I have a york affinity forced air furnace and was wondering - If the vent pipe is coming out of the top of the unit, does the pipe have to go a minimum distance before putting in the first elbow? Right now the first elbow is 4 feet straight up from the unit and i want to drop it down to 18 inches from the unit so i can make room for a humidifier. It wouldn't change the overall length of the pipe and would have the same number of joints, it would just be shifted down a few feet.