precharging the slab in the mornings and floating during peak hours. Yet, could the ground under a 4 in. (102 mm) slab provide thermal mass benefits similar to an additional 2 in. (51 mm) of concrete, especially with the tubing located at the bottom of the slab? If so, sub-slab insulation would be counterproductive because it would resist the “free” thermal mass of the ground. The transient behavior of the slab was modeled9 with a wholebuilding energy simulation program8 to evaluate the thermal mass effects of a 4 in. (102 mm) slab without insulation versus a 6 in. (152 mm) slab with insulation. The sub-slab was modeled as a 3.3 ft (1 m) layer of clay/silt: 0.51 Btu/h·ft·°F (0.88 W/m·K), 75 lb/ft3 (1201 kg/m3), and 0.52 Btu/lb·°F (2176 J/kg·K). The bottom of the ground layer was assumed to be adiabatic. Results showed that the uninsulated 4 in. (102 mm) slab and ground had at least the same available thermal mass benefits as the insulated 6 in. (152 mm) slab. For this reason, the 4 in. (102 mm) uninsulated slab was specified. • Flow rate. Standard practice in fluid flow provides for a minimum velocity to establish turbulent flow and ensure sufficient convective heat transfer.2 Based on the typical temperature rise of 5°F to 9°F (2.8°C to 5.0°C),2 5°F (2.8°C) was specified to maintain turbulence and enable a warmer supply temperature to maximize waterside economizing at the expense of increased pumping energy. • Supply-return temperatures. With the aforementioned design parameters fixed, a steady-state finite element analysis calculated that a 58°F (14.4°C) supply and 63°F (17.2°C) return would maintain a 66°F (18.9°C) floor surface temperature while providing a 15.8 Btu/h·ft2 (49.8 W/m2) cooling rate. 16 14 12 10 8 6 Sensible Loads (Btu/h·ft2) 4 2 0 –2 –4 –6 –8 –10 –12 –14 –16 Skylight Solar & Conduction Lighting Infiltration Plug Loads Roof Conduction Radiant Floor Cooling Occupants Merchandise Peak Cooling Load: 15.3 Btu/h·ft2 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 Radiant Floor Peak Cooling Capacity: 15.8 Btu/h·ft2 at 66°F Floor Surface Figure 4: Whole-building simulation of the merchandise zone across the 1.0% wetbulb design day with the floor surface temperature controlled each hour to offset each hourly sensible load. Figure 7 shows the hourly floor surface temperatures. Ventilation Air DX Coils Rack A Low Temp. Fluid Cooler 1 Rack B Medium Temp. Rack C Medium Temp. Fluid Cooler 2 IDEC Outdoor Scavenger Air Exhaust Scavenger Air Merchandise and Checkout Radiant Floor Condenser Evaporator Fluid Cooler 3 Air Cooled Chiller Figure 5: Mechanical system schematic. mechanical and refrigeration systems. Three fluid coolers rejected heat from the refrigeration racks and provided waterside economizing for the radiant floor. An air-cooled chiller ASHRAE Journal 33 Mechanical System Figure 5 shows how the radiant floor integrated into the December 2010