Can I run a PVC plumbing vent through my 150-year-old masonry chimney?

Running a PVC vent through an active chimney stack is a major code violation and a massive safety risk. Even if the chimney looks cavernous, most jurisdictions follow the rule of one appliance per flue. Mixing sewer gases from a plumbing vent with the carbon monoxide and heat from your water heater exhaust is a recipe for disaster. The heat alone is a problem; PVC starts to soften at about 140 degrees Fahrenheit, and flue gases can get much hotter than that, especially during a long cycle. You woud risk the pipe sagging, melting, or releasing toxic fumes into the house. If you were dead set on using that path, you would have to install a dedicated B-vent or a stainless steel liner for the water heater first, and even then, most inspectors won't allow a combustible plastic pipe to share that same masonry cavity. It is considered a fire spread risk. If the chimney ever had a chimney fire or even just excessive heat transfer, that PVC is going to fail imediately.

Instead of tearing up your Victorian plaster, look into an Air Admittance Valve (AAV), often called a **Studor vent**. Most modern codes allow them for basement bathrooms as long as they are accessible for maintanence and you have at least one main stack going through the roof elsewhere in the system. You can usally hide it inside a vanity cabinet or behind a small access panel in the wall. Just make sure it is installed at least 4 inches above the trap weir of the fixture it is serving. Also, while you are looking at that chimney, check if it is actualy lined. Those old masonry stacks were not built for the moisture produced by modern natural gas appliances. The condensation from the water heater will eventually eat the mortar joints from the inside out. If you are already doing work, getting a stainless liner in there for the heater is a bigger priority for the safety of the house than the bathroom vent.

I’ve been reading up on those Studor vents since you mentioned them, but I’m worried about the longevity. Do those rubber seals inside them ever dry out and let smells into the house? I really want to avoid cutting into my walls, but I don't want a stinky basement either.

Also, just curious—if heat is the main problem with the PVC, whould using a metal pipe like cast iron or copper for that vent run make it legal to use the chimney? Or is the 'one appliance per flue' thing a hard rule no matter what the pipe is made of?

Even with copper or cast iron, you still can't run a plumbing vent inside an active flue. It's not just the melting risk—any obstruction in that chimney can mess with the draft. If your water heater can't vent properly because that pipe is creating turbulence or taking up space, you’re looking at a carbon monoxide risk.

Regarding the AAVs, they're actualy pretty reliable. I've seen them last 20 years without a hitch. If the seal ever does go, you'll smell it, and it takes two minutes to unscrew the old one and pop in a replacement. Just don't bury it behind drywall without an access panel.

If you’re worried about the AAV failing, just remember they’re super easy to swap out. They literally just screw into a threaded adapter. As long as you keep it accessible—like under the sink—it’s a five-minute fix if you ever smell sewer gas down the line.

If you really can’t wrap your head around a mechanical vent, look at running the pipe out trough the rim joist and up the side of the house. I've seen guys tuck them behind a decorative downspout on these old Victorians. It saves the plaster and keeps the vent outside where it belongs.

If you go the rim joist route and run it up the exterior wall, check your local frost line requirements. In colder climates, moisture in that vent can freeze and bridge over the opening. I ussually upsize to 3 inches once I hit the exterior to prevent hoarfrost from clogging it.

Also, keep that terminal away from any windows. Most codes require the vent to be at least 10 feet away from or 2 feet above any operable window so you aren't sucking sewer gas back into the house. Since it's a Victorian, you can often tuck the pipe behind a downspout to keep it hidden.