Multiple flue vent connectors into a single chimney (C) Daniel Friedman Shared Chimney Flue Hazards & Repairs

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Shared chimney flues:

This article describes shared chimney flues, multiple heating appliances, fireplaces, woodstoves all using the same chimney venting path through a building and to outside. Shared flues can create fire and smoke spread hazards and carbon monoxide hazards in buildings.

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Here we outline how to detect shared chimney flues, how they are repaired, and we list the exceptions to the rule, showing when, in some jurisdictions, it is permitted to vent more than one heating appliance into a single chimney flue and when it is permitted to vent both oil and gas-fired appliances into the same flue.

Our photo (page top) shows three heating appliance, each connected by its own flue vent connector to a masonry block chimney. Is this OK? the answer is, it depends. On the chimney flue size and other installation details. Notice too that we are sharing the flue between oil and wood fired appliances.

These articles on chimneys and chimney safety provide detailed suggestions describing how to perform a thorough visual inspection of chimneys for safety and other defects. Chimney inspection methods and chimney repair methods are also discussed.

Shared Chimney & Shared Flue Hazards

Photograph of an unlined brick chimney flue.

A "shared chimney flue" means that the flue or pathway for venting combustion gases out of a building is shared among two or more things that need to be vented, such as a fireplace and a heating boiler. A "flue" refers to a sealed pathway out of the building to vent combustion gases, such as a metal flue pipe or clay flue tiles stacked up inside of a chimney.

A single chimney can safely have multiple flues inside the chimney, each flue venting an appliance, fireplace, or other device, provided that the chimney was properly constructed, including the necessary spacing or sealant between the individual flues.

Our photo at the very top of this page and repeated here shows an unlined chimney flue which, by luck of illumination from our flash camera, also showed us that a common flue was being used to vent two fireplaces - a fire code violation in many jurisdictions.

You can see that this chimney has two brick flues entering its common upper section. There is no flue divider in this chimney above the two flues entering in its bottom - shown in the center of our photo.

Dedicated chimney flues are required: Each device that produces combustion gases needs to have its own dedicated flue to vent combustion gases safely out of the building.

[Click to enlarge any image]

Unlined flue with missing divider (C) Carson Dunlop Associates Proper construction of chimney walls (C) Carson Dunlop Associates

Masonry flues may be too close (C) Daniel Friedman

Fire spread and combustion gas or carbon monoxide hazards: gases may be conducted from a heating appliance to another area in the building when the flue divider is missing.

For example, dangerous carbon monoxide from a basement gas fired heater or a coal stove may leak into an upper building floor through a fireplace or woodstove connected to the same flue.

As Carson Dunlop Associates [at REVIEWERS] sketches point out (above left), the missing divider from the chimney shown in our photo above and in the sketch at left may permit dangerous flue gases or carbon monoxide to enter the living area through a fireplace.

Proper construction of chimney walls in a modern masonry flue is shown at above right.

Our photo (above/left) shows that the clay flue tiles are just 1.5" apart (left flue pair) and about 2" apart (right flue pair) - too close according to the sketch above.

This is especially the case if elsewhere in the building a gas (or oil) fired heater is using the same chimney.

In the unfortunate fireplace shown below, we see that someone has "fixed" an unsafe shared chimney flue problem.

Shared chimney flue (C) Daniel Friedman

The fireplace flue was shared with the heating boiler in the basement.

To end this sharing the repairman installed clay flue tiles between the bottom of the fireplace and the upper fireplace flue.

This is no longer a working fireplace. Is it a safe flue?

Examples of Unsafe Shared Chimney Flues or Flue Vent Connectors

It was common for chimney flues to be shared in older buildings in the following ways (and probably others you can suggest to us). All of the examples in the following list are unsafe and should be corrected immediately.

Shared Flue Vent Connector and Chimney - Soot Blow-back Clue

Gas heater sharing flue with oil burner (C) Daniel Friedman

You can see in our photo of this gas fired water heater (left) that the oil-burner fired heater whose flue vent connector is shared is permitting the oil fired heater exhaust to blow backwards out through the gas fired water heater's draft hood.

It is a sure guess that at least when the oil burner is running, this water heater is not working properly and probably not safely. The underlying trouble could be an inadequate or blocked chimney or flue.

Overloaded & Improperly Shared Chimney Flue?

Multiple flue vent connectors into a single chimney (C) Daniel Friedman

Reader Comment: 09 April 2015 NHFireBear said:

What's the deal in the top photo in this article? [and shown just above]

It appears to show a wood-fired boiler (orange Itasca? WB410) and an oil-fired (green) boiler (and possibly the blue, oil-fired water heater) venting into the same flue (i.e., one connector entering above the other).

This would be a violation of both the oil-burner code and the wood-burner code. You cannot generally have any solid-fuel burning appliance venting into the same flue as oil or gas burners. NFPA 31 (2011): 6.5.25, NFPA 211 (2010): 9.8.2.

I have a recent photo of a single-wall oil vent from a floor heater to a masonry wall, having only 2-inch clearance from combustible structure, if you would like a copy to post. Shall I email you a jpg? I have marked the problem with red arrows. It is 120 dpi and sized at 6.4 inches wide, 4.8 high.


I'd like to see and if OK with you use your photo as well: our email is at the CONTACT link at page top or bottom.

The page top photo on this article is indeed an illustration of a fiasco - I agree with you and that's why we published it. I'll add your comments for emphasis.

What's interesting is the wood-oil combi boilers that I've seen with only a single flue-vent connector and thus by definition sharing a chimney.



Continue reading at SHARED CHIMNEY FLUES OK where we describe circumstances under which a flue or flue vent connector (stackpipe) can be shared - maybe, or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.


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