If you have an electric clothes dryer, it will need to be properly vented outside of your home to meet most local building codes. While an electric clothes dryer doesn't expel carbon monoxide like its gas counterpart, there are large amounts of moist air and lint coming out of your electric dryer, both of which can severely damage your home if they accumulate.
An electric dryer pulls lots of moisture out of the clothes it's drying. If the dryer isn't vented to the outside of the home, all of that moisture is being added inside. This can promote the growth of mold and mildew and cause condensation to form on your windows. In extreme cases, the excess moisture can cause the wood in your home to rot.
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All dryers are equipped with a lint trap, but not all of the lint will be caught. Some of it will escape out of the dryer vent. In situations where the electric dryer isn't vented properly, the lint will build up either behind the dryer or wherever the dryer is vented to inside the house. Lint is extremely flammable and in the event of a fire, an excess accumulation of it can quickly accelerate the fire.
Types of Venting
Smooth metal pipe venting and flexible metal pipe venting are the two different kinds of venting that you can use on a dryer and both of have their advantages. Smooth metal pipe venting offers less air resistance inside the pipe, while the flexible metal pipe venting is easier to install in tight spaces and around corners.
Most local building codes require that dryer venting, even off an electric dryer, go to the outside of the home. Not running exhaust piping at all, or directing it into an attic, basement, garage, or crawlspace is unacceptable. Run it to the roof or an outside wall. The maximum distance of the exhaust piping depends on the number of turns to ensure the dryer's blower motor is strong enough to push the exhaust all the way to the end of the venting. If you're using metal vent pipe and it's a straight run, the dryer can be up to 64 feet from the end of exhaust vent. If there are four 90-degree bends in the piping, 27 feet is the maximum distance to have from the dryer to the end of the exhaust vent. The maximum distance which you can run flexible vent pipe is roughly half the amount of the metal vent pipe.