there are allot of great HVAC installers who take the time to size and install their work right, but I hate it when someone says 'rule of thumb' on HVAC issues. 4 diffusers per ton is really vague. That's anywhere from 100 to 150 CFM per diffuser when no size of diffuser is given. Diffusers are rated on CFM, velocity, noise rating, and throw. Others might be a certain sized return per ton, or a certain size ductwork per ton. I can't count the number of times I have investitigated an HVAC system and found undersized diffusers, ductwork, or return grills...all because someone other than a professional sized the system. It really does need to have a professional design to operate correctly. And in these days of low bid, its really getting bad out there for the consumer to get a proper system. Corners are being cut at every opportunity. I have been asked by the local supply house to give classes, because they see so many contractors out there doing such a poor job. I had a next door neighbor and a pastor at church ask me what was wrong with their new A/C systems after they were installed! I would have sized it for them for nothing, but, like plumbing, everybody thinks they can do it without a professional. The best way to get a good installation is to have a pro design it and then all bidders are giving you the same system tonnage, ductwork, and returns. I've seen it done wrong too many times. Thanks for letting me vent.