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Old 02-11-2007, 11:44 AM   #1
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Furnace venting questions


I'm working on the installation of a Reznor 75K BTU hanging furnace in my garage. I'm trying to figure out the best way to deal with the venting. It's 80+% efficient and vents through 4" pipe.

I was originally thinking of venting horizontally (it has a blower for the venting), but the rule about the flue being 3' below the soffit seems to guarantee an ugly installation. I guess I'd have to come out near the soffit and make a 90 degree bend down to meet the rule. Am I understanding correctly? With an 8' ceiling, the flue would be only 5' above the ground where people could bang their heads on it too.

The other choice is to go up. My ceiling is corrugated steel with insulation above. What is the correct way to run the vent pipe through the insulation? Do I just use double wall vent pipe, or a thimble like for a wall?

Finally, is it kosher to use bends in the attic? I'd like to install the furnace pretty close to the corner of the garage, but it will be extremely tight to work there. I'm wondering about the option of getting up into the attic, then running horizontally for 8' or so to get closer to the peak of the roof where I can see what I'm doing.

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Old 02-12-2007, 01:57 PM   #2
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Furnace venting questions


Never vent a furnace down. The heat in the exhaust rises, and will not go outside, it will vent it self some other way, like inside the garage.

You can use bends in the attic, as long as the pipe is sloped up at 1/4 inch per foot of pipe. So in your case, with 8' of pipe, the side by the outdoor exhaust vent should be at least 2" higher than the furnace side.

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Old 02-14-2007, 06:45 PM   #3
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Furnace venting questions


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Never vent a furnace down. The heat in the exhaust rises, and will not go outside, it will vent it self some other way, like inside the garage.
This heater has a blower for the exhust. Are you sure going down isn't okay in that case?

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You can use bends in the attic, as long as the pipe is sloped up at 1/4 inch per foot of pipe. So in your case, with 8' of pipe, the side by the outdoor exhaust vent should be at least 2" higher than the furnace side.
Okay, thanks. What is the correct way to get the pipe through the insulation layer safely? Do I need a sheet metal box to prevent the insulation from touching the pipe, or is double wall pipe alone acceptable?
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Old 02-14-2007, 10:56 PM   #4
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Furnace venting questions


You can't go down. In an 80% furnace, the exhaust blower pulls the flames and exhaust through the heat exchangers. After that, the draft pulls the exhaust through the pipe as long as it slopes up because heat rises.
You will have to check local codes to see how to run the pipe through insulation, but I have seen exhaust pipes with high temperature insulation wrapped around them and then a layer of plastic in attics. With this wrapping on it, it should be okay to run it through the other insulation. Also you should probably use double wall pipe in the attic.
Make sure you hang the pipe from the attic ceiling to support it. How often you need do this will be in the code or the installation manual.
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Old 02-15-2007, 07:27 AM   #5
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Furnace venting questions


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You can't go down. In an 80% furnace, the exhaust blower pulls the flames and exhaust through the heat exchangers. After that, the draft pulls the exhaust through the pipe as long as it slopes up because heat rises.
Thanks. It seems kind of weird then, because this is a hanging garage heater, yet it seems impossible to utilize the horizontal vent capability if you have any sort of soffit on the garage at all. The rule is to exit 3' below the soffit, which will always be at ceiling height, which will be where the heater is hanging.

The only way around it seems to be to have a very high ceiling and just hang the heater way down, or to somehow get the pipe out the peak side of the garage and be 3' away from all soffits, or maybe to go out sideways and then up above the roof anyway...

Straight up through the ceiling appears to be the least "hack job" way to do this.
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Old 12-05-2012, 01:05 PM   #6
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Furnace venting questions


Go through the roof, but do it correctly. If you need a detail reference NRCA steep metal roof, or maybe SMACNA Manual. NEVER TURN A FLUE STACK DOWN. NEVER. You could run the flue pipe uphill horizontal indoors to gain heat from the flue pipe until you hit a point where you can work in the attic, but never have the flue pipe touch any insulation. Always have an air apace of at least 2 inches, but check your codes. Your heater should have all this info, especially Reznor, they have been around forever.
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Old 12-05-2012, 01:13 PM   #7
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Furnace venting questions


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Originally Posted by jagans View Post
Go through the roof, but do it correctly. If you need a detail reference NRCA steep metal roof, or maybe SMACNA Manual. NEVER TURN A FLUE STACK DOWN. NEVER. You could run the flue pipe uphill horizontal indoors to gain heat from the flue pipe until you hit a point where you can work in the attic, but never have the flue pipe touch any insulation. Always have an air apace of at least 2 inches, but check your codes. Your heater should have all this info, especially Reznor, they have been around forever.
This thread is almost 6 years old......he's probably 6 feet under by now.
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Old 12-05-2012, 03:39 PM   #8
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Furnace venting questions


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This thread is almost 6 years old......he's probably 6 feet under by now.
If not I'm sure he has had his toes amputated because of frost bite. The only heat that I can think of that's worse than a heater hanging high is that electric ceiling grid that was installed in some houses in the 1970s.
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Old 12-05-2012, 03:47 PM   #9
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Furnace venting questions


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If not I'm sure he has had his toes amputated because of frost bite. The only heat that I can think of that's worse than a heater hanging high is that electric ceiling grid that was installed in some houses in the 1970s.
I rented an apartment in the late 80s that had that system. the landlord was adamant about NEVER DRILL IN THE CEILING or BANG THE CEILING. It worked very well and heated the place just fine even in -20F temps.
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Old 12-05-2012, 05:51 PM   #10
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Furnace venting questions


Thats because it was radiant heat, not FHA. That kind of heat is really nice when it is run through your floor slab. Nice in the morning when your feet hit the floor. Just hope it dosent break

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