Hello, It is never a good idea to screw around with something that works. In theory it could possibly work, but will probably have hot spots. This is the reason current best design has seperate systems for each floor. You could try to close off the chute and see if it works adequately. This needs to be done on a hot day. The location of the cold air return in comparison to the stairs may be important. In theory once again air circulation needs to have the supply at one side and the return at the other side. IMO each room needs adequately return air or the room will not cool and heat properly.

They're selling snake oil. There are so manys thing wrong with this proposal. First off, why do you feel you're not getting enough airflow? Is the room they used for their demo hard to heat or cool? If the existing 370 cfm is enough to keep that room comfortable, then there is no reason to pay money to increase the airflow. "More" is not always better. Your air handling system was designed to move air as an enclosed system. OF COURSE it's gonna move more air if you rig it to operate with the fan cover door opened. But again, more is not always better. I'll bet you could stand in that back room and *hear* the air rushing and whistling out of that vent after they took the blower door off. Your system should be set up to move the required amount of air as quietly as possible. You're also risking damage to the blower by making such a drastic change to the static pressure. The fan is designed to work against some amount of resistance. Lessen or remove that resistance and you could burn your fan motor out. A return in the basement is going to bring extra humidty and musty odors into your home. The air conditioning unit would have to work harder to remove the extra humidity. It could actually cost *more* money to operate. And if the unit was not capable of removing the extra humidity, you end up with damp air in your living quarters. This results in a cold, clammy feel that makes you feel uncomfortable. If you have some rooms that are hard to heat or cool, have a reputable HVAC company *calculate* how much air needs to go to each room. It's not a guessing game, and more is not always better. I noticed that you called the original installers and they blew you off. Forget them, they don't sound very professional. Try asking some of your neighbors for referrals, or check something like "craig's list". There has to be a reputable company in your area.

Are there other returns beside this one? A 6" x 20" return by itself would be too small for the smallest furnace or AC. If there are additional returns then partially blocking this one may be alright. Will the back of the love seat be placed against the wall? If it is then see if you can pin the back skirt so it stays up and keep the love seat 6 inches away from the wall in order to allow air flow.

Paint your current return the same as the wall color and you'll notice it far less. I don't personally care for the decorative covers, as a cold air return is perfectly ok as a functional-only element in my opinion.