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.

We also provide a MASTER INDEX to this topic, or you can try the page top or bottom SEARCH BOX as a quick way to find information you need.

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 and Not-Permitted Shared Chimney Flues or Flue Vent Connectors

Coal stove venting into a common chimney with a gas fired heater (C) Daniel Friedman at InspectApedia.cpomPhoto: a coal stove (a solid fuel appliance) on the first floor of this Poughkeepsie NY home is vented into the same chimney flue as the gas-fired heating boiler in the basement below. This is an un-safe and improper installation.

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 & Chimney - Soot Blow-back Clue at Water Heater

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.

Shared Flue: Heating Boiler & Water Heater Unsafe

Water heater shares flue with a gas fired steam boiler - unsafe connection, blow-back at water heater draft hood (C) Daniel Friedman at InspectApedia.comFirst photo: in this Poughkeepsie home a gas fired water heater shares a flue with a large gas fired steam boiler.

These clues confirm that this heater is not venting safely and that there is a risk of carbon monoxide poisoning for building occupants.

  1. White salt deposits and corrosion around the draft hood on the water heater indicates back-drafting, failure to vent in some conditions, and corroded, damaged flue vent connector. It is possible that steam boiler combustion products back-flow down and out at the water heater draft hood.
  2. Flue gas spillage is occurring at the water heater draft hood, possibly because the heater vents into a very large, tall, masonry flue inside which it cannot establish a functional draft, especially if the steam boiler has been off and the chimney is cold.
  3. Unsafe modification of flue gas spill sensors: some fool has bent-up the flue gas spillage detectors / sensors to get them out of the flow path of flue gas spillage, probably because flue gas spillage was causing the water heater to turn off.

Watch out: these conditions are unsafe and risk fatal carbon monoxide poisoning.

They also tell us that someone who is not properly trained or informed in heating and chimney safety and maintenance has worked on this equipment.

Below: my photo shows the "bent-up" flue gas spill detector and the white corrosive salts around the draft hood and the flue vent connector.

Water heater shares flue with a gas fired steam boiler - unsafe connection, blow-back at water heater draft hood (C) Daniel Friedman at

Overloaded & Improperly Shared Chimney Flue?

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


There are exceptions allowed in some jurisdictions that permit separate oil and wood fired devices to share the same flue.


There, citing appropriate US codes, we note that when a gas fired appliance is permitted to be vented into the same flue as an oil fired heating appliance the smaller sized gas appliance is connected above the oil fired appliance flue vent connector.

I [DF] think the reason for that code specification or requirement is the need to avoid possible back-drafting out through the gas appliance draft hood of the combustion products from the more-powerful draft created by the oil fired appliance.

Worse, such back-drafting could cause fatal carbon monoxide poisoning by interfering with the exhaust of the gas appliance.

Question: should shared chimneys have an air vent to prevent condensation

Long metal flue run - bad draft (C) Daniel Friedman5 April 2015 Arthur said:

Four gas fire flues emerge from a brick chimney on the roof, not sure whether it is a shared chimney or not.

Two ground floor gas fires and one on the first floor are still in use, the remaining gas fire on the first floor has been removed. The builders opening has been sealed by a registered gas fitter.

Should this chimney have an air vent to prevent condensation occurring ?

But where the flue vent connector run-length is abnormally long or tortuous it may be difficult to get good draft, or where the chimney size itself (to which the flue vent connector joins) is too big or too small, the use of a draft inducer might be permitted.

Watch out: the connection of smaller and larger diameter flue vent connectors together ahead of the chimney such as shown in this photo is unsafe and improper.

The larger appliance vent may overpower and cause backdrafting at the smaller gas fired water heater at the lower right in our photo.


Arthur, if there is condensation in a chimney at levels sufficient to cause damage, the solution is not to add an air vent - a step that could be very dangerous as doing so can prevent adequate chimney draft - but to review the chimney size, height, insulating properties against the input BTUh of the appliance being vented.

Proper chimney design and sizing is what are required.

For a complete explanation see



Below: a tankless heater vented using plastic connected to a metal B-vent that is badly rusted. This is an unsafe installation. [Click to enlarge any image]

Tankless heater vented up through plastic to metal - corroded, leaky (C) Daniel Friedman at


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.

Or see CHIMNEY CODES for downloadable copies of common chimney and appliance venting codes and standards




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