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I have 2 problems that I'm hoping can be fixed with one solution. (1) an exposed drain pipe from a previous sink that is giving out pretty foul smell (2) A tankless water heater condensate pipe that is not connected to any drain but simply discharges to the basement floor (since it is close to the floor drain)

Both of these are located close to each other.

I would like to install a ptrap to fix the foul smell from the drain pipe. It seems like thre isn't one under it. I plan to connect the tankless' condensate drain into this.

Due to space constraints, I want to avoid placing the p trap at the drain pipe, but do it a few feet away (and at a height), under the tankless. See pics for illustration.

Essentially the path would start with a p trap under the tankless, slope gradually towards the drain.

I want to ensure this p trap positioning (about 3.5 feet to the right of the drain and ~2 feet above the floor) would not cause new issues. I tried reading up about p traps and understand the principle, but want to be sure this install will not cause new problems for the existing drains and bathrooms on the floor.

Tankless condensate drain

Drain pipe from old sink

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I see no big problems with what you propose. Only thing that comes out from your sketch is the angle of the pipe from the P-Trap to the existing drain line. You will be unable to get PVC fittings that can give that angle exactly. Instead you can get fittings that turn 90 degrees, 45 degrees or 22.5 degrees. Your best bet is to run horizontal from the P-Trap through the studs and then angle down at 45 degrees using two fittings to the drain line. The horizontal part of the pipe would want to have a slight slope to it to keep that part of the line drained.

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  • Don't we need some kind of exterior vent or AAV here?
    – Huesmann
    Commented Jun 13 at 11:55
  • @Michael Karas thanks for sharing the tip about the fitting angles. Did not know that !
    – SamuraiJack
    Commented Jun 15 at 3:06
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With that off-set you will need an air admittance valve (or other vent connection) at the p-trap location to satisfy plumbing code. but with the low flow rate that's expected from a condensate drain it's probably not needed.

If the existing plumbing is metal you may also need an acid trap.

you may also want to drain the pressure relief into this p-trap.

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