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First, I'm going to try and describe the current (original/builder) configuration. It seems more complicated than it needs to be, but perhaps there's a reason or code that is required. This is a kitchen sink in an island. The P-Trap is under the floor (i.e. in the basement ceiling), and vent runs under the floor to the wall, then up two stories and out the roof.

Here are a few photos of the situation, and a legend:

A. Drain from sink above. ~7" from floor to trap. Approx. 24" from floor to sink.
B. Drain pipe from trap. ~15" long.
C. Drain pipe continues from B, it contains the vent F (#2 and #3 show close ups). C runs about 7', with the vent entering approximately the centre at 3'6".
D. Drain pipe continues from C. (Partially hidden by duct work in #1).
E. Drain pipe continues from D and exits the house through the basement floor.
F. The vent, connects to the drain pipe C and runs to the wall, then up to and out the roof of the house.

Current, builder configuration

Photo 1 tries to encompass the entire system (that's below the floor, obviously the actual sink is above).
Photo 2 shows a close up of the vent connection to the drain pipe.
Photo 3 attempts to show the distance the vent is placed in the pipe. It's really awkward since it's actually above duct work.
Photo 4 shows the edge of the wall where the drain pipe C and vent F actually pass.

I included photo 4 because I'm curious why they didn't just connect the vent to C there, why run it 3 and half feet through duct work?

The issue I have, is that I'm remodelling the kitchen, and the new island with sink is in a slightly different place. The new drain needs to be about 36" to the left of the current location. I can't put the drain there because there's more duct work. I can move it about 24" to the left.

My plan is put the drain as far left as I can and put it through the floor there. Then use an elbow or 45 to connect the sink to that drain. No trap above the floor, just as direct a route as I can make it.

For the under the floor portion, I'm thinking of two options, and I'm curious which would be the safest/most efficient option.

Option Fred: Cut B and extend it through one joist to the left, add a new trap and drain connected to it there. This means the plumbing above the floor in the cabinet will be need to be longer to reach, but should require no changes to the venting. (I think).

Option Barney: B can't extend far enough to the left due to ducts, so instead, add a new drain and trap even further to the left than Fred, but have it run a long pipe (~7') to the wall, turn, and then connect into E (using a sanitary T below the current elbow visible in photo #1). Then cut C and put a cap on it. The vent will then sort of be a huge U running from the wall, 3.5 feet along the ceiling, and back again, then down to E. Perhaps it would be better to connect F directly to D and cut C/B/A out altogether. If the new drain connects to E with a sanitary T, it should be safe and effective right?

If you have a better idea than what I'm proposing, I'm all ears. I had no idea plumbing was this complicated.

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  • What plumbing code is adopted in your local area having jurisdiction (LAHJ) - in many, trap under the floor is a violation. Trap more than 18" below the fixture may also be a violation, depending on the code in use. Island venting is generally complex.
    – Ecnerwal
    Sep 6 at 0:15
  • @ecnerwal Thanks. I’m in Ontario, Canada. The previous sink had the trap under the floor. The house was built in 2007. Would banning a trap below the floor be a new rule? Though I’m inclined to put it above the floor in any case if it’s a violation of code anywhere, since there’s likely a good reason for that. I’m now planning to build a bow vent.
    – Dave Wood
    Sep 6 at 2:05
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I'm located in a different province but I believe that both of our provincial codes are based on the National Plumbing Code and are similar.

Why they didn't just connect the vent to C there, why run it 3 and half feet through duct work?

Most likely due to grade. 3" and smaller drains are required to grade at 1/4" per foot. The total fall between the p-trap and the vent connection cannot exceed that of the diameter of the pipe. So a 1-1/2" p-trap can only be 6' from the vent connection.

Regarding the p-trap below the floor: You are most likely allowed a vertical distance of 1200mm (~4') from the underside of the sink to the top of the p-trap outlet.

I see that you commented that you plan to install a bow vent, which would defiantly meet code however, your code may also allow the use of an air admittance valve in this scenario.

In this application, you can cap the existing vent line. Install the p-trap within the sink's cabinet and install the air admittance valve as high as posable with the cabinet space.

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