An 8x12 duct gives you approximately 500 cfm's(cubic feet per minute) of air. You need at least a 10" round duct to achieve the same results. You should have at least as much cold air return as you do warm air supply. It could cause you to get less air to your registers. By decreasing the size of the return it would be like covering some of your returns upstairs. A 6" round gives you 110cfm's, an 8" would give you about 250. Lack of proper return air will also cause your heat exchanger to overheat shortening its life. You will only get as much heat out as you have return air in. The blower pulls the air in and forces the warm air out. With no or little return air there will be little or no warm air out. It's like the finger over the liquid filled straw trick. Cover the top and the liquid stays in, remove the finger and you release the vacuum. Same principle.

Thanks, spot09. Your answer what about what I expected but not what I was really hoping to hear. My trunk is 12.25"x8". That must be fairly common. I searched the net and a company has a 10" adapter.

Fortunately, I think can remove an existing 6' section of trunk and use the 10" flex line adapters on the ends that remain. I'm thinking that size flex should give me sufficient air flow, don't you. My bends shouldn't be more than 15%.

The basement stays fairly comfortable so I don't think I need insulated flex especially since it's a cold air return. Thanks again for the information. Gene

Gene, Sorry for late reply but you should be fine with the 10" flex, and since it is return air there's no need for insulated flex. The only concern I have with running flex on the return is when the blower comes on it pulls or sucks the air in the flex and out the supply. I have a friend that does commercial and he told me that they won't use it because of collapsing problems, due to the suction. I have never seen it but thought it was worth a mention. Maybe because they're blower is so powerful compared to residential.

Hello, spot09. Isn't THS great for finding a sounding/advice board. It sure makes sense to be aware of what your friend said. Thanks for bringing it up.

My furnace is a 125K btu and supplies 2000 sq ft. I have and older rancher with a full basement. The air flow has never seemed hardly strong enough to sway the curtains. I'm pretty sure the motor is a 3/4 horse.

Since I will only be doing around 6' I could use insulated flex. It might withstand a little more pressure. I'll ask at Sun Supply which is a local heating/cooling contractor material supply company.

I sort of smile thinking it might be when the really cold weather hits these Western North Carolina mountains before I get started. I sound sort of like Snuffy Smith and the leaking roof. Can't work on it when it's raining. Don't need to when the sun is shining. Thanks again, Gene

Snuffy Smith.... I like that comparison!!