I came across this on an 11 month inspection. The furnace exhaust flue was obviously pointed in an upward direction. I saw this on a few of the homes in the neighborhood. Not knowing or finding any type of code violation in the IRC 2003, I concluded (through use of common sense) this arrangement was just plain ole wrong.

Red arrow depicts current level of flue pipe, Yellow line depicts where it should be (according to me) and Blue arrows depicts where water can enter. The water heater flue pipe next to it (behind it in pic, hard to see, but there), had a 45 degree elbow on it. I told client I’ll write it up, but if builder says: “Hey, there’s noooo problem, it’s good to go”, I’d still put a 45 degree elbow on it (as the water heater has). This will prevent water from enter the pipe, and running down into the furnace, ….. He agreed and said it’s a good idea. Is it taboo to put an elbow on a furnace exhaust flue ?

Any comments or code references are greatly appreciated.

Attached Images

Furnace Exhaust.JPG (109.9 KB, 53 views)

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Robert
Roberts Home Inspection Service

Robert - Most installation specs I've seen call for the exhaust to slope back to the furnace. There should be a condensate pump or other means of dealing with the condensate inside. Having said that, I've never seen one slope that much. Try finding the install specs online. For instance, in the specs I have for a Nordyne condensing furnace it says: "Vent piping must be sloped upwards not
less than 1/4” per foot in the direction from the furnace to the terminal."

John, …. Thanks for the reply. After hearing most install spec’s require the slope back to the furnace, … it still makes no sense to me. I understand the slope up to the terminating point, as that applies to any gas combusting furnace. It’s enough the furnace has to deal with its own condensate, but to deal with rain water as well ?? With heavy enough rains, that could be quite a bit of water. That flue pipe could fill up rather quickly!! I’ve always seen them horizontal. Good idea, …. I’ll certainly look it up online to see what Carrier has to say about it.

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Robert
Roberts Home Inspection Service

I believe the main reason you want a condensing furnace vent to slope back to the furnace is that the condensate is acidic (unlike AC condensate) and you don't want it dumping just anywhere outside. It doesn't have anything to do with draft.