Thread: Furnace Flue Questions

I had two furnaces on today's inspection, and now have a question on their flue set-ups. The first one, a 1987 Lennox (Model G1603/4-100-1), has an inducer fan, but they put an odd routing on the flue to get it to an existing (I'm guessing) B-vent. It's odd appearing,but I think it's probably alright.

The second one, a 2000 Lennox (Model G26Q3/4-100-5), is a high efficiency with a sealed combustion system. What threw me was that they've routed the intake so that it's piped into the exhaust stream. It looks altogether wrong to me, and I don't know how it's getting enough oxygen - but the flame was nice and blue like it ought to be. Any help would be appreciated. I got nothing from the Lennox website.

Photos of both are below. The third PVC pipe in the second photo is the exhaust from the water heater, which also had an inducer motor on it.

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Re the 2000 high efficiency furnace, what did the termination outside look like? There are vents that integrate the intake and exhaust, one inside the other, while keeping them separate.

It's called a concentric vent pipe. Did the outside termination look something like the top of this image?

Good morning: As a gas inspector I would not like to pass the vent configuration for the 1987 Lennox(that 180 degree turn certainly would make me want an annual inspection of the inducer casing & heat exchanger).If it had a natural draft water htr connected to it I would not pass it at all(that 180 config is the equivalent of abbout 11 ft straight pipe).As to the Lennox G26 thats called a concentric vent where the intake/exhaust join together & it was invented so as to only require 1 exit hole through building to outdoors for all the various reasons that may prove necessary.

Well, I'm learning something that will add to my experience. The external terminations looked alright when I was outside, so I didn't take any photos of that particular area. I don't recall anything like the illustration that JA posted (Thanks a heap!). If memory serves, the termination was just an open 90 deg. angle turned downward. I can call it suspect, and recommend further review by an HVAC tech. This furnace also has a very noisy inducer motor, so it's going to have to be checked out by one anyway.

Also, thanks PB for your insight in the older furnace's flue configuration. There is no water heater flue involved, but this looked strange enough that I wanted to ask about it. I thought it might be alright since an inducer fan was pushing the exhaust through it. There's no way I'd not comment on it if it was a naturally induced furnace. I'll probably mention that the tech should take a look at this one as well.

Thanks again for the help!