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From the 2015 UPC Illustrated Training Manual

905.5 Location of Opening.
The vent pipe opening from a soil or waste pipe, except for water closets and similar fixtures, shall not be below the weir of the trap.

This section is identical to Section 1002.4. A vent pipe connection that is below the weir of the trap (see Definitions, Crown Weir (Trap Weir), and Figure 1001.2a) allows the potential for the fixture to induce self-siphonage (see commentary for Section 1002.2 for further discussion). Therefore, lengths of trap arms are limited and slopes are restricted to prevent the vent pipe connection to be below the weir of the trap (see Table 1002.2). If the vent connection is below the weir of the trap, this indicates that either the length or the slope of the trap arm is not code compliant.

This will determine what type of fitting may be used to connect the trap arm, vent and fixture drain. For example, in vertical installations, the sanitary tee is the proper fitting to use for this connection. A combination wye and oneeighth bend or a wye will not be allowed here since the opening of the vent would be below the weir of the trap (see Figure 905.5a and commentary for Section 1002.2 for further discussion).

The above requirement does not apply to water closets and similar siphon-waste fixtures. In reality, these fixtures are intentionally S trapped so that siphonage will occur, thereby emptying the entire trap seal and its contents with every flush. These fixtures rely on a mechanical refill device (ballcock) to seal the empty S trap. Consequently, the primary function of vents serving these fixtures is to provide a source of air to follow behind the surge of water being discharged from water closets and other below-theweir waste/vent fixture connections (see Figure 905.5b).

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