Re: Venting a drain line from kitchen sink
Posted by CEA on February 08, 2004 at 20:34:18:
In response to Re: Venting a drain line from kitchen sink

In order to keep the flow of water down the drain from siphoning out the trap, the water level in the line between the trap and the vent cannot be allowed to get high enough to cut off the trap's "air supply" from the vent. Codes describe this as "the bottom of the dry vent cannot be below the trap weir." The only way to meet this requirement is to limit the vertical drop in the drain line between the trap and the vent to less than a pipe diameter. Based on that and a 1/4" per foot slope you can determine the maximum trap arm length. Codes vary, but the most conservative (UPC) only gives you 42". So the bottom line is that you have to have a "vertical" (45 degrees or more) vent connection to the horizontal drain within 42" of the trap. Also the vent can't run horizontally until you get at least 6" above the flood rim of the sink (so that if the drain backs up, the sink will overflow before the water gets high enough to cut off the vent air).

The right approach is to tee into the existing vent line at least 6" above countertop level, run horizontally in the wall toward the sink, then down vertically in the wall behind the sink. Once you make the vent connection, the remainder of the drain can run horizontally, vertically, whatever, as long as it slopes 1/4" per foot.

As an alternative you may be able to use an air admittance valve (AAV) on the trap arm instead of a vent. This is a mechanical device that opens under vacuum to let air in, and closes under pressure to keep water in. The AAV would go in the under-sink cabinet within the required distance of the trap. A lot of jurisdictions don't allow the use of AAVs so you would need to check your local code to see if this is an option.

Most codes also allow special arrangements for a sink located somewhere that is structurally impossible to vent. The usual cases are sinks on a kitchen island, or against a wall of windows. If your inspector agrees that your sink qualifies as such, the UPC allows a rube goldberg kind under-floor venting which is pretty close to what you described in your original post (for the general idea you can see http://www.legis.state.il.us/commission/jcar/admincode/077/07700890ZZ9997kR_files/image035.gif)

At this point I'd recommend a conversation with your local plumbing inspector or a professional plumber.


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