Thread: Furnace Vent Flue Too Long?

Came across a vent flue from a furnace today that ran almost horizontal for I'd say close to 20 feet, then Y'd into a combo flue with the water heater and out the roof. The furnace was in a section of the crawlspace that punched through a wall into another section of the crawlspace / basement area. Just a really long run for a pipe. I can not find anything in my Code Check that says how long a flue pipe can be without a power assist. Especially running close to horizontal. Any help would be appreciated. Thanks in advance.
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Originally Posted by Steve C.

Came across a vent flue from a furnace today that ran almost horizontal for I'd say close to 20 feet, then Y'd into a combo flue with the water heater and out the roof. The furnace was in a section of the crawlspace that punched through a wall into another section of the crawlspace / basement area. Just a really long run for a pipe. I can not find anything in my Code Check that says how long a flue pipe can be without a power assist. Especially running close to horizontal. Any help would be appreciated. Thanks in advance.
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Well, a connector cannot exceed 75% of the total vertical vent and the connector must slope up 1/4" per ft for starters. Then there is the support for this vent, which seem inadequate from here. Any more pics?

no that's it for pics. I deferred it out to an HVAC tech to confirm. You know, CYA insurance. I don't propose to be an expert on everything like some inspectors do. Thanks.
Steve

It does not look like it rises 5 inches from end to end. Were you able to check for spillage?

Originally Posted by Gary DeWitt

It does not look like it rises 5 inches from end to end. Were you able to check for spillage?

Neither end is pictured...

the run is about 20' long. my question originally was; is it too long. the answers so far dictate that it may be. i was also looking for something in my code check book, which i could not find. thanks for all the answers though gentlemen.

steve

Originally Posted by lou curley

Neither end is pictured...

Based on the length given by the OP and the apparent slope.

If you're under the IRC, reference is M1803.3, Page 498 2009 edition.
You don't give the height of the flue, so you'll have to calculate or guestimate the horizontal vs. vertical run.
Adequate combustion air is a concern for me when inspecting gas appliances, especially more than one in the same room.

From Dura Vent, "Minimize the number of turns and lateral runs, as the National Fuel Gas Code places limitations on these."

I guess you will need to look to the NFGC, the size of the gas appliance, etc.

Originally Posted by Bob Harper

Well, a connector cannot exceed 75% of the total vertical vent and the connector must slope up 1/4" per ft for starters. Then there is the support for this vent, which seem inadequate from here. Any more pics?

Originally Posted by Steve C.

the run is about 20' long. my question originally was; is it too long. the answers so far dictate that it may be. i was also looking for something in my code check book, which i could not find. thanks for all the answers though gentlemen.

steve

That vent connector looks like it is Type B gas Vent being used as a connector, Bob H. included the maximum offset (horizontal run) for a single wall connector, which does not matter that it was single wall as the Type B Gas Vent "connector" "and" "vent" combined have a maximum horizontal offset of 75% of the height from the draft hood to the type b gas vent cap outlet.

From the 2009 IRC: (bold and underlining are mine)
- 503.6.9.2 Vent offsets. Type B and L vents sized in accordance with Item 2 or 3 of Section 503.6.9.1 shall extend in a generally vertical direction with offsets not exceeding 45 degrees (0.79 rad), except that a vent system having not more than one 60-degree (1.04 rad) offset shall be permitted. Any angle greater than 45 degrees (0.79 rad) from the vertical is considered horizontal. The total horizontal distance of a vent plus the horizontal vent connector serving draft hood-equipped appliances shall be not greater than 75 percent of the vertical height of the vent.

The overall height has not been stated, however, a 20 foot offset would require a minimum height of 26.67 feet above the draft hood to the type be gas vent cap outlet. Unless that is running up through a two story house or the single story house has a very high roof, 27 feet above the draft hood puts the vent cap at about 32 feet high, and the roof is likely only about 15 feet high ... which leaves 17 feet sticking up above the roof - YIKES! If the draft hood is in the basement, the height reduces by about 8 feet, which would still leave about 9 feet sticking up above the roof on a single story house, and that vent would need to be braced above the roof at that height above the roof.

Now, if that was a basement in a two story house, the overall height would be close - providing there is no additional horizontal offset.

The key, though, is what Bob H. also pointed out: "must slope up 1/4" per ft for starters", looks to me like it fails that first test right there.

Had one yesterday in the basement of a restaurant, 25 year old standing pilot Ruud, 44 feet of flue pipe, no pitch rust, etc.
Owner said it hasn't been a problem in the last 26 years, its fine.
Of course it was fine. Between all the wall and window drafts and open sewer connections the flue pipe wasn't that big of a deal.