, including acoustical satisfaction (assessed for noise and sound privacy), with a tendency towards improved thermal comfort in radiant buildings. There is a 16% probability of temperature satisfaction superiority for occupants exposed to radiant systems.

In a third phase, we experimentally assessed the combined effect of free-hanging acoustic clouds and fans for an office room. Free-hanging acoustic clouds are intended to reduce acoustical issues of a radiant massive ceiling, yet, they will also reduce its cooling capacity. Fan-induced air movement can be used to compensate for the cooling capacity reduction and enhance thermal comfort. In the test configuration, we installed a ceiling fan between the clouds (blowing in the upward or downward direction) and small fans above the clouds (blowing horizontally) both at the ceiling level. The two types of fans were tested independently. The acoustical results showed that if the clouds covered 40-50% of the ceiling area, acceptable reverberation time (one of the metrics for acoustical quality) was achieved. The cooling capacity experiments conducted without fans showed that for 47% cloud coverage, the cooling capacity only decreased by 11%. The ceiling fan increased cooling capacity by up to 22% when blowing upward and up to 12% when blowing downward, compared to the reference case and over the different cloud coverage ratios. For variants with small fans, the cooling increases with coverage proving that the combination of a cloud and a small fan has a positive effect on cooling capacity. Elevated air motion in the occupied space can provide further advantages from a thermal comfort perspective for the ceiling fan variants. Combining fans with acoustical absorbents close to the radiant surface has the potential to increase cooling capacity while simultaneously providing improved acoustic quality.

In summary, the dissertation work has: (1) summarized the state of current research on thermal comfort for radiant systems compared to all-air systems; (2) developed and analyzed the largest database of occupant responses in buildings using radiant systems; (3) used occupant feedback to show that radiant and all-air buildings have overall equal indoor environmental quality, including acoustical satisfaction, but with a tendency towards improved thermal comfort in radiant spaces; and (4) experimentally tested two practical solutions of combined acoustic clouds and fans below a radiant chilled ceiling, herein showing that it is possible to overcome the limitation of ceiling sound absorption often associated with radiant slab systems.

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