Gas Company Told Me My Flue is Blocked

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Author – Marge Padgitt

The last thing anyone wants to hear is that their gas has been shut off by the gas company due to a blocked chimney flue, especially during winter months. 

Yet every year this happens to thousands of people.  A blocked utility flue serving a gas water heater, furnace, or boiler can mean serious problems.  A blocked flue can cause deadly Carbon Monoxide back up into the home, and is the primary reason for shutting down the system when this occurs. 

Carbon Monoxide (CO), not Carbon Dioxide, is an odorless, tasteless, colorless toxic gas that can cause headaches, fainting, flu-like symptoms, nausea, disorientation, irreversible brain damage and death. 

Just a small amount, measured in parts per million (PPM) can be very deadly to humans and pets. 

Younger children and the elderly are more susceptible to CO than adults. See  for more information.  For this reason, all homes should have a CO detector installed on each level of the home.

There are several things that can cause a blocked utility flue. 

Birds’ nests, dead birds, dead animals, squirrel nests, twigs, leaves and debris may enter an un-capped chimney flue and accumulate at the bottom of the flue at the connection to the appliances.

Over time, the debris will rise to the point that the flue is partially or completely clogged.  A simple chimney cover can prevent this from happening.  Stainless steel chimney covers are more resistant to animal intrusion and will not rust, and last longer than cheaper black steel covers.

The second most common issue is over-sizing of the flue to the appliance BTU input. 

Mid-and higher efficiency furnaces require much smaller flues than older model furnaces, but many heating, ventilation, and cooling contractors do not install a new flue liner with the new appliance.  If a qualified chimney contractor or chimney sweep is not called in to install a new flue liner trouble can quickly develop.

An over-sized flue can cause excessive condensation of acidic flue gasses, which deteriorate mortar joints, flue tiles, and exterior bricks.  Telltale white powdery efflorescence may appear on the exterior bricks along the flue’s position in the chimney when this occurs.  More importantly, the interior mortar may fall to the bottom of the flue and cause a blockage. 

In the case of older, unlined flues, the damage will likely be much worse. If the damage is too bad, partial or complete tear down and rebuilding of the chimney may be necessary. 

Often when a new high-efficiency furnace is installed and vented out the side via PVC pipe, the water heater is left to vent on its own in a flue that is now very over-sized. 

Even if the flue is clear, the toxic gasses have no way to exit the home because water heater vents are dependent on furnace venting to assist.  This is called an abandoned hot water heater, and is often overlooked by furnace installers.  In this case, a new liner sized for the water heater should be installed.

Blocked chimney flues are preventable with annual inspection, and maintenance and repairs as needed by a qualified professional chimney sweep.

~ Marge Padgitt

~ Asking for Chimney Information ~

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CHIMNEYS.COM recommends that you use these comments to better inform yourself to discuss your chimney and venting issues with a professional whom you call to your home to evaluate the issue.

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