Tommy Mac Discussions › Forums › Fix-it Forum: Home Improvement & Do It Yourself Repair Forum › venting a gas dryer › General Guidelines.
The general guidelines are that a dryer vent should not be longer than 14 feet, with a 2′ deducation for each 90 degree angle. I am a little softer and go 20 feet with the deduction. The 14′ might actually be an enforced code on new homes.
Another vital point is to not use any screws and make sure the male end points to the outside, not back to the dryer. Why? Lint catches on screws and junctions and builds up. When it builds up, a fire can start inside the dryer that will blow off the door throwing flaming clothes inside the house. A cause of fires each year.
My general advice for you is that you should consider all outside wall options, including closets for the dryer. You sound like you have no choice, it sounds to me like you have to choose no.
A safer venting would be through the roof, given a single story dwelling over crawlspace.