The builders or previous owner of my house had the water that drains out of the A/C going through a 1" pipe that was simply sitting inside a 2" pipe (cut just 2" above the slab). A foul odor was emanating from the gap between the two pipes. After I discovered it, I was worried that merely sealing the connection between the pipes would only result in the gases and smell going up into the central A/C and furnace.

I've always wanted a garage sink, and I'm thinking why not kill two birds with one stone. I could use the trap from the sink to also isolate the A/C drain. The existing drain did not have a vent pipe, however there is one 3' away where the washing machine is connected.

The image below shows what I have in mind. Will this work?

schematic showing a proposed sink connected to existing drain and extension off existing vent of a different drain

Edit: Assume the S-shape is actually a poorly conceived P-trap. But how does the long tail of the P need to connect to the vent and drain, anyway?


2 Answers 2


Yes, that is a good way to accomplish all three goals: Provide a drain for the condensate, install a sink, and do it all properly with a trap and a vent. I have this: see photo below.

Some important notes on your salmon colored vent: You need some horizontal pipe between the trap and the connection to the drain. What you are building is actually a new drain stack. Your trap should connect to it with a wye and the vent should rise vertically from it, at least a little. If there are any upstairs fixtures draining into the main riser then it's a wet vent and your new connection should be with a wye pointing downwards. Any water that collects anywhere in your new pipes must drain away, there must be no low points that collect water. The details of this can't be shown in your simple diagram and they are behind the wall in my photo.

A note on condensate traps: Your condensate line should also have a trap just after it exits the air handler. Your new trap isolates the sewer line. The upper trap isolates the low-pressure chamber of your air handler from the environment and without it your new sink drain will act as a return air vent and your air handler might not drain properly. Here's much more on that topic.

Note from the photo, adapting the condensate line to the drain line is a bit fussy and cannot be done with standard lightweight under-sink polypropylene fittings. You can avoid all this by draining the A/C into the new sink, if you don't mind the ugly stains that will cause.

enter image description here

  • If the AC drains into the sink, that will also prevent negative pressure in the AC drain line from sucking out the sink's trap, as it could only suck air from the side of the sink. Unless we want a column of water to remain in the AC drain line, to weigh it down from sucking air continuously? I'm a bit confused which is the goal there. The pre-existing setup would have been the worst of both worlds depending on whether the sewer line the pipe was in was empty or full.
    – Dennis
    Dec 30, 2022 at 21:23
  • Referring to the photo in your other recent post your current situation is puzzling. I can't tell where the cooling air chamber is or where it's condensate pan is but if the silver hose is connected at one end directly to the condensate drain you have a crazy reverse trap that would cause about six inches of water to collect inside your A/C. I can explain more about the negative pressure problems (it's not strong enough to suck water out of a trap) but I'd like to understand the setup better.
    – jay613
    Dec 31, 2022 at 0:00
  • 1
    The six inches or so of incline in the silver hose is due to my having pulled the tall skinny PVC pipe it's connected to out of the floor. The silver hose had a slight decline as originally "installed" (using air quotes as many things in that closet are quite janky). I was in too much of a hurry to get the sewer-gas-spewing hole capped off and keep it thus to get a good picture of the original condition.
    – Dennis
    Dec 31, 2022 at 4:11
  • Ok @Dennis so you should replace that silver hose with a proper A/C trap like this, this, or this, The third one is transparent and has a cap so you can see if it's empty or dirty, fill it at the beginning of the season and clean it with the included brush. That REPLACES the silver hose. [ ... 1/2 ]
    – jay613
    Jan 1, 2023 at 21:33
  • [ 2/2 ... ] You then use PVC to above the trap as shown in the above photo, or you could use tubing or hose, and for the new sink's tail piece use on with a dishwasher connection and a hose clamp. NOTE .... the "open air" includes the sink drain, its tail piece, and the tubing BETWEEN the two traps. The new top trap has two important functions (prevent air intake from the garage and prevent air flow through the condensate line from interfering with drainage). The bottom trap plays the usual function of sealing the sewer.
    – jay613
    Jan 1, 2023 at 21:37

As I see it you have two options. The design in your drawing is an S trap which is against code, at least in the US.
Option 1:
You will need a P trap there which means running a horizontal drain out of the P trap with a 1/4" per foot slope and tapping into the washing machine drain line on the left. If you use a 1 1/2" P trap you'll be fine with a 36" run to the existing vent. Once you connect to the vertical washing machine drain with a sanitary tee you won't need to run the proposed vent. The existing vent should suffice.
Option 2:
If you want to use the drain under the sink you will have to have a minimum horizontal run after the P trap (assuming 1 1/2" pipe) of 3" (per code) before the proposed vent which can be tied into the drain with a sanitary tee.

enter image description here

  • I wasn't trying to imply an S-trap or P-trap in particular. It was just a limitation of drawing this in a Paint app, or a limitation of my imagination in how to draw "a" trap. However you do make a good point. In the P-trap, what happens to the right where the drawing ends? A tee into the vent and drain?
    – Dennis
    Dec 24, 2022 at 22:28
  • I'm in Oklahoma, US.
    – Dennis
    Dec 24, 2022 at 22:29
  • See my edited answer above.
    – HoneyDo
    Dec 24, 2022 at 23:22

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