How Old Floor Drains Work

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Floor drains provide a way for water to escape the home.
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Floor drains are an effective way to remove excess water from a home. In many older homes, floor drains are part of the basement design, built in to prevent water damage to the house and its contents. Many new homes also utilize these types of drainage systems. Old floor drains work passively, their design based on principles of physics.


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The basic mechanics of an old floor drain are simple: Gravity moves the water from the house. The floor drain is installed at the lowest point in a floor built to slope toward it. The drain drops the water into a sewer system that carries it from the house along with waste water from other drains that feed into the system. If the home has a septic tank, some old floor drains may lead instead to drainage ditches, dry wells or leach fields, to avoid flooding the septic system. Newer floor drains may also utilize pumps to move water when necessary.


The drain trap is an important part of the floor drain. The traps used in floor drains are like those used under kitchen and bathroom fixtures. The drain pipe dips down in a U that is always filled with water, whether flowing or still. The water-filled trap prevents noxious sewer gases from backing up into the home. Drains with a trap also must have a vent to the air outside the home to maintain neutral air pressure in the system, allowing water to flow freely.


Cleanout Plugs

Cleanout plugs are a feature of most floor drains. The cleanout plug is a rubber stopper, held in place by a bolt and wing nut. A cleanout is an opening inside the floor drain that allows access for removing blockages in the pipes. The cleanout is normally kept plugged. The plugs can get rusted out and damaged by corrosion. When this happens, the plug must be replaced. In some floor drains, the cleanout bypasses the U trap, so the plug must be maintained in good condition to keep sewer gases out of the home.


Unclogging a floor drain is no different than unclogging other wastewater pipes. Blocked floor drains can sometimes be cleared using a plunger. In other cases, using the cleanout plug may be necessary. More advanced methods may call on a plumber's skills and equipment. In some cases, inserting a water hose into the drain as far as it will go and turning it on may be sufficient to flush the debris out.