How To Look At A House
McGarry and Madsen's home inspection blog for buyers of
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Why is garbage disposal not allowed at a combination waste and vent (CW&V) under a sink?
Wednesday, June 20, 2018
“A combination waste and vent system shall not receive the discharge of a food waste grinder” is clearly stated at P3111.1 in the International Residential Code (IRC) and the Residential Edition of the Florida Building Code (FBC). But we see them installed together fairly often, like in the photo above. One plumber we know suspects that some local building officials confuse a combination waste and vent, that uses oversize pipe to allow extra room for air to reach the trap to equalize pressure, with an island vent, which is a tall loop configuration of smaller diameter pipe.
We cannot find anything definitive as to why garbage disposals are not allowed, but suspect that it is because the larger diameter pipe of a CW&V may reduce the depth and rate of flow of the liquid in the drain, and it might not be enough to float the larger chunks of food waste produced by the disposal down the pipe.
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