What is the general minimum size of an air gap, measured vertically above the flood level rim?
- Equal to the diameter of the supply outlet's effective opening, and never less than 1 inch
- Twice the diameter of the drain outlet of the receptor, and never less than 2 inches
- Twice the diameter of the supply outlet's effective opening, and never less than 1 inch
- Three times the diameter of the supply pipe upstream, and never less than 6 inches
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C. Twice the diameter of the supply outlet's effective opening, and never less than 1 inch
The basic rule is two times the effective opening of the supply outlet with a 1 inch minimum. A gap equal to one diameter is too small to stop water from bridging during a vacuum.
Reference: ASME A112.1.2 air gap principles; EPA Cross-Connection Control Manual (EPA 816-R-03-002), ch. 4
A faucet with a 3/4-inch effective opening discharges into an open tank away from any wall. What minimum air gap is needed above the tank's flood level rim?
- 3/4 inch
- 1-1/2 inches
- 1 inch
- 2-1/4 inches
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B. 1-1/2 inches
Two times the 3/4-inch effective opening is 1-1/2 inches. One inch is only the floor that applies when twice the opening would come out smaller than that.
Reference: ASME A112.1.2 air gap principles; EPA Cross-Connection Control Manual (EPA 816-R-03-002), ch. 4
A supply outlet with a 3/8-inch effective opening fills an open sump away from any wall. What minimum air gap applies?
- 3/4 inch
- 3/8 inch
- 1-1/8 inches
- 1 inch
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D. 1 inch
Twice 3/8 inch is 3/4 inch, but an air gap is never less than 1 inch, so 1 inch governs. Stopping at 3/4 inch forgets the minimum.
Reference: ASME A112.1.2 air gap principles; EPA Cross-Connection Control Manual (EPA 816-R-03-002), ch. 4