Radiant conditioning has a long history going back to ancient China, thousands of years before the Roman Baths. It takes advantage of the fact that for low ambient air velocities, radiant heat transfer between the clothed body and surrounding surfaces is equal to or greater than convective heat transfer of the clothed body. For most buildings, the interior surfaces of the exterior partitions, exposed on their outer surfaces to the weather, will have the most extreme temperatures in the enclosure. Radiant conditioning balances the radiant interaction between occupants and enclosure, both by offsetting the radiant effects of the exterior partitions, and by interacting radiantly with these surfaces to bring them closer to the desired temperature.
Radiant delivery of space conditioning is attractive because of its inherent transport efficiency and because of its space efficiency. Because of the greater specific heat and density of water, heat can be transported to or from a space using water with between 10% and 20% of the amount of transport energy required when using air as the medium for moving heat. Similarly, the space requirements for hydronic transport of heat within the building are significantly less than those for transport by heated or cooled air. Avoided ductwork, air-handling capacity and architectural accommodation for these systems often can offset much of the cost of the radiant delivery system.
For hydronic transport to be successful, the coupling between the transport medium, and the space must be maximized. To maximize this coupling, radiant conditioning systems often use the most extensive surfaces in the building, the floor and the ceiling. These surfaces have the advantage of convective coupling to the room air and radiant coupling to the room surfaces and occupants. Because of the radiant coupling between the surfaces and occupants, the cold interior surface temperature of extensive glazed areas or other lightly insulated partitions can be offset by warm ceilings or floors.
Radiant heating with floor slabs with extensive, low temperature surfaces has the additional advantage, when heating a high space, of minimizing thermal stratification. The maximum temperature of stratified air is limited by the maximum surface temperature within the space. Small surface area, high temperature convectors, while nominally of adequate capacity for a given heat loss, will generate significant buoyant plumes of high temperature air, resulting in high temperatures aloft, and unacceptably lower temperatures in the lower occupied regions. Stratification of high spaces during heating is inefficient, because the excessive temperature of the upper unoccupied regions of the space created as a by-product of maintaining comfort conditions in the lower occupied region, resulting in excessive heat loss.
Radiant cooling floors, on the other hand, tend to drive stratification. To some extent, this stratification can be beneficial, because only the occupied lower space of a high space need be conditioned to comfort conditions. However, the capacity of a cooling floor is limited because it does not tend to generate convective circulation of the cooled air around the space. Lowering the temperature excessively (below 68°F [20°C]) to increase cooling capacity can result in discomfort due to thermal asymmetry (feet much colder than head) or to condensation.
Thermally active slabs are most effective in the cooling mode when the circulating water removes absorbed solar heat gain directly from the slab. With a lower limit on the temperature of the floor surface of 68°F (20°C), and a room temperature of 76°F (24°C), the maximum cooling capacity of a floor can be less than 10 Btu/ft2·h (31 W/m2·h). European comfort standards allow a maximum air temperature of 79°F (26°C), giving a minimum floor capacity of 13.5 Btu/ft2·h (42 W/m2·h). On the other hand, with 40 Btu/ft2·h (126 W/m2·h) of absorbed solar flux, the cooling floor can remove more than 95% of absorbed solar flux, and maintain the floor less than 2°F (1°C) above ambient air, compared with a more than 23°F (13°C) rise for the inactive floor.
A final advantage of thermally active floor slabs is the possible direct manipulation of the building interior mass. Typical passive thermal mass applications in buildings, because of the limited thermal coupling between the air and the mass, require significant interior air temperature variations to “drive” the interior mass. Thermally active slabs, on the other hand, can be “driven” directly, to pre-cool the space before occupancy and to minimize space condition excursions due to limited conditioning capacity. Radiant floors may access a minimal amount of mass if insulation separates the topping slab/floor finish assembly from the building structure, or a much larger fraction if no thermal barrier separates the two. While radiant slab systems are often accused of slow response to applied loads, the mass of the system, and its close thermal coupling to the space, results in mitigation of peak loads, alleviating the need for quick conditioning response.
The typical installation for a thermally active slab consists of high density polyethylene tubes embedded in a concrete floor slab. Cool or warm fluid, depending upon conditioning mode, flows through the tubing to provide the conditioning. The tubing is often covered by a topping slab, which, in turn, is covered by the floor finish material (Figure 1). If heat transfer through the slab below the tubing is not desired, then a layer of board insulation is inserted between the structural slab and the tubing/topping slab layer. For slab on grade, the insulation may be inserted below the structural slab. Water is circulated to each loop of tubing through a manifold that serves multiple floor loops (Photo 1). For capacity control, the system may be configured as a constant flow variable temperature system or a variable flow (multizone pulsed constant flow) with constant fluid inlet temperature setpoints for heating and cooling modes. The author almost always uses the variable flow system because it allows inexpensive individual zone control with only the addition of two-way control valves. A constant flow system with variable inlet temperature requires a pump and a mixing valve or a variable temperature fluid source for each temperature control zone. Zones in these systems tend to be much larger and cannot so easily track solar patterns across the floor of a large sunspace.
Citation: ASHRAE Journal, vol. 55, no. 1, January 2013