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The codes are very specific in regards to the proper placement of vents to insure that the entire DWV (drain,waste & vent) system will remain at or very near standard atmospheric pressure during normal operation, however, many local codes require that during construction we must perform a 'Static Head Test" to insure there are no leaks in the DWV system.
To conduct a static head test all drain openings are sealed with test plugs and a vertical riser is installed (often they use the main vent stack) so that the top of the riser is 10' higher than the top of the highest opening on the DWV system.
The entire DWV system is then filled with water to the top of the test stack and it must maintain that full level for a period of one hour, after which all the test plugs are removed and we may proceed to the trim out portion of the plumbing job, where the fixtures are attached to the DWV system.
Water produces .434psi per ft of vertical rise so with a 10' riser the actual test pressure of a static head test would be approximately 4.34psi on the main drain.
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