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Anti-freeze loops protect areas in a building or outside a building that are prone to freezing. A check valve isolates the anti-freeze loop from the rest of the fire sprinkler system. There is also a main drain and a fill cup to assist in the draining and refilling of the loop. Anti-freeze loops are filled with a variety of different liquids, the most common being: propylene glycol, Glycerine and ethylene glycol.
If the fire sprinkler system is connected to a public water supply, ethylene glycol should not be used. Also, systems that are constructed out of CPVC plastic pipe should only be filled with Glycerine. NFPA requires the testing of antifreeze loops on an annual basis. AFP tests the anti-freeze solution with an refractometer to ensure that the solution will stand up to freezing temperatures.