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Old 12-25-2012, 12:21 PM   #1
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Furnace flue pipe goes up and then down


I was under the house dealing with a plumbing issue and I noticed something that didn't look right to me. The home has a crawl space with dirt floor and low clearance. Suspended from the rafters is a Goodman 95% high efficiency furnace. I guess it was installed by the pervious owner and looks fairly new.

The flue pipe looks to be two inch PVC. It rises about three inches over and eight or ten foot run to make it over a beam and then drops about six to eight inches over a ten foot run so it can exit through the skirting.*

I'm not familiar with these high efficiency furnaces. I've never seen PVC used as a flue pipe and I've never seen a flue pipe go up and then down. Should I be concerned?

Greg

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Old 12-25-2012, 12:32 PM   #2
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Furnace flue pipe goes up and then down


http://www.heatinghelp.com/forum-thr...for-flue-gases

~ POSTED ON ANOTHER THREAD ~ Says it better and with more clarity than I could sit here and write. Plenty of info here for you to consider. ~

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Old 12-25-2012, 01:01 PM   #3
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Furnace flue pipe goes up and then down


Janice,

Thank you for the link, but that rather long article does not answer my question and is not really relevant. That is a large commercial installation of water heaters not a residential installation of a furnace. Yes, they both used PVC, but that really does not matter. All it really tells me is that PVC can be used in some instances for flue gas. Not all that helpful.

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Old 12-25-2012, 01:08 PM   #4
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Furnace flue pipe goes up and then down


High efficiency furnaces (90+%) use PVC for exhausting, and are required to be sloped toward the furnace.

How much of a dip (angle down and length) do you have? Could be a problem.

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Old 12-25-2012, 01:29 PM   #5
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Furnace flue pipe goes up and then down


In my area a PVC flue can slope down but only if:

1. The slope is towards the furnace
2. It slopes in to an in line condensate trap which then drains into an approved drain
3. It slopes to the outside to that the condensate will pour outside from the end of the pipe
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Old 12-25-2012, 01:35 PM   #6
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Furnace flue pipe goes up and then down


It slopes back towards the furnace for the first ten feet but ends up in the joist bay to get over the 6x6 horizontal beam that the joists sit on. On other side of the beam the pipe is in the joist bay. Then there two 45 degree elbows to bring the pipe back below the joists and out through the skirting. So the pipe goes up three inches as it travels away from the furnace and then down six or eight inches towards the skirting. Not so much a dip, but a hill.

I've been trying to read about this type of furnace and it seems the slope back to the furnace is so the condensation can be drained. The drain pipe (1/2 inch PVC) feeds into a small pump that pumps the water out the same side of the house but several feet away from the flue pipe.

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Old 12-25-2012, 01:43 PM   #7
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Furnace flue pipe goes up and then down


The installation manual should tell you specifics but as long as the slope that goes outside terminates in a way that the water can properly drain out it sounds like it may be done properly
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Old 12-25-2012, 01:52 PM   #8
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Furnace flue pipe goes up and then down


Thanks. There is some documentation in a metal pocket on the side of the furnace. Maybe there is a manufacturer support number.

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Old 12-25-2012, 02:01 PM   #9
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Furnace flue pipe goes up and then down


Hopefully. But to me the "hill" in the flue pipe slopes down into two allowed locations ( outside and into the furnace) however it is allowed in my local codes. You should also check your local code and what the manufacturer recommends. Some manufacturers don't want the flue pipe sloping down at all other than towards the furnace. Others say its ok to even pipe the flue straight down and then to the side. It all depends
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Old 12-25-2012, 02:26 PM   #10
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Furnace flue pipe goes up and then down


The half and half sloping to and from the furnace to the outside can also be a problem for vent termination icing in freezing weather and nuisance PS lockouts.
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Old 12-25-2012, 04:01 PM   #11
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Furnace flue pipe goes up and then down


How,

I'm a temperate climate, so freezing is not an issue. What do you mean by PS lockouts?

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Old 12-25-2012, 05:25 PM   #12
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Furnace flue pipe goes up and then down


If your areas temp never drops below 32, you probably will never have a problem with it. Its not within code for that furnace to be ran that way. but, in reality, it won't hurt in mild climate areas.
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Old 12-25-2012, 05:34 PM   #13
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Furnace flue pipe goes up and then down


A "hill" will not usually cause an issue. A "valley" on the other hand is bad news. PS lockout = Pressure Switch lockout. You furnace will not fire because it doesn't sense correct intake/exhaust flow,
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Old 12-25-2012, 05:39 PM   #14
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Furnace flue pipe goes up and then down


Quote:
Originally Posted by chitownken View Post
A "hill" will not usually cause an issue. A "valley" on the other hand is bad news. PS lockout = Pressure Switch lockout. You furnace will not fire because it doesn't sense correct intake/exhaust flow,
LOL, I like your Avatar.
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Old 12-26-2012, 02:44 AM   #15
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Furnace flue pipe goes up and then down


In freezing conditions, steady dripping from the vent can freeze and start to restrict the vent air flow. The furnace has a pressure switch which measures the venting air flow and will turn off your furnace if such a thing happens..

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