ASHRAE Journal - January 2013 - 34

Floor Finish Polystyrene Insulation Nylon Staple Structural Concrete Slab

Topping Slab (0.75 in. Minimum Over The Top of the Tubing) Edge Insulation
©Robert P Ellington, Robert Preston + Partners .

PEX Tubing

Figure 1: Construction configuration for standard heating/cooling floor with polyethylene tubing embedded in concrete slab. commodation 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 byproduct 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
34 ASHRAE Journal

Photo 1: Radiant floor distribution manifold showing individual polyethylene tubing loop connections. 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).4 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.

Thermally Active Slab System
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
ashrae.org January 2013



ASHRAE Journal - January 2013

Table of Contents for the Digital Edition of ASHRAE Journal - January 2013

ASHRAE Journal - January 2013
Contents
Commentary
Industry News
Letters
Meetings and Shows
Feature Articles
Long-Term Commercial GSHP Performance: Part 6: Maintenance and Controls
Thermally Active Floors, Part 1
Technology Award Case Studies:
Aquathermal Systems
Standing Columns
Data Centers
People
Emerging Technologies
IAQ Applications
Engineer's Notebook
Washington Report
Refrigeration Applications
Classified Advertising
Advertisers Index
ASHRAE Journal - January 2013 - ASHRAE Journal - January 2013
ASHRAE Journal - January 2013 - Cover2
ASHRAE Journal - January 2013 - 1
ASHRAE Journal - January 2013 - 2
ASHRAE Journal - January 2013 - Contents
ASHRAE Journal - January 2013 - Commentary
ASHRAE Journal - January 2013 - 5
ASHRAE Journal - January 2013 - Industry News
ASHRAE Journal - January 2013 - 7
ASHRAE Journal - January 2013 - 8
ASHRAE Journal - January 2013 - 9
ASHRAE Journal - January 2013 - 10
ASHRAE Journal - January 2013 - 11
ASHRAE Journal - January 2013 - 12
ASHRAE Journal - January 2013 - 13
ASHRAE Journal - January 2013 - 14
ASHRAE Journal - January 2013 - 15
ASHRAE Journal - January 2013 - Letters
ASHRAE Journal - January 2013 - 17
ASHRAE Journal - January 2013 - Meetings and Shows
ASHRAE Journal - January 2013 - 19
ASHRAE Journal - January 2013 - 20
ASHRAE Journal - January 2013 - 21
ASHRAE Journal - January 2013 - 22
ASHRAE Journal - January 2013 - 23
ASHRAE Journal - January 2013 - Long-Term Commercial GSHP Performance: Part 6: Maintenance and Controls
ASHRAE Journal - January 2013 - 25
ASHRAE Journal - January 2013 - 26
ASHRAE Journal - January 2013 - 27
ASHRAE Journal - January 2013 - 28
ASHRAE Journal - January 2013 - 29
ASHRAE Journal - January 2013 - 30
ASHRAE Journal - January 2013 - 31
ASHRAE Journal - January 2013 - Thermally Active Floors, Part 1
ASHRAE Journal - January 2013 - 33
ASHRAE Journal - January 2013 - 34
ASHRAE Journal - January 2013 - 35
ASHRAE Journal - January 2013 - 36
ASHRAE Journal - January 2013 - 37
ASHRAE Journal - January 2013 - 38
ASHRAE Journal - January 2013 - 39
ASHRAE Journal - January 2013 - 40
ASHRAE Journal - January 2013 - 41
ASHRAE Journal - January 2013 - 42
ASHRAE Journal - January 2013 - 43
ASHRAE Journal - January 2013 - 44
ASHRAE Journal - January 2013 - 45
ASHRAE Journal - January 2013 - 46
ASHRAE Journal - January 2013 - 47
ASHRAE Journal - January 2013 - Aquathermal Systems
ASHRAE Journal - January 2013 - 49
ASHRAE Journal - January 2013 - 50
ASHRAE Journal - January 2013 - 51
ASHRAE Journal - January 2013 - 52
ASHRAE Journal - January 2013 - 53
ASHRAE Journal - January 2013 - 54
ASHRAE Journal - January 2013 - 55
ASHRAE Journal - January 2013 - 56
ASHRAE Journal - January 2013 - 57
ASHRAE Journal - January 2013 - 58
ASHRAE Journal - January 2013 - 59
ASHRAE Journal - January 2013 - 60
ASHRAE Journal - January 2013 - Data Centers
ASHRAE Journal - January 2013 - 62
ASHRAE Journal - January 2013 - 63
ASHRAE Journal - January 2013 - People
ASHRAE Journal - January 2013 - Emerging Technologies
ASHRAE Journal - January 2013 - 66
ASHRAE Journal - January 2013 - 67
ASHRAE Journal - January 2013 - IAQ Applications
ASHRAE Journal - January 2013 - 69
ASHRAE Journal - January 2013 - 70
ASHRAE Journal - January 2013 - 71
ASHRAE Journal - January 2013 - Engineer's Notebook
ASHRAE Journal - January 2013 - 73
ASHRAE Journal - January 2013 - 74
ASHRAE Journal - January 2013 - 75
ASHRAE Journal - January 2013 - Washington Report
ASHRAE Journal - January 2013 - Refrigeration Applications
ASHRAE Journal - January 2013 - Classified Advertising
ASHRAE Journal - January 2013 - Advertisers Index
ASHRAE Journal - January 2013 - 80
ASHRAE Journal - January 2013 - Cover3
ASHRAE Journal - January 2013 - Cover4
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