Is your dryer safe and efficient?

by John Cranor of Cranor Inspection Services, Glen Allen, VA  804-747-7747 

Read ASHI published dryer exhaust article here


Is your dryer costing you more to operate than it should? Is your dryer exhaust system safe?

Dryer exhausts ducts are all to often not given due respect, rarely given much consideration in the design stage, often installed haphazardly, and never maintained.

Generally speaking the dryer exhaust system is rarely given another thought after installation.

The “thinking” on the importance of dryer exhaust ducts needs to change!  Homeowners need to understand what the issues are, familiarizing themselves with the modern standards, know how to recognize symptoms of potential hazards and most importantly make improvements when necessary.

Clothes dryers have historically either been located in basements or generally within a short distance from an outside wall that made maintenance less critical and the potential for trouble minimal.

Today’s homes have laundry rooms located practically anywhere in the home. Often the dryer is installed on upper levels of the home with long concealed runs with several elbows (as opposed to short efficient runs of duct often utilized in older homes i.e. out the close exterior wall or a short distance through the crawl space).  Due to the tightness of modern homes, the proper operation of the dryer exhaust has become even more critical, opposed to older homes. 


Why exhausting is critical?  


Moisture …….A typical load of laundry has one gallon or more of water to dispose of.   Because of the moisture, exhausting the dryer to the outside is a must since indoor exhausting can create conditions to support mold growth and degrade indoor air quality.   See photo below.


Carbon Monoxide Proper exhausting is of particular importance with gas dryers.  In addition to moisture and lint a gas dryers exhaust duct carries the by products of combustion, including carbon monoxide.


Clogging…. A large amount of lint is produced during the drying process. The lint can restrict or block the dryer duct. A poorly exhausting dryer is not only less efficient; it can also be a fire hazard due to the extreme flammability of lint.   Clogging can also cause moisture to accumulate and seep out the duct, leading to moisture damage.


What causes the fires?

The U.S. Consumer Product Safety Commission estimates that in 1998, clothes dryers were associated with 15,600 fires, which resulted in 20 deaths and 370 injuries.

Lack of maintenance is the major contributing factor but poor installation and/or the use of improper duct material all have rolls in leading to fires.   A dryer’s lint filter only catches a fraction of the produced lint some are better than others.  Overtime the lint accumulates reducing airflow causing the dryer to operate at elevated temperatures and increasing the chance of something malfunctioning and/or catching the lint on fire.  Dryer fires usually start beneath or inside the dryer. The draft from the dryer then forces the fire into the exhaust duct and, in many cases, causes a house fire.  The probability of the fire spreading greatly increases with the use of plastic or Mylar (foil) duct, especially plastic. 

U.S. Consumer Product Safety Commission has published research that indicates that a 75% blocked dryer exhaust duct elevates the exhaust air temperature of the average electric dryer 89% more than its normal operating temperature with an unblocked duct.

Problem Conditions

Most often the dryer is pushed as close to the wall as possible, this very often crushes or kinks the transition duct, which immediately slows the flow of air and creates a trap for lint to collect.   The transition duct should not exceed 8 feet in length, should be UL approved material (not plastic), should be limited to a single length, and should not have any part concealed within construction.  Periscope type connectors (sometimes called Banjos) eliminate the common problem of kinked and crushed transition ducts behind the dryer. Metal foil tape should be used on the ends of the periscope to seal any cracks.  Air leakage disrupts the efficient flow of air.

Dryer exhaust ducts in many homes tend to sag, (not guided through the rafters and/or properly supported) or makes sharp turns or humps up. The sharp turns, humps, and sags create turbulence preventing all the moisture and lint from blowing out.  The common spiral ribbed flexible ducts by mere design create turbulence and reduces airflow.  Then all too often the termination sleeve gets bent inward during connections, creating a dam for lint to snag.  Some older termination hoods are restrictive by design, not allowing the damper to fully open.     These common problem conditions along with exceeded the recommended duct lengths and not doing routine maintenance elevates the potential for lint to become trapped within the duct system leading to blockage.

What’s being done to help reduce dryer fire risks?

The slow elimination of dangerous dryer exhausting products from the market over the last two decades has been the most significant change or step toward reducing fires.   Years ago, the standard dryer duct was a white vinyl wire coil duct. Not only was the vinyl susceptible to overheating and melting, the coil construction created turbulence, reducing air flow leading to lint accumulations within the ridges.

A Mylar-covered wire coil duct replaced the white vinyl duct. Mylar is a shiny metallic-coated polyester film (often called foil), and is more heat-resistant than vinyl, but still a potential problem for many of the same reasons.

The use of metal exhaust duct is now the standard, recommended and/or required by Underwriters Laboratory (UL), the Association of Home Appliance Manufactures, the Consumer Product Safety Commission, most (if not all) dryer manufacturers and the building code.  The use of all-metal (rigid or semi-rigid) dryer exhaust duct helps to achieve optimal airflow reducing lint buildup and reduces the operating temperatures of the dryer.

Warning Signs!

  • Clothes take an unusual long time to dry.
  • Clothes are hotter than usual at the end of the cycle.
  • Outside of dryer is unusually hot
  • Damper or flappers on exhaust termination doesn't open or barely opens when dryer is on.
  • Laundry room feels warmer or more humid than normal.
  • Unexplained moisture stains in concealed dryer exhaust duct area.
  • Burnt smells in laundry room

Current industry standards

  • The dryer exhaust system must be independent of all other systems and convey the moisture and any products of combustion to the outside of the home.  The duct must not exhaust to attics, crawl spaces, basements, chimneys, the cavity of any wall or any interior room. 
  • Dryer duct must be at least 4" diameter or at least the size of the dryer outlet.  The exhaust duct must not extend into or through HVAC ducts or plenums.  The exhaust duct system should be supported and secured.
  • The maximum length for a clothes dryer exhaust duct should not exceed 25 feet.  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.  This does not include the transition duct.
  • The dryer exhaust duct should be constructed of rigid metal (aluminum or galvanized steel).  The interior of the duct should be smooth surfaced with the joints running in the direction of the airflow.  There should be no sheet-metal screws, rivets or any other fastener used to connect the duct joints. The use of fasteners that extend into the airway will catch lint and obstruct airflow.  Clamps or foil duct tape should be used to secure joints.
  • Outside termination must be equipped with a back draft damper, which prevents moisture/air intrusion and/or the entry of small animals.  The termination must not have a screen covering the exhaust outlet.


Clothes dryers should always be exhausted in accordance with the manufacturer’s instructions. If the make and model of the dryer is known, the manufacturer’s instructions override other standards.  Many manufacturers installation instructions differ and exceed the standards i.e., many exceed the 25 feet duct length (as much as 3 x), some specify to avoid 90 degree turns, some approve semi-rigid duct but specify it should be fully extended, some specify that semi-rigid cannot be used in concealed areas, some specify clamps at joints and say no tape, some specify tape or clamps, at least one specifies that ducts in unconditioned spaces should be insulated. 

Good Practice
  • If at all possible, rigid aluminum or rigid galvanized steel duct should be used (especially if concealed). If flexible metal duct must be used, use the semi-rigid type.
  • The dryer duct should be as straight and short as possible. Minimize 90-degree turns. Sharp turns cause backpressure and create resistance to airflow. Two 45-degree bends are more efficient than one 90-degree.  For best performance separate all turns by 4 feet of straight duct.
  • Duct joints should be wrapped with foil tape to make air and moisture tight.   Avoid the gray duct tape, it will deteriorate over time.
  • The hood should have at least 12 inches of clearance between the bottom of the hood and the ground or other obstruction. The hood opening should point down.
  • Ducts should always have adequate support especially at each joint.
  • A dryer duct should never exhaust near the fresh air intake of
    a high efficiency furnace, water heater or any HVAC intake.
  • Rigid seamed exhaust duct ideally should be installed with the seams up to prevent any accumulation of condensation from seeping out.  When possible horizontal runs of dryer ducts should slope slightly downward (1/4” per foot) toward the exterior termination to reduce the possibility of condensation accumulating and collecting lint. 
  • In cold climates, insulating the dryer exhaust duct in unheated spaces may help to limit the condensation from forming inside the duct and collecting lint.  Note: At least one dryer manufacturer specifies this in their installation instructions.
  • Exhausting a dryer near or next to an air conditioning or heat pump condensing unit should be avoided.  The expelled lint can collect and clog the condenser fins and will likely require continual condenser cleaning.  
Be aware
  • The common Mylar (foil) and white plastic spiral flexible ducts contribute to poor dryer performance.   These spiral ducts are no longer recommended and will void the warranty of most (if not all) dryers if used.
  • The use of PVC pipe is not recommended and against all standards and specifications - see photo below.
  • Magnetic dampers on exhaust hood are restrictive and should not be used on dryer exhaust systems.
  • Venting up or through the roof although possible should be a last choice. These systems clog up faster, demand close monitoring and routine cleaning for safety and efficiency.
  • A common problem with venting through a roof is that the wrong type of termination is used.    
  • Dryer exhausting out through the roof should be ducted to a special dryer roof vent without screen, or tight louver, or any type of filter that can clog with lint.  The roof vent or commonly called "Roof Jack" must be equivalent to a 4” wall termination in regard to resistance to airflow, back flow prevention and should require little or no maintenance to prevent clogging. The termination must be air tight on the attic side so damp air don't blow back into the attic.  I have two photos below, one is the correct roof termination and one is a commonly used incorrect type. 
  • The older dryer exhaust hood terminations have a 2 ½” hood opening which are restrictive by design, they are often clogged and stuck open. The newer versions have a full 4” hood opening or a series of flappers allowing unrestrictive exhausting and improved performance.
  • Mobile home exhaust ducts also must be secured to home and should not discharge under the home.
  • Clothes dryers require makeup air; if the laundry area is too tight it will effect the operation and function of the dryer exhausting.  Another important point is that dryers operating in utility areas with gas appliances that lack sufficient makeup air can create a dangerous back drafting (CO) problem.
  • Dryer sheets leave a residue on the dryer lint screen, limited use and frequent cleaning (with soap and water) is recommended.

   Correct type of roof vent

               Wrong                                             Correct


What about vent buckets or lint traps?


A vent bucket (AKA lint trap) sold in hardware stores everywhere is a device that you fill with water and connect to a clothes dryer duct and the water collects the lint.  Although commonly used, especially in older homes, they are not recommended.  They have a limited ability in catching the lint and they allow gallons of water to be pumped into the home creating a potential air quality issue.  Whether or not a true problem will arise from this will depend on many factors such as, the climate, the house, how often used, etc.    Along the same principle, you occasionally see a woman stocking hose being used to filter lint, which is considered a fire hazard!



Dryer Exhaust Booster

Booster fans can be used to exceed the maximum 25 feet or help with vertical applications when necessary.  The recommended location of a booster fan is a minimum of 15 feet from the dryer outlet, if mounted closer it may develop enough pressure to lift or pull wet lint into the fan impeller resulting in excessive lint loading in the fan.   The best location for the fan to be mounted is as close as possible to the termination.  (Exception: If a secondary lint filter is installed between the dryer and the booster fan, the mounting location can be within the minimum distance.  The fan must be mounted securely and must be accessible to perform recommended maintenance.  If the fan is not readily accessible from the dryer room, it is recommended that a permanent label be placed near the dryer transition duct stating that a remote booster fan is used   A pressure switch activates booster fans automatically and the fan cycles on and off in 10 minute intervals.  Angled and horizontal mountings are possible but vertical mounting is recommended to reduce condensation build up in the fan.  Horizontal mountings require a ¼” hole drilled in the bottom (along with the proper drain insert and drainage tube) to allow condensation to drain.   The diaphragm pressure switch should be positioned vertically for optimum sensitivity.  

In Conclusion

Close monitoring and maintenance of dryer exhaust systems are more necessary (or more critical) today than in the past, newer houses tend to have dryers located further away from an outside wall. These new locations mean dryers tend to be vented longer distances and ducts are often installed with sharp turns and bends to accommodate the structure of the home. As a result, dryer ducts often have more places where lint can collect and animals and birds can hide or nest.  Also there are more gas dryers being used today, which is another reason why dryer exhaust duct systems warrant careful maintenance.

The dryer duct is sometimes concealed and making it difficult to evaluate but an alert home owner should remain watchful of the warning signs and knowledgeable of the issues. Call for professional help when you need it.

People die and property is destroyed as a result of inadequate or clogged dryer ducts. Unlike many issues that homeowners face, this one has a relatively easy and economical solution.  If it’s dirty…clean it!  If it’s not up to standard or no longer considered safe …. have it replaced or corrected!   

Safety suggestions 

  • Have dryer and exhaust ducts inspected and cleaned annually.
  • Always follow dryer manufacturer’s recommendations.
  • If the exhaust duct system is not up to current standard, have it replaced or corrected.
  • It is good practice to always run the dryer long enough so that the last few minutes push only hot dry air into the duct. This will help to clear out any remaining moisture.
  • Install a fire extinguisher in laundry area but not over the dryer.
  • Install smoke alarm in laundry area.
  • Install CO alarm in laundry area (Gas dryer only).
  • Clean lint filter before every cycle.
  • Clean lint filter screen periodically with soap and water.
  • Replace a damaged lint filter.
  • Monitor exterior termination for flapper function and airflow.
  • Use caution not to crush or kink transition duct behind dryer.
  • Never operate a dryer while sleeping or away from home.
  • Concerns should be dealt with immediately.

Found during a Home Inspection - mold/rot - caused by a dryer duct


An example why you don't use PVC for a dryer duct


CALL John Cranor at 804-747-7747 for your home inspection needs.

Contact for dryer exhaust service.

Contact Tri-Co Appliance Service for dryer repairs  804-346-8134