It really depends on the situation and every home and need are different. In the basement overcooling situation, you gotta do it for the comfort. Some people have insulation between the basement and living space so you don't want to lose your heating to that area. Then there is the problem with imbalanced pressures. If there are supply duct openings, where is the air going to go? If the basement is closely coupled to the area with a return (upstairs?) and you don't mind a little air down there to dehumidify, or warm it up a little bit, leave it alone. If the basement is sealed off from the return upstairs, it may leave the area more closely coupled to the outdoors. It will be pressurized and the air will tend to leak out through the openings and cracks to nature. That will, in turn, depressurize the living space and suck unconditioned air from random cracks and openings up there. It might even come down some flues. If that is going on, you gotta fix it. You can do it by adding a return or sealing the supplies. Ducting systems are complicated. I hate them.

Given that you're renovating I would say it's a good time to do it. It can raise the costs dramatically depending on a myriad of factors. If what you have is flex duct / going back with flex duct the costs shouldn't be too extreme, but this can lead to boot replacement and vent replacement so that can increase costs further than just duct work alone. You should replace with at least R8 flex duct if you are going back with flex. This can pose challenges because circumference wise these ducts will be larger than what you have now most likely. By using a minimum R8 insulated flex duct, this will increase the efficiency of the system, marginally in most cases depending on climate. It also wouldn't be a bad idea to possibly add an additional new filtered return at a known hot spot / cool spot within the structure. (hot or cool --dead air space that might be getting trapped, by adding a filtered return to this spot it will aid in the comfort factor of the structure.)

I had posted my plan a while back. Here is the part of the plan that shows the older space and the dropped ceiling area in the addition is the shaded. Currently, the HVAC guys propose one supply vent on each side of the fireplace and no ducts leading to the other side of the space. There are no soffits or closets in the older space. Those shadowed vertical bars you see on the right side are the exposed beams and they show how far the eaves extend. Thanks

Elmer's inquiry deserved more of a answer than your chest-thumping, self-promoting, California-bashing commercial. The climate around Katy, Texas is certainly not "one in a million." It's similar... Just "similar" but not exact... meaning different. Me and Mr. Fudd's disagreements are well documented on this board the story continually evolves from duck to rabbit season and back again. One in a million is a generalized comment... a saying. But you Mr. Ross love to find something to argue about for the sake of arguing. Distinctly different? That's rich. I find it humorous to what people like to nit pick over. It doesn't change anything. I don't change anything about the comment I made. You can disagree if you want, I still say Katy, Texas is indeed "one in a million"... at least. Ray is a one-person company. He lives in the replacement contractor world. When someone is without heat, air conditioning or hot water, it's often the first replacement contractor to arrive that "wins." Not hot water... I am not a plumber. Texas licensed for AC and heat only (HVAC). Yes essentially that is the "service" I offer as well as the part of the world I live in. Air conditioning and Furnace repair service in Katy, Texas area. When I say "service" I mean it. Every day, including summer holidays, weekends. Repairs also include equipment replacement. I myself install all new equipment I sell. I don't sub out work to sub contractors unless the contract specifies that (which is extremely rare situation). If you don't like the way I do things you're free to look elsewhere. I'm not here to tell you what your ears are aching to hear, I am here to tell you like it is. (Firmly and exponentially based on experience.) You don't have to like what I say, most of the time it's not pleasant due to a variety of reasons. In some aspects similar to the confrontational aspects of this forum boards conversations. Anyone who has viewed my posts know I come thick skinned. Disagreements do not phase me. If you want a gimmick, chase a gimmick. I could not offer the service I offer if my approach didn't work after all these years. I'd be running back around chasing my tail. You'd never get thru when you try to call me. Seriously? Think about it. Chest pounding: If you want to call it that it's your opinion. You're free to have it. My experience is my experience. 26 years. Easily verified. 13 years running my own company. Again easily verified facts. Essentially it's an advertisement. I am not paid to be here. I am here drinking my morning coffee filling in that time. If this isn't advertising my business what purpose would I have to be here? I don't come here to argue with the metamucil fan club that propetuates this board. Ha, ha... you know who you are. People search for me. All I have to do is leave bread crumbs so people can find me. AC repair service Katy, Texas area. Everyday. When I say something I mean it. Not a spam service. Don't sell your info. Don't pester you. You initiate the service you want. It's not that difficult when you know what you're doing. If you want a 20 something or 30 something coming to your home, you have to ask yourself how much experience does this guy really have? If that's what you want go for it. I've worked for companies like that... having to go back behind some newbie over and over and over. Used in not very fun ways. Follow my rules. I've been doing this longer than some of you have been alive. My methods work. Experience. What is it worth? Who else on this board in all the world listed as a pro HVAC service provider has my work ethic? Name one besides me. (beside the mini split warehouse spammer) Truth is stranger than fiction. There are people out there who read these threads and never post due to this kind of argumentative climate so frequent here. How could I know such a thing? I service the Katy, Texas area.