In high school I had a teacher who, when exasperated, used to say,  "Boys I don't like to cuss, but "%$*@#" sometimes you make me".  And so it is with some Dryer Vents.

.Dryer vent runs should be as short as possible. Time was dryer vents were no problem. The dryer was on an outside wall and the vent went straight out and was very, very short. As a matter of fact the shortest pipe possible was even too long and had to be cut down to fit. But now for the sake of convenience utility rooms are many times placed in the middle of the house making it nessesary to be vented straight out the attic and on to the roof. Not only is the dryer fighting gravity to push the lint out, it must fight other factors. First of which is that most builders don't give a second thought to what type of vent they use to terminate the run. Most use an ordinary house vent which has a fine mesh screen on it. It onloy takes a few years to build up to a point where no air can escapse, no matter how strong the dryer blower is. At this point it takes clothes two or three cycles to dry a load of clothes. And even worse it will cause the hating element to burn out requiring a costly service call. 

Some are tempted to go on top and rip off the screen and do nothing else to service the vent. This is not a good idea because what that does is open up the dryer vent to other problems. Birds will get in and next in the vent clogging it up completely.

Some builders put on a vent that is especially manufactured for dryer vents and hope they are doing right. But there is a huge disconnect between roof top dryer vent makers and we who clean them. I have yet to find one that allows for easy cleaning. Most require removing, altering and reinstalling to make it work properly.

We have run into some vents that are impossible to clean properly. Those are the ones  that go in the wall turn 90 degrees down then 90 degrees to the outside wall. Once it runs to the outside wall it turns to go up and turns again to go out the wall. In effect you have a 'U' with a long bottom. What happens is as the lint is extracted along with the humidity it never makes the turns and stays in the bottom of the 'U'. It putrifies in there and water collects stopping the vent from doing its job. We have seen vents with 5-10 gallons of water in them.

We suggest that these runs be rerouted to make cleaning a routine matter. One home we did that had a bad vent was like this. The utility room was on one end of the home and it was vented in the slab to run out the opposite end of the home. It was a run of about 50 feet. No explanation could be given as to why it was done that way except for the sobriety of the builder. We qucikly rerouted the vent and 'voila' no more problem.