Radiant Heating & Cooling Systems: A Theoretical Discussion and Literature Review

Supervisor: Murray Hodgson
Researcher: James Higgins

Overview

Radiant heating and cooling systems have been widely used in buildings throughout history. Primitive versions of heated flooring, dating back as far as 10,000 B.C., circulated flue gases from wood or coal-fired stoves under raised floors of buildings. The modern version – heated or cooled water circulated in pipes embedded in room surfaces – was developed by the British in the early 1900’s and has since benefited from a century of modern scientific development. Radiant heating systems are used extensively in Asia and Europe, though North America has only recently caught on.

Thermally Active Building Systems, or TABS, refers to the active heating or cooling of a building’s structure, a building practice with large benefits over heating or cooling solely using forced air convection. These radiant systems exchange heat with building inhabitants through both radiation and convection. The temperature humans feel, termed ‘operative temperature’, is affected equally by the mean radiant temperature of surfaces close to us and the air temperature. Therefore TABS can increase human comfort, while greatly reducing the energy required to heat or cool a building.

Compared to mechanical convection heating/cooling systems, TABS distribute heat using water instead of air. Water has a higher thermal conductivity and heat capacity than air, meaning that it acquires heat faster and at a steadier temperature (allowing more efficient heat source operation), is able to store more thermal energy per unit mass (and volume), and releases heat more readily. Water’s high thermal conductivity and density combine to make for a high convective heat transfer coefficient, and thus, a high rate at which it can transfer heat to the slab by convection from the fluid to the pipes.

The above advantages of hydronic energy transport result in a lower required mass transfer of water through a thermal slab system than air in a purely convective HVAC system. Therefore less energy and installation space is required. The heat transferred to a slab is stored until it is discharged into the building, with only small pathways for losses.

Radiant systems are considered to be low temperature heating and/or high temperature cooling systems. Because the systems operate at a water temperature close to room temperature, they increase the heat/cooling source’s coefficient of performance (a measure of efficiency) and reduce distribution losses.

Because radiant systems operate solely by surface heat exchange (convection and radiation), the surface condition is of great importance. Surfaces are typically finished with a hard, bare layer of concrete or wood, which makes for a very acoustically reflective surface. This increases reverberation in rooms and generally decreases the quality of the sound environment.

This report reviews literature on how thermal slabs work, their advantages over conventional HVAC systems, current design standards, modeling techniques, and acoustic solutions. Other possibilities for making radiant surfaces more sound absorptive without unacceptably diminishing their thermal performance are briefly discussed.

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Higgins&Crocker Report 2012 [Radiant Slabs]