As part my inspection routine, I always look at appliance connections. This picture shows an improper dryer exhaust system.
This dryer is vented with a flexible plastic duct. This should never be used for dryer venting. The vented air can be quite hot, especially with gas-burning dryers. This material will melt or burn very easily if temperatures become too high, which is especially possible with lint buildup creating a fire hazard. Lint buildup is likely due to the narrowing of the duct at corners and along the length. These comments apply to foil ducts as well.
The proper material is rigid stainless steel duct, or semi-rigid stainless steel duct, as permitted by installation instructions. This creates a smooth duct for air and lint to exhaust easily as well as a resistance to much higher temperatures.
The path of the duct is also important. The rule of thumb is that ducts should not exceed 25 linear feet. This is to be reduced by 5 feet for a 90 degree elbow and 2.5 feet for a 45 degree elbow. The pictures duct essentially has four 90 degree elbows, leaving only 5 linear feet to run the duct.
Lastly, this duct is connected with actual duct tape. In spite of its name, duct tape should almost never be used on any ducting, particularly if the ducts will carry hot air. It can cause air leakage and it is not heat resistant. Dryer ducts should be connected with aluminum tape or appropriate ring clamps. Note that the use of screws will promote lint build-up and will not provide an air seal at the connection.
An improperly installed exhaust system can cause your dryer to run less efficiently. It can reduce the lifespan of the dryer and result in humid air escaping into your home. In rare cases, it can lead to fires.
Always ensure your appliances are installed in accordance with the manufacturers instructions which can be found in the owner’s manual or by contacting the manufacturer.