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Old 11-09-2011, 06:40 PM   #1
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venting sizing


i have purchased 120000btu furn and exhaust and intake call for 3" pvc from 70' to 100'. my question is can the vent size be reduced to 2" if total lenght of pipe is 14' with three elbows the friction would be greatly reduced is there a mathematical caulculation available for this

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Old 11-09-2011, 06:50 PM   #2
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furnace calls for 3" discharge.....cannot reduce it.....should not reduce it.....

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Old 11-09-2011, 07:38 PM   #3
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Yep. Velocity of the air thru it will change and you can get pressure switch tripping problems or dangerous flame liftoff from the burners or they won't light properly. Gas inspector won't allow it either. In the install manual they should give you a minimum length of pipe (equivalency) which needs to be followed as well. May have to add extra elbows to get that. Usually around 15' minimum equivalent pipe. Short radius elbows are twice as restrictive as long radius I believe.
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Old 11-10-2011, 04:33 AM   #4
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There are some brands that allow short runs of 2" vent for that size furnace. York/Hamilton is one. For example their TM9V (2 stage variable speed) models allow a maximum run of 30 equivalent feet of 2" pipe for the 120000 size. Some other brands/models may allow similar. Equivalent means you need to add extra "feet" for elbows. A regular 90 degree elbow adds 7 feet, a sweep 90 degree elbow adds 5 feet, 45 degree reg/sweep elbows add 3 1/2 and 2 1/2 respectively. So if it is very critical to use 2" you might try to return your furnace and get a brand that allows the 2".

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Old 11-10-2011, 05:45 AM   #5
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The trend I am seeing is that with almost all units going to be a minimum 95% efficient VERY soon they are going to 3" venting because to get a proper flame and draft and lighting it works better or is necessary. Rheem and Lennox are going that route and units as small as 70,000 BTU are using 3". The design engineers are designing more efficient burners and need more air it seems to accomplish that. 3" has a huge advantage in that it is a LOT less prone to plugging with snow/ice /debris and the kids can drop golf balls and small toys in the vent w/o plugging it as easy. 2" and you are hooped.
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Old 11-10-2011, 05:58 AM   #6
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Yes, I noticed that trend, too. I was looking closely at Rheem when I was looking for a 80000 size furnace but the 3" vent was problematic for my install. The 96% efficient York unit with about 30 equivalent feet (65 feet max spec) of 2" fit a lot better and works great so far. Bay vent keeps golf balls and hopefully birds and such out!
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Old 11-10-2011, 06:06 AM   #7
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Quote:
Originally Posted by roydon View Post
i have purchased 120000btu furn and exhaust and intake call for 3" pvc from 70' to 100'. my question is can the vent size be reduced to 2" if total lenght of pipe is 14' with three elbows the friction would be greatly reduced is there a mathematical caulculation available for this
Are you sure you need 120K furnace? what was in there before?
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Old 11-10-2011, 06:06 AM   #8
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I am a Lennox tech but am looking to put a basic Rheem in my cash strapped Sisters house next Spring. Probably going to have to work with the 3". I really like the Bayvent and plan to use it for her. Lennox now has their version of it and I imagine everyone will have one soon. Looks nice and flush and cosmetic rather than elephant ears the other way. The newer units are getting more install friendly as some brands allow venting out 2 different walls and taking the supply from a crawlspace or attic (if properly vented) and exhausting up the old chimney. B4 we had to stay in the same pressure zone and no more than 12" apart.
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Old 11-10-2011, 06:51 AM   #9
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The York was very installer friendly. I think it allows the vent/supply options you mention. Also, it did not require an external condensate trap. And the inducer rotates to accomodate venting out either side w/o needing to mess with elbows in the cabinet. Only thing about this I didn't like was that with the inducer in position to vent out the right side there was very little vertical difference from the vent collar drain to the nipple that connects this to the internal condensate management system. But it works so I guess I can't complain.
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Old 11-10-2011, 04:14 PM   #10
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Make sure you put an open tee in the drain line after the trap and drain the AC after the furnace into that line ideally. Tee allows it to siphon better and not airlock and you don't want AC water accidentally getting into the furnace. Some brands are very picky about that setup. I like their clear collector box.

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