The main construction objective was affordable scalability, necessary to implement the design in a fleet of new stores in hot, dry climates. All-Air System PPD MRT Dry-Bulb Temperature Relative Humidity Metabolic Rate Clothing Insulation Air Speed 7% 76°F 76°F Radiant-Cooled Floor and DOAS 5% 72°F 76°F 50% 1.7 met (Walking Around) Radiant-Cooled Floor and DOAS 8% 72°F 78°F Why Radiant Floor Cooling? The project engineers designed a radiant-cooled floor with a DOAS because it satisfied the performance and construction objectives. Maximize Evaporative Cooling All-air systems typically supply 55°F (12.8°C) air that requires 45°F (7.2°C) refrigerant or chilled water. Radiant-cooled floor systems typically maintain a 68°F to 72°F (20°C to 22.2°C) floor surface temperature2 that requires chilled water supply of 55°F to 60°F (12.8°C to 15.6°C). The warmer supply temperature increases system efficiency by (1) extending the operating range for water-side economizing; and (2) enabling a warmer refrigerant suction temperature under chiller operation. Decouple Ventilation from Occupant Conditioning Radiant floor cooling can meet most or all of the sensible cooling demand in a retail store. A separately controlled DOAS can supply ventilation air at space-neutral conditions or cooled to meet any latent and/or additional sensible load. Shift Peak Cooling Demand charges and time-of-use tariffs can significantly increase electricity bills. Radiant floor cooling can leverage thermal mass to offset peak demand without compromising thermal comfort. A morning charge at lower dry-bulb and wet-bulb conditions further capitalizes on water-side economizing or more efficient chiller operation with lower condensing temperatures. Improve Thermal Comfort Thermal comfort models such as the Fanger Method4 account for the role radiative heat exchange plays in thermal comfort.5 The Fanger Method calculates the predicted percentage dissatisfied (PPD) metric; the percentage of occupants who will be uncomfortable under given conditions.6 Standard design practice attempts to maintain an overall PPD December 2010 0.5 clo (ASHRAE Standard 55-20047 Summer) 70 fpm (Walking Around) Table 1: Thermal comfort comparison.5 Standard Radiant-Cooled Floor Installed Radiant-Cooled Floor 6 in. O.C. 1/2 in. 260 ft Bottom of Slab 4 in. None 58°F 5°F 0.79 gpm/Loop Tube Spacing Tube Diameter Loop Length Tube Depth Slab Thickness Edge and/or Sub-Slab Insulation Supply Water Temperature Water Temperature Rise Maximum Flow Rate 6 in. to 9 in. O.C. 5/8 in. 300 ft 1.5 in. to 2 in. Below Slab Surface 6 in. 0 in. to 1 in. Foam Board 55°F to 60°F 5°F to 9°F 1.2 gpm/Loop Table 2: Standard versus installed radiant-cooled floor design. 6 in. Slab Plastic Ties 5/8 in. Tubing 4 in. Slab 1/2 in. Tubing Gravel 6 in. Spacing Foam Board Insulation (Optional) Gravel Clip Strip Figure 1: Standard radiant-cooled floor (left) and installed radiant-cooled floor (right). for general comfort at less than 10% (greater than 90% satisfaction). The radiant floor and DOAS actively regulate both dry-bulb temperature and mean radiant temperature (MRT); an all-air system actively regulates drybulb temperature only. Table 1 shows how a lower MRT of 72°F (20°C) can meet the comfort criteria at a higher drybulb temperature of 78°F (26°C). Scalability Over the past few years, much effort has been expended to make radiant floors scalable and economical. The focus has been on reducing the labor required to lay and fasten the tubing in place. A radiant floor OEM worked with the project engineers to develop a preconfigured module to reduce installation time. Condensation Considerations Condensation was not considered an issue for three reasons: (1) Sacramento is in a dry climate (ASHRAE Climate Zone 3B); (2) the internal latent loads are minimal (occupants are the main source of moisture); and (3) the grocery ASHRAE Journal 29