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From the 2015 UPC Illustrated Training Manual

801.2 Air Gap or Air Break Required.
Indirect wastepiping shall discharge into the building drainage systemthrough an air gap or air break as set forth in this code.Where a drainage air gap is required by this code, theminimum vertical distance as measured from the lowestpoint of the indirect waste pipe or the fixture outlet to theflood-level rim of the receptor shall be not less than 1 inch(25.4 mm).

Air gaps are required by most plumbing codes for dishwashers and for water softeners. They are needed to prevent any possible cross-connection between your potable water system and your sewage system.

An indirect waste pipe does not connect directly to the drainage system, but conveys liquid waste by discharging it into a plumbing fixture, receptor or receptacle, which in turn is directly connected to the drainage system. Various types of fixtures, appliances and equipment are required to discharge their wastes to the drainage system by indirect waste.

There are two types of indirect waste piping:
1. Indirect waste that discharges to the receptor through an air break
2. Indirect waste that discharges to the receptor through an air gap. There are two types of air gaps:
a. Water distribution air gap.
b. Drainage air gap.

An air break provides a physical separation between the fixture indirect waste pipe and the receptor. If allowed by the code, the fixture indirect waste pipe may terminate below the flood level rim of the receptor, creating a low inlet into the receptor. An air break simply requires that there be no direct physical connection between the fixture indirect waste pipe and the receptor (see Figure 801.2a).

The drainage air gap is not the same as the water distribution air gap. The water distribution air gap is defined in Chapter 6 and determined by Table 603.3.1 (see Figure 801.2b). The water distribution air gap minimum size is also one inch but also requires that the air gap height be a minimum of two times the diameter of the indirect waste pipe if the size is over one-half of an inch.

The drainage air gap (see Figure 801.2c) is defined as the vertical distance as measured from the lowest point of the indirect waste pipe or fixture outlet to the overflow rim of the receptor. This distance is a minimum of one inch. There are no further requirements, unlike those that exist for the water distribution air gap.