my new dryer vent was installed down into my crawl space approx. 12" then a 90, then about 5' horizontal to the outside termination. problem was it was to low to meet the 12" height requirement from the ground at the termination. called the installer back, and he just went vertical again for about 6" then back out. so my question is before i call for another inspection..is it ok to vent down then horizontal, then back up? it creates a u type shape. is it permissable since it only goes back up a short distance?