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.
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.
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.
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.
Some home builders think it is a good idea to increase the dryer vent line from 4" to 5". I guess they think it will help with air flow. Unfortunately while the larger duct line will help when moving air in a HVAC system, it causes problems when used for dryer ducting.
All dryer ducting must be a minimum of 4" in diameter. Clean, unobstructed, frictionless ducts encourage air flow efficiency, quickens drying times, adds longevity to clothing's life and reduces utility bills. The DryerFlex meets the 4" diameter specs.
Exhaust ducts shall terminate on the outside of the building or shall be in accordance with the dryer manufacturer's installation instructions. Exhaust ducts shall terminate not less than 3 feet in any direction from openings into buildings.
Be sure to keep 90-degree bends to a minimum. Every 90-degree bend is the equivalent of ten feet of vent run. Never install a dryer vent which has a bend greater than 90 degrees. Sharp bends restrict airflow and will lead to excessive lint buildup.
The guideline for vent length is a maximum distance of 35 feet subtract 2.5 feet for every 45° bend and subtract 5 feet for every 90° bend. There are rules governing the size, shape, connection methods, and terminations of dryer vents, and as an inspection company we rarely see every rule followed.
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.
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.
In most states, indoor dryer vents are not legal. Colorado's residential code, for example, initially states that dryers must be vented according to manufacturer instructions, which could allow for indoor or outdoor venting. The next section, however, says that dryer exhaust vents shall convey moisture outdoors.
It is not safe. Vent hoses are necessary for dryers to operate properly because they allow heat, lint, and humidity to be vented outdoors. When a dryer is used without one of these vent hoses, moisture and lint can build up inside the appliance and even cause a fire.
Route the dryer vent horizontally to the home's outside wall. Duct work should pitch 1/4-inch per foot to the outside. This keeps moisture out of the pipe. For basement laundry areas where your ductwork will run vertically to the outside wall, you may need to use 90-degree elbows.
[NFPA 54:10.4. 5.3] Exhaust ducts for Type 2 clothes dryers shall be installed with a clearance of not less than 6 inches (152 mm) from adjacent combustible material.
Ensure the window is open enough for hot air to escape, but remember, this isn't a safe long-term solution and can still pose fire and moisture risks. Indoor Venting Option: If you can't vent outside, you can use an indoor vent kit.
The minimum duct air velocity during normal operating conditions should be at least 1,200 FPM (feet per minute) to keep lint moving through the vent. (In a 4" (102 mm) diameter pipe, this requires at least 105 CFM of air flow.)
Here's the brief: A dryer vent can typically run up to 25 feet for electric dryers and around 35 feet for gas dryers, with adjustments for bends and turns along the venting system.
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.
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.
Under no circumstances should a plumbing vent pipe be installed with a 90 degree angle PVC joint.
For most dryer models, you can't run the exhaust duct more than 25 feet from the dryer to the exhaust port. Ducts should be as short and straight as possible to reduce lint buildup. Read on to learn more about the nuances of correctly venting a dryer so you can get your laundry dry and keep you and your family safe.
Better Airflow: One of the primary benefits of using a hard pipe for dryer venting is that it provides better airflow. The smooth interior of a hard pipe allows air to flow freely, while the ridges and creases in flexible accordion piping can cause restrictions in airflow.
There is only code to dictate the height of the exhaust above exterior ground. There is no code for interior height. Standard puts the top of the vent pipe typically around 12" off the floor.
PVC is plastic and not conducive for dryer vents. When lint travels through a PVC pipe, static electricity created by the plastic will pull the lint to the sides. Over times this causes a 4” pipe opening to shrink dramatically. This leads to poor airflow which creates drying exhaust issues.