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To be honest, much of the confusion over whether a drawing shows a code compliant installation of a grease interceptor or not, can be the result of boilerplate drawings provided by manufacturers in submittals or installation instructions. Manufacturer's want to provide guidance for a broad range of installations without providing detailed drawings for every single installation variable that can exist. When it comes to traps and vents for grease interceptors and the fixtures discharging to them, it's incumbent upon an engineer or contractor to identify and comply with local code requirements regardless what a manufacturer's drawing shows.

The Uniform Plumbing Code (UPC) does not permit a grease interceptor to serve as a fixture trap and also prohibits the installation of a vent between the air intake on the flow control and the grease interceptor. The above diagram would not be compliant with the UPC. There needs to be a trap and vent between the fixture and the vented flow control fitting.

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The only solution for compliance with the UPC is to add a vent for the trap on the fixture upstream of the vented flow control fitting.

The unit shown is certified to ASME A112.14.3 Type C (without external flow control) with a built-in or integral flow control and does not require an air intake.

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PDI G101 and ASME A112.14.3 Type A certified interceptors are required to have a vented external flow control installed upstream of the interceptor.

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The drawing shows a PDI G101/ASME Type A grease interceptor connected to a the three compartment sink that is trapped and vented, an external flow control with air intake and a vent on the downstream side of the grease interceptor.

The International Plumbing Code (IPC) 1002.1 (exception 3) allows a grease interceptor to serve as a fixture trap - where it is intended by the manufacturer to serve as a trap - for a single fixture or a combination sink of not more than three compartments so long as the vertical distance from the outlet of the fixture to the inlet of the interceptor is not more than 30 inches and the developed length of the waste pipe from the most upstream fixture outlet to the inlet of the interceptor does not exceed 60 inches.

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That being the case the interceptor does not require a vented external flow control - jurisdictions unfamiliar with this type of interceptor often question drawings like this and understandably so.

Once in a while, I get an email that contains a drawing or a photo of a grease interceptor installation with the question, "does this look right?" or, "what's wrong with this?" Usually the email-sender wants to know if the trapping and venting is correct.  Below is a good example of a questionable installation, but what's wrong with it?

Since all semi-automatic draw-off grease interceptors are certified to either PDI G101 or ASME A112.14.3 Type A (that I am aware of) an external vented flow control must be shown for compliance with the IPC.

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The UPC does not include provisions for AAVs except as a part of an "Engineered Vent System" under section 912.0, much to the chagrin of engineers around the country. Many states that adopt or adapt the UPC have added an allowance for AAVs so you will have to check with your state to see if they are approved.

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This is not compliant with the IPC since the code does not allow double trapped fixtures. The code does allow the interceptor to serve as a trap (assuming that this interceptor is intended to serve as a fixture trap) thus adding a trap to the fixture upstream of the interceptor creates a double trapped fixture installation. Either adding a vent to the trap or removing the trap on the fixture altogether would solve the problem.

Model plumbing codes require each fixture discharging into a grease interceptor be individually trapped and vented, and require the installation of a vent downstream of the grease interceptor.  Standards also come into play when it comes to determining the type and location of vents upstream of a grease interceptor.

What's important to remember about AAVs is that they only allow for the relief of negative pressure in the drainage system. Therefore, when using an AAV to vent a grease interceptor, it's important that the drainage system has provision for the relief of positive pressure to ensure proper flow.

This drawing for a PDI G101/ASME Type A grease interceptor appears to be compliant with the IPC.  Assuming the interceptor is intended to serve as a fixture trap, there is no requirement to install an additional trap and vent between the fixture and the interceptor.

The drawing shows a PDI G101/ASME Type A grease interceptor with an external flow control with air intake, however it also shows a trap on the fixture but no vent for the trap.

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PDI G101 mandates the installation of an vented external flow control. The vent on the flow control is an air intake.  As the waste stream flows through the orifice in the flow control device negative pressure is created, drawing in air from the air intake which is intended to mix with the waste water as it enters the grease interceptor aiding in separation efficiency.

This installation is compliant with both the IPC and the UPC since it shows a trapped and vented fixture discharging through an approved interceptor without external vented flow control with a vent installed downstream.

For compliance with the UPC this drawing would have to show both a vented external flow control and a trap and vent for the fixture.

The multi-compartment sink is not trapped and vented - UPC does not allow a grease interceptor to serve as a fixture trap; and