Reuben's Home Inspection Blog

Keeping your clothes dryer safe

August 7th, 2012 | 1 comment

According to Underwriters Laboratories, clothes dryers are responsible for approximately 15,000 home fires each year.    It’s not hard to believe.  Improperly installed and improperly maintained clothes dryer ducts are one of the most common issues that home inspectors find, but it’s not that difficult to keep your clothes dryer safe.  Today I’ll discuss dryer duct installations and maintenance.


In Minnesota, the installation requirements for clothes dryer ducts can be found in Section 504 of the Minnesota Mechanical Code.   I won’t go over all of the requirements, but I will mention a few of the items that most people have questions about.

Dryer duct length – the maximum allowable length for a clothes dryer duct is 25′.  Each 90 degree turn in the duct is worth 5′, and each 45 degree turn is worth 2.5 feet.  For a typical basement in Minneapolis or Saint Paul, the clothes dryer has a 90 degree turn in the duct right behind the dryer, then another 90 degree turn at the ceiling.  Assuming it’s a 7′ ceiling, this leaves 8′ of run before the dryer duct needs to terminate at the exterior, according to these requirements.

While many clothes dryers have a longer run than this, it’s not always a problem.   There’s an exception to the rule which says “where the make and model of the clothes dryer to be installed is known and the manufacturer’s installation instructions for such dryer are provided to the code official, the maximum length of the exhaust duct, including any transition duct, shall be permitted to be in accordance with the dryer manufacturer’s installation instructions.”.  

In other words, if the dryer manufacturer allows a longer duct, no problem.

I’ve read a lot of dryer installation manuals, and in every case the dryer manufacturer allowed for a much longer run than what’s allowed by code; for instance, a 29″ Maytag clothes dryer allows for a 100′ duct when two elbows are used.

I once inspected a condo conversion building in Saint Louis Park where the contractor had attached a placard to the location where the dryer would go, warning that the maximum length of the drier (sic) vent was limited to 30′ with three elbows.

Dryer vent placard

Dryer duct construction – dryer ducts need to vent to the exterior, be made from metal, be at least 4″ in diameter, and have a smooth interior.  The entire duct needs to be supported and secured, and no screws are allowed on the joints because they could accumulate lint.  Flexible materials, such as foil, plastic, and semi-rigid metal aren’t allowed.  Those are all common materials used for a dryer transition duct – the material that can be used to get from the dryer to the duct.

Dryer transition duct

The terminal for the dryer needs to have a backdraft damper, and no screens are allowed at the dryer exhaust.  When screens are installed, they get clogged with lint.  This reduces the performance of the dryer and creates a potential fire hazard.

Clogged dryer duct

Clogged dryer terminal

I’ve heard concerns about pests getting in to the clothes dryer duct if a screen isn’t installed, but if the backdraft damper at the exterior is kept clean, this shouldn’t be an issue.

Transition Ducts are a big enough topic to deserve their own post.  I’ll write about these next week.

Clothes Dryer Maintenance

Clothes dryer maintenance is actually quite simple: keep it clean.  Dryer lint is flammable, and the more that accumulates in the dryer and the duct, the greater the risk.  The most obvious and routine part of this is keeping the lint screen clean – it should be cleaned after every load.

Periodically check the damper at the exterior to make sure it’s clean; when lint accumulates at the damper it will eventually cause the damper to stay open.

Dirty dryer terminal

If you’re unfortunate enough to have a dryer that exhausts through the roof… I’m sorry to hear it.  Someone needs to get up there on a regular basis to clean the damper.

If you have a dryer duct that passes through a concealed or infrequently visited space, such as an attic or crawl space, you should check on it periodically to make sure that everything is still properly connected.  A disconnected clothes dryer duct will exhaust a ridiculous amount of lint and moisture in to the home.

Disconnected dryer duct in crawl space

Take a peek behind your dryer with a flashlight periodically – it’s quite common for the dryer to come disconnected from the duct, which makes a big mess of flammable lint behind the dryer.  The dryer duct itself needs to be cleaned periodically as well.  You can find instructions for cleaning your own dryer duct here –  This sounds like a good project for the next rainy weekend.

I’ll have plenty more to discuss next week, on the topic of clothes dryer transition ducts.

Reuben Saltzman, Structure Tech Home Inspections


Dryer Safety

January 17th, 2009 | No comments

The U.S. Consumer Product Safety Commission estimates that in 1998, there were 15,600 fires related to clothes dryers.  These fires caused 20 deaths and 370 injuries.  Dryer ducts are one of the items that I most commonly find problems with when I inspect homes, but keeping your dryer safe is usually an easy thing to do.  The two problems I find are improper installations and lack of maintenance.

Improper installations on dryer ducts are rampant.  The most obvious, and possibly most common defect, is plastic dryer ducts.   Plastic is not allowed for dryer ducts because lint can easily accumulate in the duct when it sags, and plastic will not contain a fire.

Plastic dryer ductClogged Dryer Duct

Flexible foil ducts are allowed, but these are little better than plastic.  Any plastic dryer ducts should be considered a potential fire hazard, and should be replaced with rigid metal.  If flexible material must be used, a maximum of eight feet is allowed.  Use the metal type, like the kind pictured below.

Semi-rigid metal duct

Another common installation defect is the length of the ducting.  The Minnesota State Mechanical Code states that dryer ducts may be no longer than 25 feet, and for each 90 degree turn, you subtract 5 feet.  Most dryer ducts make a 90 degree turn as soon as the duct comes out of the dryer, run up to the ceiling about 8 feet, and then make another 90 degree turn at the ceiling.  This adds up to 18 feet, and the duct hasn’t even run horizontally yet!  Some dryers are designed for longer ducts, and the code allows a longer run if the dryer is made for it, but you need to check the dryer installation instructions.

I find most problems with the length of dryer ducts at condos and townhouses, but I have also come across a few interesting solutions recently.  At a condo in Saint Louis Park, the placard pictured below was attached to the wall – I was very impressed!  At a condo building in Saint Paul, an inline pressure-activated fan had been installed in the dryer duct to allow the duct to be much longer than 25 feet.  Sorry for the picture quality – I only had my phone with me when I came across this.

Dryer (Drier?) Duct PlacardDryer Duct Booster

The last common problem I’ll discuss is blocked or clogged ducts.  This is typically caused by ducts made of improper material or ducts that are too long getting clogged with lint, but can be prevented with regular maintenance.  If there is a screen cover at the dryer exhaust (such as the one pictured below), remove it – these are unnecessary on dryers, and they are not  allowed.

Clogged Dryer Vent

I once inspected a dryer exhaust at a 20 year old townhouse because the homeowner complained that their clothes got very hot in the dryer, but never dried.  I found about three inches of lint packed at the exhaust because a screen was clogging it! Clean the lint filter between each load, and clean your dryer duct periodically, or have the cuct cleaned professionally.

Reuben Saltzman, Structure Tech Home Inspections – EmailMinneapolis Home Inspections