direct vent exhaust vent sizing question

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New Member
Jan 7, 2023
Boston, MA
Hi Experts - I am replacing a 20 year old Fireplace Xtrodinaire (Travis Industries) direct vent NG insert with a new insert (choice TBD). The current insert has a co-linear vent (4" exhaust, 3" intake) that is vertically run up a 2 story chimney. It seems most newer DV inserts have 2 x 3" colinear vents. I'm hoping I can just use a metal 4" to 3" reducer to solve this mismatch. My thinking is that there should be no harm in having a larger exhaust run (4") up the chimney. I could understand why one would never want to vent a 4" exhaust into a 3" vent pipe (as one might constrict the flow). However, I can't see anything with venting a 3" exhaust into a 4" pipe. I'm trying to avoid having to run a new set of vent piping for the new insert. Thanks in advance for any opinions.
You can try using a 3” to 4” increased in your exhaust, & it may work fine, but I suggest you run a short section of 3” liner up inside the existing 4”, maybe 4’ long. Getting a draft flowing is easier in a smaller diameter. If it doesn’t perform correctly, then you’ll have to run a new 3” on the exhaust side. Check with your local hearth shop. They may have some short pieces leftover from their installs.
Thank you for the great suggestion (I did not consider the differences in draft for 3" vs. 4"). Since it's a straight run from the bottom of the 4" vent to the top of the chimney, I can probably get several feet of 3" vent pipe up there. Just curious, what would be a sign of poor performance because if insufficient drafting? Bad color or behavior on the flame?
If it doesn’t draft correctly, it won’t pull in enough combustion air. It just won’t burn right. It might not work at all.