Since the dryer removes the water from wet clothes by heating them, the warm and moist air must be released for achieving the end result. The air is released through a simple exhaust system which consists of a duct connecting the appliance to the outside of the house. There is a cover on the outside opening. The warm air carries lint as well. Over time, it sticks onto the walls of the duct and causes pressure on the air. This can lead to fire without proper exhaust system cleaning.
A single load of clothes does not produce a large amount of lint. But as you keep using the dryer, these light yet sticky fibers get built up into ever thicker layer. This happens gradually and fairly slowly in most cases. The rate at which the lint layer grows depends on various factors from the frequency of using the dryer to the amount of types of dried clothes. The good thing is that when the build-up increases to a dangerously high level, there are clear signs of trouble.
One of the first signs of excessive lint accumulation is that the clothes are still damp after a normal drying cycle. They may have slight musty smell as well. You will need to run the appliance for longer in order to get the desired result. Once you take the clothes out, they will be quite hot. It is easy to notice the difference between normal dryer operation and functioning with clogged exhaust system. The important thing is to opt for dryer vent cleaning as soon as you notice the problem.
There are other bothering signs which are quite easy to notice. The room where the dryer is will be much hotter while it operates. The same applies to the walls of the appliance as well. The room air may also feel more humid as well as warmer. If you notice smell of burnt, you must stop the appliance immediately and have the vent inspected and cleaned. If there is damage, the required dryer duct repair must be made right away.
As you know, the lint filter has to be cleaned after every drying cycle. If you do this, you will easily notice when something is wrong. In case the vent has become excessively clogged, there will be a larger than the usual amount of lint built up on the filter. You can also do a simple exam. You need to go outside and see how much the flap opens during the operation of the dryer. If it does not open fully, it is time for air duct cleaning.
Finally, it is best not to wait until you notice these bothering signs. Based on previous experience, you should schedule the regular cleaning of the dryer's exhaust system.