Is this a good way to plumb a garage sink and stop existing sewer gas smells?

Question

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?

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?

Answer 1

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.

Answer 2

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.