Hello again, the 18" round duct, 8 feet long is 15% small for return air requirements of 350 CFM per ton of cooling capacity. 350 x 4 = 1400 CFM
Thats 792 feet per minute CLICK HERE FOR THE CALCULATOR
Thats 15% above the recommend maximum velocity, but with no 90 degree bends and only 8 feet long it will be passable in most cases depending on the size of the supply air ducts and the *total external static pressure
of the duct system.
If the supply air ducts are all 15 to 20% undersized then the system will be restricted for net air flow..and that would make a significant difference in the systems over all performance.
But if the supply air ducts are not undersized as well, then the 18" return duct, only 8 feet long and with no 90 degree bends will be passable... not ideal at all, but passable. This is a common problem with two story houses where getting large enough spaces to run duct work is a problem.
Do your schedule D calcs on the entire system, with pressure drops figured at the registers per the specs on the particular registers used, and the supply air duct sizes and lengths, and the get the total static pressure drop... compare that to the total external static pressure capabilities of the air handler or furnace used.
If you do not exceed the total external static pressure of the air handler or furnace there will generally be no problem at all... however if you do exceed the total static pressure capabilities of the air handler or furnace by even 10%, you can suffer a loss of air flow in the 20%* range and that will definitely affect performance and leave the system more prone to icing the coil when the house is warm inside and the outside temperature is below 75F... an iced coil stops all air flow.
The issue here is the practical issue of how to fit large enough duct work into the structure given the limited space available as the contractor stated. It would be good if you find a way to run an additional return air duct at least 12 inches in diameter.
If you rate my response *positively I can hold the question open for follow up.
( It is 350 CFM per ton in Florida, very low air flow per ton, because of the very high humidity there... the humidity (steam essentially) carries tremendous heat with it, allowing for less air flow, so we can run very easily at 350 CFM per ton, as compared to 400 to 500 CFM per ton in dryer climates)