Replacing a Three-Compartment Sink in a Tavern Kitchen
You are replacing a 3-compartment sink in a tavern kitchen. Each compartment measures 16 x 20 x 12 inches. The old sink was direct-connected with 1-1/2-inch pipe to a drum trap under the floor, with no vent on the sink trap; a floor sink 6 feet away was used only for the mop bucket. The new sink comes with a pre-rinse sprayer, and the owner is buying a new undercounter dishwasher with a 6-foot drain hose and no air gap. There is a 1-1/2-inch vent stack 5 feet away and a 2-inch floor drain with a trap primer nearby. The owner wants the dishwasher to discharge into the third compartment and wants the sink hard-piped exactly like the old one to save money.
1. The owner wants the sink direct-connected as before. What do you do?
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A 3-compartment sink in a food establishment normally has to discharge indirectly - through an air gap or air break to a receptor - so a backed-up building drain cannot rise into a compartment holding food-contact ware or sanitizer solution.
The existing arrangement already carries two problems you would be inheriting:
- Drum trap: generally prohibited where an accessible, properly vented trap can be used, and it is a grease and debris trap you cannot clean easily.
- Unvented sink trap: it will siphon and break the seal, which is exactly the sewer-gas complaint you get called back for.
So: install a properly trapped and vented receptor (floor sink or air-gap receptor) and terminate the sink drain with a real air gap or air break above the receptor's flood-level rim. Confirm the local requirement with the AHJ before cutting. If the owner refuses, document the refusal in writing and do not make the direct connection.
2. Size the receptor and trap, and show the arithmetic.
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Compartment volume: 16 x 20 x 12 = 3,840 in³; 3,840 / 231 = 16.6 gallons per compartment. Three compartments total about 50 gallons, but a person empties one at a time, so design for the largest single discharge.
Receptor: must accept that dump without overflowing and must have a genuine air gap - its rim has to sit below the sink's indirect waste outlet, not level with it. A 2-inch floor sink receptor is the usual minimum here.
Trap: 1-1/2-inch or 2-inch, sized to the load rather than oversized. An oversized trap runs partly full and does not self-scour; an undersized one chokes on solids.
Then verify: the receptor's outlet and the branch drain must carry the dump rate, the receptor must be vented, and the trap must be accessible for cleaning. Confirm the fixture-unit load against the line it ties into - the sink is not the only fixture on that branch.
3. Can the dishwasher discharge into the third compartment, and what has to be in place if it does?
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Only with backflow protection, and for a commercial unit most inspectors want more than a hose in a sink.
- Air gap or air break: the discharge must terminate above the flood-level rim of the receptor with a real gap. A common solution is a deck-mounted air-gap device, or a dedicated indirect receptor.
- Hose: stay within the manufacturer's length limit; excess hose holds debris and smells. Do not coil 6 feet of hose into 2 feet of space.
- Pump lift: confirm the machine's drain pump can lift to the air-gap height - many undercounter units are rated for a specific maximum.
- Load on the trap: the sink trap now handles the dishwasher discharge as well, so confirm trap and drain size for the combined load.
If the machine is listed for an air gap and the owner wants to skip it, that is a refusal you document rather than a connection you make. A siphoned dishwasher is a contaminated-ware problem, not a convenience problem.