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Heating & Air Conditioning

PVC Vent Through Chimney Flue

15 years ago

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?

Thanks

Featured Answer

heimert

15 years ago

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.

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Comments (7)

bluegrassguy
Original Author
15 years ago
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.
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Boxerpups, I am thrilled to hear from you. I so appreciate your taking the time to research pics for me. It is MUCH more noticeable than the Broan pic you shared. I would be very happy if it were to look like any of the samples you posted. I'm also glad for the encouragement that it's possible to have the two pieces smooth and flat -- I swear talking to the contractor and carpenters made me feel like I was going crazy... as if my expectations were outrageously high. They insist that the pieces aren't manufactured properly, whereas the tech support team said they have received zero complaints of this type about this vent, with the caveat that the pieces are very easily bent. I don't know who/what to believe. Ahhh... this too shall pass. And, at some point it will all be done to my satisfaction.
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The general rule for gas stoves requires an exhaust fan capable of 100CFM per 10,000 BTU. For your stove, I bet you're looking at 700-1000 CFM. Plus it has to be capable of exhausting 20 feet up. That is a high-performance exhaust that requires a 10" diameter flue. If the existing flue is smaller, don't even consider using it. All these specs should be available from the stove manufacturer and fan supplier. Don't take the contractor's word on these requirements. You're not just exhausting grease, but CO. High-end stoves require high-end exhausts for reasons other than looks. Here is a link that might be useful: Rangehoods
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This is not a DIY job. Multiple gas burning appliances attached to the same flue can under certain scenarios cause the creation of CO (carbon monoxide a colorless, odorless gas that kills). These appliances do not create CO on their own. They create CO2 (carbon dioxide). Notice the similarity of these two substances CO - CO2 The difference that separates the two is 1 molecule of oxygen. Because these appliances burn gas, they need oxygen to burn. They will take oxygen from around the area in which they are installed. If there is a flue gas vent problem what do you suppose can happen? (Clearly there is a problem because it's dripping.) If this problem causes a drop in available oxygen in the air, you have the makings of a deadly combination. Because combustion air of these appliances will not be able to get the oxygen they need for full, complete combustion the result is exhaust that consists of carbon monoxide, the higher the PPM count the deadlier it is. If you can't afford a new furnace with a newly run flue to the outside separated from the water heater flue, save your money until you can do it right. Note: Newer higher efficient furnaces 90% or higher can be vented with PVC plastic run horizontally (They are called condensing furnace). You can not do this with your current furnace.
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zl700
15 years ago
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
im140.6
15 years ago
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.
jaf_industries_yahoo_com
15 years ago
To answer your last question, there is a cap that will cover PVC pipes that look good.
endlessfixing
14 years ago
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?
paulbm
14 years ago
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

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