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.
M1502.
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.
Ideally, your dryer should have exhaust vents that lead outside to keep your laundry area clean and less humid. The most common place to vent your dryer is horizontally through the exterior wall near the dryer, which is the ideal placement.
Dryer vents should be sealed with foil tape as they can withstand the high temperatures encountered during machine use. Other tapes, even duct tape, are susceptible to failure under the extreme heat. Non-foil tape can also catch on fire.
Although dirty, this break allows heat and moisture to escape and limits your immediate risk of a dryer fire. PVC piping will not do this which leads to water pooling and severe clogs. These clogs will cause your dryer to overheat and become a potential hazard.
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.
Dryer venting tips and practices
Flexible transition hose between the dryer and the wall outlet should be either the foil type or the aluminum flexible duct (most preferred). Do not use the plastic or vinyl. Concealed ducting must be rigid metal (galvanized or aluminum) duct.
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 situations where outdoor venting is not an available option, the vent bucket serves as an efficient indoor solution. It collects lint and moisture, ensuring they do not disperse throughout your living area. It is particularly convenient for apartments or temporary setups.
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.
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.
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.
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.
The dryer needs to be installed near an external vent that has been fitted into an external wall, allowing the damp air to escape outside. Thanks to the external vent, there is no need to worry about empting water from the machine.
I've only seen and used 4 inch dryer vent piping. Various construction platforms call out 4 inch for dryer venting, 3 inch for ceiling fans (bathroom exhaust).
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.
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.
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.
Rigid Metal Ducting: Rigid metal ducting, such as aluminum or galvanized steel, is widely considered the gold standard for dryer vent systems. These materials are non-combustible and can withstand high temperatures, reducing the risk of fires.
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.
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.
In summary, while flexible accordion piping may seem like a convenient option for dryer venting, a hard pipe provides better airflow, fire safety, durability, and aesthetics. Investing in a hard pipe may cost a bit more upfront, but the long-term benefits make it a worthwhile investment for any homeowner.