Antifreeze Loop in a Parking Garage With a Shared Riser
A 3-level open parking garage uses a wet system on the two upper levels and an antifreeze loop on the lowest level, which dips to 20°F. The antifreeze loop uses a listed antifreeze solution at a concentration the manufacturer lists for −20°F, and the loop is 1,200 ft of 1-inch pipe with 40 sprinklers. The building's single riser also serves a small office on level 1 that is heated. The contractor proposes to fill the entire system with antifreeze "to be safe" and to use a single backflow preventer at the riser. The AHJ has asked for the antifreeze documentation and a hydraulic calculation showing the antifreeze loop does not compromise the rest of the system.
1. Should the entire system be filled with antifreeze? Explain the code and practical issues.
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No. Antifreeze systems are permitted only where piping is subject to freezing, and the solution must be a listed antifreeze of the correct concentration. Filling the heated office and upper levels with antifreeze wastes solution, increases cost, and - more importantly - changes the fluid properties and discharge characteristics of the sprinklers. NFPA 13 limits antifreeze use and requires the concentration to match the lowest expected temperature (here −20°F). The correct design is an antifreeze loop limited to the freezing level, isolated by a check valve or a dry/wet boundary, with the heated areas on plain water. The AHJ's request for documentation is appropriate: the designer must provide the manufacturer's listing, the concentration, and the hydraulic calculation.
2. What must the hydraulic calculation show for the antifreeze loop, and what is the risk if the concentration is wrong?
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The calculation must show the antifreeze loop's demand using the solution's properties (density and viscosity differ from water, affecting friction loss) and confirm the loop's demand plus the rest of the system is met by the supply at the required pressure. It must also show that the antifreeze loop does not reduce pressure or flow to the upper levels. If the concentration is too weak, the loop freezes and bursts; if too strong, the solution may not be listed for the sprinklers or may discharge differently. The designer should verify the listed concentration for −20°F, confirm the sprinklers are listed for antifreeze service, and provide the calculation with the safety margin before the AHJ approves.