Dryer Vent Safety

Clothes dryers remove moisture from damp clothes by circulating hot air around them as they spin in a rotating drum. The heat comes from an electric heating element or a gas burner. Certain heavy loads of clothing may hold over a gallon of water, which turns into airborne water vapor during drying and exits the dryer and home via an exhaust duct, commonly referred to as a dryer vent.

A vent expelling moist air to the outside of the home must meet several requirements:

1. Ensure it is connected. The connection is typically located behind the dryer but may also be underneath it. Inspect carefully to confirm it is indeed connected.

2. Ensure it is not restricted. Dryer vents often consist of flexible plastic or metal ducts, which can be easily kinked or crushed where they exit the dryer and enter the wall or floor. This is frequently an issue since dryers are usually situated in compact spaces with limited room. Vent elbows are available that can turn 90° in tight spaces without hindering exhaust airflow. Any restrictions should be noted in the inspector's report. Airflow restrictions pose a potential fire hazard.

3. One reason restrictions are a potential fire hazard is that, along with evaporated water vapor from wet clothes, the exhaust stream carries lint—highly flammable particles of clothing made of cotton and polyester. Lint can build up in an exhaust duct, reducing the dryer’s capacity to expel heated water vapor, which then accumulates as heat energy within the machine. As the dryer overheats, mechanical failures can cause sparks, igniting lint trapped in the dryer vent. This situation can lead to the entire house catching fire. Fires generally start within the dryer but spread by moving through the ventilation duct, burning trapped lint, and following its path into the building wall.

   
   

According to statistics from the National Fire Protection Agency and the Consumer Product Safety Commission, house fires resulting from dryers occur more frequently than many people think, with approximately 15,000 fires annually linked to inadequate lint removal and maintenance of household clothes dryers. Fortunately, preventing these fires is quite simple.

The recommendations outlined below reflect International Residential Code (IRC) SECTION M1502 CLOTHES DRYER EXHAUST guidelines:

M1502.5 Duct construction.Exhaust ducts shall be constructed of minimum 0.016-inch-thick (0.4 mm) rigid metal ducts, having smooth interior surfaces, with joints running in the direction of air flow. Exhaust ducts shall not be connected with sheet-metal screws or fastening means which extend into the duct.

This implies that the previously used flexible, ribbed vents are now obsolete. If detected during an inspection, they should be identified as a potential fire risk.

M1502.6 Duct length.

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 maximum length of the exhaust duct does not include the transition duct.

This means that vents should also be as straight as possible and cannot be longer than 35 feet. Any 90-degree turns in the vent reduce this 35-foot number by 5 feet, since these turns restrict airflow.

 A couple of exceptions exist:

1. The IRC will follow the manufacturer's instructions, so if the manufacturer allows for a longer exhaust vent, it is permitted. An inspector likely won't have the manufacturer's guidelines, and even if they do, verifying adherence to them goes beyond the scope of a General Home Inspection.

   
   

The IRC permits the installation of large radius bends to minimize restrictions at turns. However, verifying compliance necessitates engineering calculations as outlined in the ASHRAE Fundamentals Handbook, which is certainly outside the scope of a General Home Inspection.

M1502.2 Duct termination.

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. Exhaust duct terminations shall be equipped with a backdraft damper. Screens shall not be installed at the duct termination.

Inspectors will see many dryer vents terminate in crawlspaces or attics where they deposit moisture, which can encourage the growth of mold, wood decay, or other material problems. Sometimes they will terminate just beneath attic ventilators. This is a defective installation. They must terminate at the exterior and away from a door or window. Also, screens may be present at the duct termination and can accumulate lint and should be noted as improper.

M1502.3 Duct size. The diameter of the exhaust duct shall be as required by the clothes dryer’s listing and the manufacturer’s installation instructions.

Look for the exhaust duct size on the data plate.

M1502.4 Transition ducts.Transition ducts shall not be concealed within construction. Flexible transition ducts used to connect the dryer to the exhaust duct system shall be limited to single lengths not to exceed 8 feet, and shall be listed and labeled in accordance with UL 2158A.

Required support for lengthy ducts is covered by the following section:

M1502.4.2 Duct installation.Exhaust ducts shall be supported at intervals not to exceed 12 feet and shall be secured in place. The insert end of the duct shall extend into the adjoining duct or fitting in the direction of airflow. Exhaust duct joints shall be sealed in accordance with Section M1601.4.1 and shall be mechanically fastened. Ducts shall not be joined with screws or similar fasteners that protrude more than 1/8-inch into the inside of the duct.

Furthermore, makeup air for the laundry room must be provided in an amount equal to the total – in cubic feet per minute (CFM) – of the dryer vent fan and any laundry room fans when both are running. This could reach up to 300 CFM, depending on the size of the laundry room. This air must come from some source. If the door is closed and there is no window, it may lead to issues such as longer drying times and decreased dryer vent flow, potentially causing lint buildup in the exhaust vent, which is a fire hazard.

Typically, an inspector may not be familiar with specific manufacturer recommendations or local codes, and therefore cannot verify the dryer vent's compliance with them. However, they can identify issues that might need correction.