ASHRAE Journal

ashrae.org

February 2013

space. This latter approach is appropriate
for many projects, because condensation
avoidance strategies will address locationspecific condensation hazards.
The final tool for application to the system is a standard building energy modeling
platform. A simplified version of the thermally active slab is input into the simulation
program, incorporating pumping power and
cooling/heating capacity. Depending upon
the platform, sophisticated workarounds
may be required to capture the cooling capacity variation of the system in response to
solar heat gain. The primary intent of the use
of this tool is to validate the energy savings
from reduced transport energy of conditioning heat to and from the space.
Figure 3: National Rural Utilities Cooperative Finance Corporation headquarters
The result of this design process will be lobby temperature distribution output from CFD analysis.
a floor configuration with zoning consistent with the solar patterns on the floor, tube spacing consistent with the maximum heating or cooling load, ventilation air sufficiently dry enough to avoid condensation, and
supplementary heating or cooling provisions to handle any
loads beyond the capacity of the radiant floor. Analysis of
intermediate operating conditions should be performed in
addition to the peak heating and cooling analysis. These
conditions include high humidity, low sensible load operation, high solar, low exterior temperature operation, and
maximum and minimum occupancy operation. Although
these subsequent analyses may not inform the physical configuration of the radiant slab system, they may inform the
control sequence design to ensure both comfort and condensation avoidance.
A final important part of the design process is coordination of the thermal characteristics of the architectural ele- Photo 1: William Jefferson Clinton Presidential Library showments of the space. An important consideration is the ther- ing bamboo plywood flooring over thermally active slab.
mal resistance of the floor finish. Materials with excessive
thermal resistance will increase the temperature differential
Finish Material
Capacity (Imperial) Floor Temperature
between water in the tubing and the floor surface. Ideal materials include stone pavers, ceramic tiles and terrazzo. Ac1 in. Stone Paver
20.1 Btu/h·ft2
74.9°F
ceptable materials include dense wood finish materials such
as processed bamboo plywood bonded directly to the floor,
77.2°F
1.25 in. Wood Floor
17.3 Btu/h·ft2
linoleum and polymeric floor coverings. An example of bamboo plywood finish flooring above a thermally active slab is
shown in Photo 1, at the William Jefferson Clinton Presiden- 5/8 in. (15.9 mm) PEX tubing; 9 in. (229 mm) on center; 300 ft (91.5
m) roll; 2 gpm (0.12 L/s) per loop; 61°F (16.1°C), inlet water; 20 Btu/
tial Center. This building has more than 50,000 ft2 (4647 m2) ft2 (63.1 W/m2) absorbed sunlight; 75°F (23.9°C) space temperature.
of this active floor system.
Thick porous finish materials such as cork or carpet are not Table 1: Floor performance with various floor coverings.
desirable, although the author has completed several projects
with dense indoor/outdoor carpeting without pads. In that case, a minimum density of 120 lbs/ft3 (1925 kg/m3). Lightweight
both sides of the floor slab were active surfaces so that the ther- concrete has greater thermal resistance due to its much higher
mal resistance of the floor covering was less important to over- porosity. The design goal is to create the least resistance pathall heat transfer to the space. The thermal resistance of the top- way feasible for heat flow between the water in the tubing and
ping slab is also important for maximizing heat transfer across the occupied space. Floor characteristics and performance of
the floor. The topping slab should be composed of concrete with high and low transmission floor finishes are shown in Table 1.

ASHRAE Journal - February 2013

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

ASHRAE Journal - February 2013 - 38

ASHRAE Journal - February 2013

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