Video · Furnaces and Heating
Furnace PVC Vent Pipes: Designing a New Layout From the Old One
A high efficiency furnace vents through PVC because its exhaust comes out cool and wet, not hot. That lets the old metal flue go, but the new PVC pipes need their own path to the outside, a steady slope and a condensate drain.
Job footageSwiftPro
- Topic
- Furnaces and Heating
- Length
- 1:40
- Posted
- July 19, 2026
- Shot by
- The SwiftPro crew, Northern Virginia
What you’re seeing in this video
Same visit, a different job on camera: documenting the existing system so a new one can be designed around it. The SwiftPro rep holds up a phone and photographs the furnace plenum, the sheet metal box on top of the furnace, then the trunk ductwork branching off it toward the rest of the house.
The furnace sits facing the wall with its metal flue pipe running out through the wall right beside it. The rep lays out the idea: rotate the furnace 180 degrees so it's easier to reach, rework the ductwork to match, and drop that old flue completely, because a high efficiency furnace won't use it. The inside work, they agree, is the easy part.
They step outside to look at the front of the house where the new PVC line would leave. Then the rep promises the homeowner he'll confirm every measurement and come back with a design that fits the house as it is.
We'll go look at the basement, measure that. Okay, so what I'd like to do is have this taken out Okay. 180° and have a high efficiency line go it out the front like the kind of the basically right out through the stuff here. And then we get rid of this completely cuz we wouldn't need it anymore with a high efficiency furnace.
Okay. Oh, they got this thing turned right towards the wall, don't they? BTU, 80%. All right, I'm going to see if I can get my install manager on the phone.
I want to talk to him. Hey [install manager]. Hey. His thought was basically to flip it around a little bit so it's a little more accessible, not, you know, tight.
You can kind of see there it's tucked turned right towards the wall with that pipe running right out here and then out this wall. So, his thought was redo the ductwork. Obviously, there'll be some changes with the ductwork here, turn the thing, then run the the new high efficiency flue pipe out like he was thinking the front. Yeah, that all looks good.
We just got to be 3 ft away from the window, dryer vent. Okay, let's go take a look. Yeah, I don't think you can be close to that gas meter, though. So, this is the side.
What do you think about that? I don't know if we can make the turn cuz the joists are running the wrong way. You're worried about the proximity to the gas meter? Yeah, I'm pretty sure it's 3 ft with that, too.
Yeah, let me just I mean, I would just call it in and let me just double-check the plans on that gas pipe. There's that sub board in that joist space right there. I will. I will.
And I'll confirm measurements. But other than that, you don't really see any issues, right? Like from what you saw there. We should be able to just reconfigure ductwork, make it work.
Yeah, inside's easy. Yep. All right, thanks.
Why it matters in your house
- Most furnaces rated 90% and up use two PVC pipes: one brings outside air in for combustion, the other carries exhaust out. That sealed combustion means the furnace stops pulling heated basement air up a flue and outdoors.
- Retiring a metal flue has a side effect. If a gas water heater shared it, the flue may be too big for the water heater alone, so the chimney needs a liner or the water heater needs its own vent.
- Photos and measurements of your plenum and ductwork are what make an estimate accurate. If a quote skips them, you're more likely to see surprise sheet metal work show up on install day, along with the bill for it.
Why do high efficiency furnaces use PVC vent pipe instead of metal?
Because the exhaust is cool. A condensing furnace pulls so much heat out of the flue gases that they leave cool enough for plastic pipe, often well under 140 degrees, and water condenses inside the pipe. That slightly acidic water would corrode a metal flue over time, and PVC handles it fine. The condensate also needs its own drain line or a small pump to reach a drain.
Want it done at your place? See Furnaces and Heating at SwiftPro or call (703) 997-9757.
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