direct vent exhaust vent sizing question

[wavejd]

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.

 

[DAKSY]

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.

 

[wavejd]

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?

 

[DAKSY]

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.