Multiple flue vent connectors into a single chimney (C) Daniel Friedman Flue Vent Connectors / Stackpipes for Heating Boilers, Furnaces, Water Heaters & Chimney Flues
Horizontal & sloped connections between heating appliances & chimneys: materials, sizes, lengths, codes

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Flue vent connector specifications & fire clearances: this article describes flue vent connectors used to connect heating appliances to a chimney in order to safely deliver combustion gases to a chimney for venting outside the building.

Flue vent connectors are not the same thing as a chimney and they have their own safety and installation requirements including slope, materials, diameter, and clearance from combustibles.

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Flue Vent Connectors - Heating Boilers, Furnaces, Water Heaters & Chimney Flues

Flue vent into chimney flue blocked by debris(C) Carson Dunlop Associates

Definitions of Chimney, Factory-Built Chimney, & Flue Vent Connector

Because some readers may be confused about the difference between a "chimney" and a "flue vent connector" we include some simple definitions below. On our illustration at left we have marked the chimney, chimney flue, and flue vent connector - adapted from a Carson Dunlop Associates illustration used later in this article.

[Click to enlarge any image to see more detail]

Chimney means a primarily vertical shaft enclosing at least one flue for conducting flue gases to the outdoors.

Factory-built chimney means a chimney consisting entirely of factory-made parts, each designed to be assembled with the other without requiring fabrication on site.

Vent connector (as applying to heating or cooling systems) means the part of a venting system that conducts the flue gases or vent gases from the flue collar of a gas appliance to the chimney or gas vent , and may include a draft control device. - 11/13/2013

These definitions are important because the construction, use, and safety requirements as well as building codes and standards and fire clearances are different for these different components.

Flue vent connectors for "B" or "L" Vents: A vent connector connects gas equipment to a flue or chimney. Appliances having draft hoods and installed in an attic or concealed space must use Type B or L connectors. Appliances installed in basements can use Type B or Type L or metal pipe (.0304" thick) or aluminum pipe (.012" thick). Gas fired equipment should not be connected to any flue serving solid fuel appliances.

Multiple gas vents into a single flue: Where two or more vent connectors enter a common gas vent, chimney flue or single wall metal pipe, the smaller connector shall enter at the highest level consistent with available headroom and clearance to combustibles. Avoid unnecessary bends and secure all joints with sheet metal screws or other approved means.

Chimney Blocked at the Flue Vent Connector

Watch out: Debris falling down a "dead end flue" easily blocks the flue vent connection and chimney (illustrations below). The result can be rapid and fatal
CARBON MONOXIDE - CO poisoning of building occupants. .

Carson Dunlop Associates [at REVIEWERS] sketch (above) shows fallen bricks blocking the bottom of the flue so that even where a "dead end" flue was not constructed - that is, where a chimney cleanout was installed - falling debris can so fill up the chamber at the bottom of the chimney flue that the effect is the same: a blocked chimney flue and the production of carbon monoxide.

Flue vent into chimney flue blocked by debris(C) Carson Dunlop Associates Debris blocking a chimney thimble (C) Daniel Friedman

A blocked flue case study is reported

Flue Vent Connector Too Far into Chimney

Flue vent into chimney flue (C) Carson Dunlop Associates

Carson Dunlop Associates [at REVIEWERS] sketch shows what happens if the flue vent connector is jammed too far into the chimney flue.

Draft may be blocked, resulting in improper and potentially dangerously unsafe heating equipment operation.

For example, inadequate draft on a gas fired appliance makes the production of potentially fatal carbon monoxide gas likely.

The symptoms of this mistake are about the same as a blocked flue, and like a blocked flue, the diagnosis requires some disassembly and inspection of the flue vent connector and chimney by an expert.

Proper Joint Connections for Single Wall Metal Flue Vent Pipes

Flue connector section joint requirements (C) Carson Dunlop Associates Gas vent home made connection (C) Daniel Friedman

Metal flue vent connectors such as shown in Carson Dunlop Associates [at REVIEWERS] sketch (above left) should also be joined with the female (wider) ends facing "up" so that condensate and creosote that may form inside of the flue connector remain inside of the flue pipe. Our photo (above right) shows that foil tape was used to join two flue vent connector pipes of different diameters from a gas fired heating appliance (not shown). Foil tape is not a suitable substitute for the necessary adapter to join the two different diameter flue vent connectors.

It's easy to spot an improperly-joined flue vent connector pair of sections: you will often see stains from leaks oozing out at the section joints.

Flue vent connector horizontal length limits

Long flue in a crawl space (C) Daniel FriedmanHorizontal vent length limits: The horizontal length of a vent connector to a natural draft chimney or vent serving a single draft hood appliance shall not be more that 75% of the height of the vertical portion of the chimney above the connector. [NFPA 1992 (7.10.10)]. We show some long flue vent connector runs at

The maximum horizontal length of vent connectors per the GAMA tables is limited to 1.5 feet per inch of diameter with a provision of a 10% reduction in capacity for each multiple of the length permitted. The venting tables must be consulted to size an adequate venting system.

Watch out: allowable flue vent connector length calculation can be a bit more complex than that rule of thumb, as the Q&A below illustrates.

Our photo shows a very long dead flue vent connector joining a heating appliance to a dead end chimney.

Reader Question: is there a Simple rule that relates allowable horizontal flue length and flue vent diameter?

What is the maximux allowable horizontal length of a category 1 appliance vent connector in feet for each inch of its diameter. - S.W.

Reply: Not quite so simple, here are the factors that determien allowable horizontal flue length:

From your question, a simple, straight answer on allowable flue length per inch of flue diameter is not quite possible. Take a look at the National Fuel Gas Code NFPA 54, or ANSI Z223.1, for example, Tables G, and you'll see that the tables consider the following factors that set the allowable horizontal flue length or run:

An example (from a 1988 copy of the National Fuel Gas Code):

a 6" diameter Type B double wall connector with a 15' lateral run and a 15' high chimney can vent 198,000 BTUh of capacity.

So a simplistic equation that gives allowable horizontal distance of a flue vent connector or vent expressed in feet per inch of diameter simply would be wrong and potentially unsafe, risking a carbon monoxide or equipment operating problem.

You will want to consult the proper tables either in one of the codes or in the heating equipment manufacturer's equipment installation procedures (the manufacturer would be expected to have the final say).

Reader Question: what's the code on distance and clearance for metalbestos chimneys & vents vs oil storage tanks?

I am installing a pellet boiler and will need a second metal chimney as I am leaving my oil burner as a backup. I live in maine. The only place I would like the metal asbestos chimney would run 40” away from my oil tank. Do you know what the code is on this? I am having a heck of a time getting any answers regarding this? - R. B. Windham ME 2/27/2014

Reply: heat flue vent length maximum distances vs. clearance distances from oil tanks or other building features

R.B. If you are asking about the chimney itself, insulated metal chimneys typically require a one-inch clearance from combustibles.

  1. The required separeation or clearance distances for flue vent connectors (generally single-wall uninsulated metal "flue pipes" or "stack pipes" that connect the heater to the chimney) is discussed
  2. The required separation of metal or insulated chimneys themselve from varous building components or combustibles is explained
    for height above roofs,
  3. The required separation of heating appliances (like an oil burner) from heating oil storage (the oil tank) is elaobrated at ABOVE GROUND OIL STORAGE TANK (AST) INSPECTION

If you were asking about the distance or run length for the flue vent connector between the chimney and the heating appliance, that's a different story.

Suppose we wanted to run across an entire building from the heater to the chimney: the maximum horizontal length should not be more than 75% of the chimney height above the vent connector, or 100% if the chimney is insulated. So to cross a 40 foot basement from a heater to a chimney you'd need a chimney at least 40 feet tall for the system to vent properly by natural draft. But an option that may work acceptably in that case is to include a draft inducer fan such as the type made by Field Controls.

Flue Vent Connector Loose, Leaky, Not Sealed at Chimney

Flue vent into chimney flue loose leaky <C) Carson Dunlop Associates Leaky flue vent connector at chimney (C) Daniel Friedman

Carson Dunlop Associates [at REVIEWERS] sketch (above left) shows a loose flue vent connector at the chimney. This is one of the most common chimney and venting defects we observe in buildings.

This defect can often be seen by careful visual inspection of the flue vent connector at the chimney. Our photo (above right) shows our client pointing to a flue vent connector that was not sealed at the chimney thimble.

The results of a loose or leaky vent connection at the chimney include inadequate chimney draft (unsafe heater operation) and leakage of potentially dangerous combustion gases into the building.

Unsafe Rusted Hole in Metal Flue Vent Connectors

Rusted metal heating flue (C) Daniel Friedman

We suspect that the root cause of this unsafe metal heating flue is that it was routed out of the building at or below ground level - into a dead-end chimney.

Water from roof spillage or surface runoff have rusted out the flue vent connector.

This is an unsafe installation even before we think about the added hazards of fire clearances and adequate draft.

Rusted through metal stack pipe (C) Daniel Friedman

This flue vent connector (photo at left) has rusted through from water leaking down the chimney into the vent connector elbow.

On an oil-fired system we expect soot to blow out of this opening at system startup and during the system run cycle the opening may interfere with proper system draft. The opening leaves a fire risk should a spark blow out of this opening.

On a gas-fired system a flue vent connector with a rust hole increases the chance of inadequate draft and dangerous carbon monoxide release in the building.

This flue vent connector elbow,rusted through with a large hole, needs to be replaced and more, we need to identify and repair the source of water leakage into the chimney that caused this damage.

Proper Flue Vent Connector Slope

Flue connector slope requirements (C) Carson Dunlop Associates

Metal flue vent connectors such as shown in Carson Dunlop Associates [at REVIEWERS] sketch should:


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