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Island sink drain venting procedures: this article describes the special venting requirements for island sinks or peninsula sinks found in some kitchens.
Here we explain that because there may be is no nearby vertical wall through which to run a conventional vertical plumbing vent up through the building roof, a different approach is needed for routing the sink drain, waste, and vent piping to avoid slow noisy (gurgling) sink drains and to minimize the risk of smelly, unsanitary, and even dangerous sewer gas back-venting through the sink trap.
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An island sink is a sink located (typically) in a free-standing countertop that does not abut a building wall. That open location can make conventional plumbing vent connections tricky.
There are several approaches to successful island sink drain line venting
Synonyms for island sink vents such as the piping arrangement marked (B) in our sketch above include "Chicago Loop" or "Island Fixture Vent". Also see DEFINITION of YOKE VENT discussed separately in PLUMBING VENT DEFINITIONS, TYPES
Island sinks or other plumbing fixtures or water-using appliances installed in an island or peninsula (e.g. a dishwasher in a kitchen island counter) require special provisions for proper venting of the plumbing drain. The plumbing trap(s) are roughed-in above the floor level and vented (using a vacuum breaker vent such as those we discuss here) placed as high as possible and in no case lower than the height of the drain board or sink's flood rim height.
The plumbing vent for an island sink or fixture is returned back dow and connected to the horizontal portion of the sink drain piping immediately downstream from the vertical portion of the drain as is illustrated in the schematic above on this page.
The two island sink photographs just above show a recent (2014) island sink installation in Franz Josef, New Zealand. You'll note that the shallow drainboard tees into the sink drain above a single trap. A vacuum breaker vent is indicated by our red arrow. [Click to enlarge any image]
The returned vent is connected to the horizontal drain through a "y" branch fitting and is provided with a "foot vent" taken off the vertical fixture by means of a Y-branch fitting immediately below the floor. This foot vent extends through the floor to the nearest partition wall and from that point extends upwards inside the partition wall through the roof to open air above the building as does any other plumbing vent.
Alternatively the foot vent may be connected to other building plumbing vents provided that connection point is at least six inches above the flood rim of the island sink or other plumbing fixtures being served.
Plumbing fixtures originally installed with no vent connection to which a (sometimes permitted) local plumbing vent/vacuum breaker (such as a "V-200 plumbing vent - instructions shown above) usually installed right under the sink to permit air to enter the plumbing drain line as needed but intended to keep sewer gases from exiting that same drain - a one-way valve. If the valve is defective or "stuck", inadequate, or improperly installed this problem may occur.
At PLUMBING VENT DEFINITIONS, TYPES we point out that there are distance limitations between a plumbing fixture trap and the nearby vertical vent stack.
As we discuss at PLUMBING DRAIN NOISE DIAGNOSIS, if the horizontal distance between a plumbing fixture and the vertical vent piping is too great, the fixture may not drain properly, producing slow drainage or gurgling noises.
[Click to enlarge any image]
Poor drainage is not just an annoyance, it can be unsafe since there is also the risk that the poorly-vented plumbing fixture will lose the water from its plumbing trap, then permitting sewer gases into the building.
As we show in Carson Dunlop's sketch, the distance allowed between a plumbing fixture (actually the fixture plumbing trap) and the vertical vent piping varies between a minimum and maximum as a function of the pipe diameter.
When we inspected the island sink shown at page top we saw this garbage grinder mostly hidden by clutter under the sink. How were the garbage grinder and dishwasher draining and venting in this location?
The air gap provided by the connection shown at above right may successfully drain the dishwasher but there is no provision against trap siphonage nor back-venting of potentially dangerous sewer gases back into the building through this opening.
At KITCHEN & BATH DESIGN GUIDE as well as in other InspectApedia articles we describe special requirements to assure functional design and operation of island sinks and peninsula sinks.
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