The maximum developed length of a clothes dryer exhaust duct shall not exceed 35 feet from the dryer location to the wall or roof termination. The maximum length of the duct shall be reduced 2.5 feet for each 45-degree (0.8 rad) bend, and 5 feet for each 90-degree (1.6 rad) bend.
Codes and Standards
In the United States, most home inspectors refer to the International Residential Code (IRC) M1502 for dryer venting. However, some states may have additional regulations. The International Mechanical Code (IMC) 504 also includes extensive clothes dryer requirements.
Termination of dryer venting must be to the exterior with a proper hood or roof cap equipped with a backdraft damper. Small orifice metal screening should not be part of the hood or roof cap as this will catch lint and block the opening in a very short time.
The most common place to vent your dryer is horizontally through the exterior wall near the dryer, which is the ideal placement. Other common locations are through (but not into) the basement or the attic/roof.
A summary of the typical codes relating the dryer venting is as follows: Dryer vent systems shall be independent of all other systems and shall convey the moisture to the outdoors. Terminations shall be a minimum of three feet from property line and 12" above the ground and not exhibit any type of screen.
Keep flammable materials and vapors, such as gasoline, away from the dryer. Place dryer at least 18 inches above the floor for a garage installation. Failure to do so can result in death, explosion or fire.
The exhaust duct should be rigid metal 4 inches in diameter and no more than 35 feet in length or as specified by the duct manufacturer. This length should be decreased by 2.5 feet for every 45-degree bend the duct makes and 5 feet for every 90-degree bend the duct makes.
DO NOT terminate dryer vents in crawl spaces or attics. Dryer vents that terminate in crawl spaces or attics allow for moisture to deposit and encourage mold growth and health hazards. Dryer vents must terminate outside of the building.
Some dryers can exhaust beyond 25 equivalent feet without a booster fan. Dryer Duct Booster Saves significant energy - reduces drying times 50% or more. Reduces lint build-up, dust & fire hazards by maintaining proper velocity in duct runs up to 150 equivalent feet.
The International Association of Certified Home Inspectors (InterNACHI) establishes length requirements for dryer vents. The length cannot be more than 25-feet; it should also be as straight as possible. If there is a 90-degree angle in the vent, then it shouldn't be more than 20-feet.
Fire Hazards: Running a traditional dryer without a vent leads to dangerous lint buildup, increasing the risk of fires. Moisture Damage: Without proper ventilation, trapped humidity can encourage mold and mildew growth, affecting indoor air quality and structural integrity.
Vent elbows are available which is designed to turn 90° in a limited space without restricting the flow of exhaust air. Restrictions should be noted in the inspector's report. Airflow restrictions are a potential fire hazard.
You can use a 4-inch flexible aluminum foil duct that is UL 2158A listed and marked for safe dryer venting. Look for a product that is constructed from heavy duty, fire resistant aluminum foil with a high density, corrosion resistant wire helix. Remember that only ducts 8 feet or less can be UL listed and marked.
PVC or plastic ducts are common because they're easy to install but they can trap lint. For clothes dryer venting, a 4-inch, rigid aluminum or galvanized steel duct pipe is a better option. They let air flow more freely and reduce lint buildup.
Duct vents at exterior should be a minimum of 3 feet from any other opening; window, door, other vents, etc. Duct vents at exterior should be at least 6 feet from the A/C condensing unit. Duct vents should never be terminated in the attic or crawlspace. Never combine dryer venting with any other venting source.
On the back of your dryer, you will find the unit's ventilation hose. The hose then leads to the exhaust vent, which will typically be located outside your home.
Dryer cords are usually a maximum of six-feet-long as the code requires that an outlet be close to the connected appliance. Check to see if you need a 3-prong vs. 4-prong dryer cord and take that into consideration when shopping for new appliances.
Venting a dryer through the roof is possible, but there are important considerations to remember. This method can be practical when sidewall venting isn't an option due to your building's layout. However, overall, sidewall venting is considered the safer option.
M1502.
The passageway of dryer exhaust duct terminals shall be undiminished in size and shall provide an open area of not less than 12.5 square inches (8065 mm2).
In conclusion, while traditional electric dryers do require a vent to the outside to expel moist air, advancements in technology have brought ventless options to the market.
The flexible nature and expandable configuration allow most installs to occur without any modification to the duct or the wall penetration. A pair of jumbo zip ties and you're good to go…at least for a few months until all of the internal bumps and ridges begin to clog with lint.
A: The tape is flammable and burns very well so I would not recommend it.
Here's a quick rundown: If you can't vent your dryer outside, there's a risk of fires due to lint buildup, a chance for mold to grow, your laundry room might get too hot, and drying times could increase, possibly causing damage to your clothes and higher energy bills.
Some dryer vent installations also have an external lint trap, which is also known as a secondary dryer lint trap or a 4″ In-Line Dryer Lint Trap.
Long vertical runs can result in greater humidity and lint accumulation, which will reduce drying efficiency. Vertical runs may require the installation of a booster fan to maintain effective exhaust air flow.