Definition & hazards of dead end chimneys & flues.

This article describes dead base or dead end flues and explains why they can cause heating equipment operating difficulties or why they may be unsafe.

A dead end flue is one in which the flue vent connector joins at the very base of a chimney.

Debris falling down the chimney can quickly block the flue, prevent proper removal of exhaust gases from the building heating equipment, and can be dangerous. We describe how to spot dead end chimneys, how to recognize trouble signs, and what to do about dead end chimney venting systems.

Dead End Flues or Dead Base Chimney Flue Hazards

Dead-end flues, chimneys whose flue stops right at the point of entry of a thimble for a woodstove or heating appliance are inherently more dangerous than a conventional flue which extends several feet further past the thimble. Dead end flues are quickly and easily blocked by any debris that might fall down a chimney.

[Click to enlarge any image]

If a chimneys venting a gas-fired appliance becomes partly or totally blocked, the appliance is likely to rapidly produce very dangerous, potentially fatal carbon monoxide (CO).

Be sure that chimneys of this type are inspected and cleaned annually and be sure that you have working CO detectors as well as smoke detectors in the building.

The dead end flue in this photo (above) can be determined because we're venting that long run from a heating boiler into the bottom of a chimney that's just above ground level in a crawl space.

Looks as if we've got some asbestos paper wrapping the flue - another matter to be addressed.

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.

Dead End Chimney Flue Clues & Examples

• Absence of a chimney cleanout - the chimney may stop right where the flue vent connector enters the chimney.
• Combustion chamber backpressure burns on the heating equipment (photo at left) may indicate that the chimney is blocked (or there are other draft problems such as an open chimney cleanout door)
• Old houses built on stone foundations with no original central heating.
  
  Often when a basement was later excavated or deepened to install a heating boiler or furnace, the only connection into a chimney flue was an entry point at or close to ground level above the basement because the original chimney flue did not extend below ground level.
  
  Look for a chimney that sits atop a stone foundation.

• Flue vent connectors (stack pipes or smoke pipes) routed through crawl spaces often enter the bottom of a chimney flue. (Photos above on this page).
• Masonry fragments in the cleanout: if you open a chimney cleanout-door or pull the flue vent connector from a dead end chimney flue, in either case you are looking at the very bottom of the chimney flue.
  
  If you are inspecting a masonry chimney, be alert for discovery of masonry fragments when inspecting or cleaning the bottom of the flue. If a piece of brick, masonry block, concrete, or clay flue tile liner is pulled out of your chimney, ask "where do you suppose this came from?"
  
  If the masonry scrap fell during construction of the chimney it may mean nothing.
  
  But if it fell because the chimney has been damaged, perhaps by water, frost, or during cleaning, then you probably have an unsafe chimney flue - more investigation is in order, promptly.
  
  Certainly if you believe that the masonry scrap found at the bottom of a flue fell during original construction, and if you remove it during cleaning, you should never find another piece in the flue bottom. If you do, the flue has been damaged anew and it is unsafe.

• Chimney use history: if a chimney has been used for a variety fuels, coal, oil, wood, there may be a greater accumulation of debris at the chimney bottom, a mixture of creosote, soot, or even masonry fragments from a flue damaged by corrosive gases and condensate in the flue.
  
  Our photo (left) shows a truly abandoned dead end flue chimney. We changed this building to a direct-vent heating boiler and abandoned the chimney entirely rather than continue to face chimney blockage problems.
• Chimney draft problems: if someone has installed a draft inducer fan because they couldn't get enough draft, the problem could be a blocked flue.
  
  Look for back-pressure burns on the combustion chamber. Look for a defeated or sealed-up barometric damper.
  
  See details
  
  at DRAFT REGULATOR, DAMPER, BOOSTER.
• Chimney leak history: leak stains inside the building attic or on any floor, if they are traceable to leaks at a chimney, are reason to be concerned for possible water damage to the chimney as well as to the building.
  
  Further investigation may be warranted.
• Chimney cap history: if a chimney has spent part of its life with no cap installed, there is extra risk of water damage to the flue interior. In a masonry chimney damage may appear as frost cracking of the upper flue liners or masonry.
  
  In any chimney, there may also be water damage to the heating appliances being vented by that chimney.

Dead end chimney flues are likely to lead to the rusted-out flue vent connector such as we show here.

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.

Experienced heating service technicians are expected to be well aware of the hazards of dead base chimneys & flues, and as our client points out in our photograph (left), the service tech may leave a note to tip off the technician who arrives for the next service call.

To the heating service tech a dead base chimney means

• Watch out for blockage at the chimney - be sure to open and clean it out
• Watch out for draft problems on the heating equipment

Below we illustrate another clue that may indicate a dead base chimney - no visible cleanout door below the chimney thimble where the flue vent connector attaches to the flue, and water leak stains directly below the thimble.

Watch out: Just below we illustrate another scary problem often found in deep, hard to access crawl areas: a dead base chimney and a fallen-off or disconnected flue vent connector or "flue pipe" - presenting a fire hazard, a flue gas poisoning hazard, improper heating operation, generally an unsafe heating system.

Why would this problem be more likely here? Because the connection of the flue vent connector to the dead base chimney is deep into a hard-to-access crawl space where nobody wants to go.

Be sure to inspect such locations with care.

Photo below: the partial blockage of the bottom of this thimble entry into a chimney flue shows why model codes and chimney standards require at least 12" of clearance between the bottom of the thimble opening into a flue and the bottom of the flue itself.

The leak stains below this thimble tell us of another concern: water leaks down the flue.

International Fuel Gas Code (2015) Chapter 5 Chimneys and Vents

501.9 Chimney Entrance

Connectors shall connect to a masonry chimney flue at a point not less than 12 inches (305 mm) above the lowest portion of the chimney flue.

503.10.10 Chimney connection.

Where entering a flue in a masonry or metal chimney, the vent connector shall be installed above the extreme bottom to avoid stoppage.

Where a thimble or slip joint is used to facilitate removal of the connector, the connector shall be firmly attached to or inserted into the thimble or slip joint to prevent the connector from falling out.

Means shall be employed to prevent the connector from entering so far as to restrict the space between its end and the opposite wall of the chimney flue (see Section 501.9). 

Solutions for the Dead End Chimney or Dead Base Flue Problem

• Safe chimney maintenance: regular annual maintenance must include opening and cleaning the chimney base as well as checking/cleaning the flue vent connector where a dead end flue chimney is in use.
  
  Also monitor heating equipment operation for signs of blocked flue passages such as indications of inadequate chimney draft.
• Relocate the flue vent connector connection point to higher on the chimney:
  
  Where feasible, move the flue vent connector to a higher entry entry point in the chimney, replacing the original dead base entry point with a chimney cleanout door.
  
  This option will rarely be feasible or no one in their right mind would have connected the heater to the chimney bottom in the first place.
  
• Eliminate use of the dead base chimney entirely by converting the heating equipment's venting system to a direct through wall power vent system.
  
  See our photo and for details of this approach
  
  see DIRECT VENTS / SIDE WALL VENTS.

...

Continue reading at CHIMNEY CLEANING PROCEDURE - topic home, or select a topic from the closely-related articles below, or see the complete ARTICLE INDEX.

Or see these

Recommended Articles

• ABANDONED CHIMNEYS
• CHIMNEY FLUE INSPECTION CAMERA
• CHIMNEY INSPECTION DIAGNOSIS REPAIR - home
• DIRECT VENTS / SIDE WALL VENTS

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