PVC Vent Through Chimney Flue

bluegrassguySeptember 4, 2008

In looking at ventilation options when upgrading our old furnace with a 95% high-efficiency model, the most straightforward path for the PVC venting is through the nearby flue which is dedicated to the current furnace exhaust. Am having trouble visualizing what the PVC ventingt would look like coming out of the flue - the flue is one of 4 flues together in the center of the (1-story) roof which all have matching black caps on them. Is it going to look really tacky, lol? Are there caps that can cover the PVC?


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I'm not an HVAC guy, but I had thought that the exhaust for the high-eff wasn't supposed to be so long.

Anyway, you can paint PVC. And I don't think it would stick up inordinately.

    Bookmark   September 4, 2008 at 2:23PM
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Well, good question - both contractors I've had indicated it's probably the best option. I've tried searching for info on maximum vertical length, but haven't found anything yet. Maybe someone else on here knows.

    Bookmark   September 4, 2008 at 3:28PM
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You will find the venting table in the install manual.
Usually stated as max distance with 4 elbows, every 45 is worth 2.5' and a 90 is 5' deductions from max length.

It is rare today to see a furnace with venting max less than 60'

Max is vertical or horiz, only concern is a continuous pitch back to appliance

    Bookmark   September 4, 2008 at 5:16PM
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I'm doing the same thing in the install of the Lennox G71P from my current 80% Tempstar. The install crew will remove the existing ducting and install 2-3" PVC's inthe chase. They connect to a concentric stack for the roof, meaning they are merged together to a special exhaust stack. I believe the Lennox part #71M80 and is called a 1.5" concentric vent kit. http://tech.lennoxintl.com/C03e7o14l/RAfFXCJCv/ehb_g43uf_0611.pdf some photos pn page 11

I will actually have 2-of these because I am installing a AO Smith Vertex 100 (96%) which needs the same.

    Bookmark   September 5, 2008 at 2:20AM
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To answer your last question, there is a cap that will cover PVC pipes that look good.

    Bookmark   November 3, 2008 at 11:02PM
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Restarting this discussion- does anyone have photos or additional info about venting 95% efficiency gas furnaces through a chimney flue?

Got 2 bids for high eff furnaces. Both recommended Lennox G61MPV. One company said furnace will be vented via PVC out front wall of (brick) house- horizontal distance of about 10 feet. Would exit below hall window near front door.

Another recommended venting through existing chimney flue. Chimney service just put steel cap on fireplace flue and confirmed there is room on furnace flue for 2 PVC pipes, but questioned why vent is vertical rather than horizontal. Also said PVC out chimney would be ugly on old house.

Can anyone comment on venting through chimney versus wall?

    Bookmark   August 18, 2009 at 3:22PM
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you said you had 4 cap for 4 flues, then later you said that one contractor was wanting to run two 2" pvc pipes up the old flue. If any of those other flues are putting out gas fumes they could go down the intake pvc pipe to your 90+
furnace and that could stop your furnace. That intake pvc pipe has to get clean air. Later paulbm

    Bookmark   August 18, 2009 at 10:16PM
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