Clothes dryer safety is about installation and maintenance

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 installation requirements 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 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.