![]() ![]() But often, installers don’t install pipe at the minimum slope, just as they wouldn’t stand at the very edge of a cliff to enjoy the vista. It’s stating that you can only use up to an 8-foot length if the pipe is run at the minimum allowable slope of 1/4 inch per foot. ![]() They may also see that the slope column references 1/4 inch per foot, but they are likely to assume that it’s just reiterating the minimum allowable slope for a 2-inch drain.īut the slope column is saying much more than that. When builders look only at the table without reading the text, they find that a 2-inch-diameter drain can run a maximum distance of 8 feet to the vent. This basic scientific statement always trumps the simple length taken from a table and is the real key to preventing a situation where a siphon could form. However, this table was-and still is-accompanied by a very important statement: “The vent connection shall not be below the trap weir.” Unfortunately, that statement is now much easier to overlook. For simplicity, the maximum distance between a trap and its protecting vent has been presented in the form of a table for decades. The section of the code that pertains to the maximum trap-arm (also called a “dirty arm”) length has strict provisions designed to prevent a siphon from forming in a drain. A vent is supposed to maintain air in the cross-section of the pipe to prevent a siphon from forming. ![]() If the entire cross-section of a drain is able to fill with water downstream of the trap, a siphon can be created for a sufficient period of time to pull enough water out of the trap to render it ineffective. In a drain, one of the primary purposes of a fixture vent is to break the siphon. ![]() The flow of liquids has a lot of science to it, and anyone who has ever siphoned water from a fish tank or a swimming pool knows that a liquid can be drawn from a vessel without any power other than what appears to be magic. ![]()
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