An HVAC Furnace Drain Pipe Is Clogged

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A clogged drain pipe on a central air and heating furnace causes problems inside the house, and can create significant damage if not repaired. The good news is that do-it-yourselfers can handle the job without professional help.

Spotting a Leak

A clogged furnace drain pipe often shows itself in a pool of water on the floor directly in front of the furnace. If you remove the access panel on the furnace, you may also see leaking water leading a trail up to the unit's evaporator coils. The leak may also show itself in a water-stained ceiling underneath the system. This water is supposed to drain out of a drain line on the outside, normally near the air conditioner compressor.


The biggest danger caused by a clogged drain pipe, aside from the unit not operating properly, is water damage. The leak can lead to extensive, and expensive, repairs to ceilings, floors, carpet and walls.

Unclogging the Pipe

You can unclog the pipe from inside, at the furnace, or outside, where the drain line ends. If you do it outside, place the end of the hose of a wet/dry vacuum over the end of the drain line. Wrap duct tape around the hose/drain line connection so the air can't escape out around the smaller drain line. Usually, you need to use an extension cord so the vacuum hose can reach the drain line. Turn the vacuum on to suck out the line for about six seconds, or longer, if needed. If you approach a clog from inside, you can blow out the line, using either a water hose without a nozzle, or compressed air. Also, you may need to clean dirt out of the condensation pan underneath the evaporator coils. Turn off the unit's power first. The cooling unit, which contains the coils, normally sits on top of the furnace.


The system's drain line should be regularly maintained to prevent a clog from building up, including cleaning out the condensation tray. One effective method to prevent clogs is to pour one cup of bleach in the drain line's access hole, near the furnace. Also, clean or change dirty HVAC filters. If this filter is covered with a layer of dirt and clogged, incoming dirt sticks to the evaporator coils instead, and ends up in the drain pipe.


Christopher John

Christopher John has been a freelance journalist since 2003. He has written for regional newspapers such as "The Metro Forum" and the "West Tennessee Examiner." John has a Bachelor of Arts in journalism from Memphis State University.