Is there a certain number of registers needed per square foot of finished space. I am finishing out the unfinished part of my basement and there are two existing registers. I am not sure if I need to branch off these and add more. Also there is no cold air return in this area. Should I add one. If so what size? The area I am finishing is aprox 920 sq ft. The ceilings are 9 1/2 feet tall in all but 140 sq ft. They are 8 ft tall in the 140 sq. ft area. I am putting R13 insulation in all of the walls and R19 in the ceiling between the upstairs and downstairs. There is already R11 in the floor of the upstairs. The existing ducts are from a 2 ton heat pump unit that also heats and cools about a third of the upstairs. There is also a 4 ton unit that heats and cools the other two thirds of the upstairs and the other finished portion of the basement area that is divided from the unfinished portion that I am working on with walls and doors. The portion that I am finishing out now(the 920 sq ft) will be all open. All total these two units will be heating and cooling about 5000 square feet when I finish out this section. Is this adequate. I hope I have given you enough info to give an informed answer. If not just let me know what info you need. Thanks.

5000 sf


225,000 btu's
And about 6 tons ac so yes that should be enough. You need around 4 six inch supplys per 750 sf

Thank you. So as I calculate that, I will need 4.91(5) registers to accommodate the 920 sf I am finishing. What about the return air. The indoor Air Handler is right in the room I am finishing(in a closet) so It will be fairly simple for me to tap right into the side of the box below the handler and place the vent in the closet wall at floor level. The forced air vents will all be in the ceiling. Will that work? If so, what size return air vent would you suggest for 920 sf? Thanks for your help from a fellow West Virginian.

if you are going to put the supplys in the celing make sure you get square registars, so they will throw air to the floor, round one will just blow across the celing. Return air you will need about 500 cfms. So 10 x 14 or 12 x 14 will work, Wv? What town?

Thanks for the info. I do appreciate it. We live in Hurricane/Teays Valley. What town you from? Thanks again.

Morgantown. Hope some of the info helped, if you need more just ask

Thanks! You have helped tremendously. Go Mountaineers!!

Its not the amount of grills you put in a space to make it cool or heat. You have to have a system that is sized for the load its trying to heat or cool. If I added 2 , 6 inch ducts to a system that was 3 tons and sized for that and added those I would be under sized by 1/2 ton. When a home owner tries to add or subtract ducts from a system that is sized for a certain area it only comes back to haunt them.

2 6'' reg undersizing by half ton?

O.K. So now I am a little confused. Let me tell you what I have(and I was a little confused on the layout of this system to begin with, but have been told it is proper/just different). I have a two level house with the first floor being all above grade and the lower level being a walk out basement open to the outside on two sides. I have a 4 ton unit that is set up to heat and cool 1650 sf of the first floor, with 10 registers and 3 return air vents(10" x 32")& 1500 sf of the lower level with 8 registers and 3 return air vents(10"x32", 7"x25", and 20"x20"). I then have a 2 ton unit that is set up to heat and cool 990 sf of the upstairs with 6 registers and 4 return air vents(10"x32", 10"x32", 10"x16", and 10"x16"). In the upstairs the two systems seem to overlap, meaining that part of the 4 ton unit serves a 30x30 room that is open to a 22x30 area that is being served by the 2 ton unit. Now is where the new section I am finishing out comes in. On the side of the house being served by the two ton unit I am finishing out an additional 920 sf. There are two existing registers already in this area, with no return air vents. I just need to know how many registers I should add this area and what size return air vent. I guess I just want to make sure I don't overload the system or underserve the area I am adding vents to. Thought I had it figured out until T-Top posted. Thanks for all the help.

I have no idea what he was talking about. Other then you need 400 cfms per ton of ac, But your system was sized for you house it is not like you are adding a addition. The main thing you need to be concerned with here is not ac it is the heating. Being in the basement will stay cool. Are these units heatpumps or do you have gas heat? What size is this room? 26 x 36?

Thanks. They are electric heat pumps. The room I am working on is 22 x 45 minus about 70 sf of understeps area. Yes it does stay very cool down stairs in the heat of summer. I close off most of the vents(not all) downstairs in the summer and reverse the process in the winter. That seems to balance things out(is this advisable?) I would assume that the system was designed to accommodate the entire house as a completed unit. The previous owner was the contractor that built most of our neighborhood and he lived here for 8 years, so I would assume he had it designed correctly. He used the unfinished basement as a workshop, so he never finished it out, but he did have the two existing vents put in on that side. Thanks again for all your help and info.