For existing antifreeze sprinkler systems installed under previous editions of NFPA 13, where a modern backflow prevention device is not present, a small hole was required to be drilled in the clapper of the check valve. This 1/32 in. (0.79 mm) hole serves as a pressure relief mechanism. It prevents the system from becoming pressure-locked due to the thermal expansion of the antifreeze solution, which could inhibit water flow into the system upon sprinkler activation. This practice is now superseded by the requirement for listed backflow preventers in new installations to protect the potable water supply.

B. 1/16 in. (1.59 mm): This dimension is not the specified size for the check valve weep hole in the standard; it would permit excessive leakage.

C. 1/8 in. (3.17 mm): This is the minimum orifice size for a pressure relief valve on a gridded wet pipe system, not the hole in an antifreeze system check valve.

D. 1/4 in. (6.35 mm): This size is far too large for this application and would compromise the function of the check valve by allowing significant backflow.

1. NFPA 13, Standard for the Installation of Sprinkler Systems, 1999 Edition.

Section 4-5.2.2: "A listed check valve shall be provided in the connection to the antifreeze system. A 1/32 in. (0.8 mm) hole shall be drilled in the clapper of the check valve." This section explicitly states the requirement for the specified hole size in the check valve for systems installed under this code version.

2. NFPA 13, Standard for the Installation of Sprinkler Systems, 2022 Edition.

Section 8.6.2.1: "Where the water supply to a sprinkler system is from a potable water source, a backflow prevention device shall be installed." This reference shows the modern requirement, explaining why the practice of drilling a hole is obsolete for new systems but relevant for "existing" ones as per the question.

3. Jensen, J. (Ed.). (2016). Fire Protection Systems (2nd ed.). Oklahoma State University.

Chapter 5, Antifreeze Systems: Courseware from OSU's Fire Protection and Safety Engineering Technology program discusses historical and current antifreeze system design. It notes the older practice of drilling a 1/32-inch weep hole in the check valve clapper to relieve pressure from thermal expansion, a method replaced by modern backflow preventers. (Specific reference to lecture notes on legacy system components).