Can I pull a vertical vent off this horizontal drain line for a floor sink?

In Chicago, you can't just drop a tee into a horizontal line and run the vent straight up. That’s considered a flat vent below the flood level rim, and it’s a major no-go becasue waste can easily splash up and clog the vent opening over time. You need to roll that vent off at a 45-degree angle using a wye or a combo til you get at least 6 inches above the flood level rim of the floor sink. Since you're working with copper, you’ll want to use a DWV wye and a 45-degree street elbow to get your vertical rise started while staying out of the way of those old fittings.

As for the sizing, stick with the 2-inch for the vent. While some IPC-based codes allow you to drop down to 1.5 inches for a single fixture vent, the Chicago Plumbing Code is much more restrictive on commercial builds. If the drain is 2 inches, the inspector is going to expect a 2-inch vent. Reducing it might save you a little space in those cramped quarters, but it’s a fast way to fail an inspection and have to sweat out all that expensive copper again.

You also need to watch your **trap arm distance**. For a 2-inch line in Chicago, you’ve generaly got a 5-foot limit from the trap weir to the vent connection. If you’re pushing past that to reach the 4-inch hub, you'll need to move the vent closer to the sink. Since you're dealing with existing brass and cast iron, make sure you're using the right shielded couplings if you're transitioning materials. Chicago inspectors are sticklers for those heavy-duty bands on commercial jobs.

Don't try to cheat the vertical jump by using a sanitary tee on its back either. It’s tempting when space is tight, but a sanitary tee in a horizontal run is only for a vertical-to-horizontal transition of waste, not for pulling a vent off a drain. Stick to a wye or a combo and you will be fine.

Reminds me of working the old loop vents back in the day. If you're messing with that old brass, watch your torch. Overheat those old threads trying to sweat a new adapter in, and you'll be chasing pinhole leaks for a week. I’ve seen kids try to use a regular rubber boot on a commercial line, but in the city, you better have those heavy-duty Mission bands or don't even bother calling for an inspection. Back when I started, we'd have just poured a lead joint and been done with it. Just keep your flame away from any old horsehair or insulation if it’s still hanging around—that stuff is a headache.

I had to squeeze a 2-inch copper vent past some ancient cast iron last spring. One thing that helped me was using a heat shield blanket instead of just winging it with the torch. In those tight Chicago commercial ceilings, it's way too easy to scorch a joist or pop an old joint you aren't even working on. Also, make sure your hangers are rock solid before you solder. Copper is heavy, and if the pipe sags while the joint is cooling, you're gonna have a hairline leak that’s a nightmare to find later.