I have recently had three different contractors out to see about switching to a high efficiency furnace. Two said they could do the horizontal vent and one said he could not due to a crowded joist space and the distance of the run. (9" joist height over 24') Is the quarter inch drop per foot the standard on the vent, and are there exceptions? I can't quite get that on 24'. What is the maximum distance the vent can run? Also, there is only room for a single vent system. Any ideas?