Yesterday I had found a site explaining how to calculate CFM's needed per room based on room size, air turnover, etc, but today I can't find it. WAIT....maybe I just found another one. It says to Calculate Volume of Room, then to multiply by required air changes and gives a table. For residential it says Bedrooms are 5-6, Basement 3-4, Kitchen 7-8, Family 6-8 and Bathrooms 6-7. This says then divide that number by 60minutes and you get the Required CFM.
I also looked again and two rooms actually have 7" round ducts feeding them, not 6" as I originally said. The kitchen and the pantry do have 6". So there are two 7" round ducts and two 6" round ducts feeding off the 8x14 trunk.
Here is what I know if it helps at all: (now based on the calculations I found)
1) Bedroom #1 (located on corner of house so two walls are exterior) is using 7" round duct and is 10x16x9, so it's 1440 cubic feet. Based on formula above that says I need 144cfm for this room. 30' from furnace to register with 3 90's, if you count flowing into side of register and where it exits the furnace. I believe 7" duct is 160cfm, so would that need stepped up or will running straight off furnace as 7" suffice? The 3 90's are off furnace, coming off trunk and going up to joist bay. It goes into the side of the 9x10 floor mounted register, no sure if that counts as a 90. This room has a return air grill.
2) Bedroom #2, center of house one wall is exterior) is using 7" round and is 10x11x9, so 990 cubic feet. Formula says I need 99cfm for this room. Duct is 19' from furnace to register, which is about a 12" square wall mounted register. (keep in mind old house) There are 3 90's off of this as well, again the furnace, off the trunk and then into the register box that feeds the wall register. It looks like that should be well under the 160cfm of a 7" duct, so can I stay with 7" on this one? Could I get away with stepping it down to 6"? This room tends to be one of the coolest in summer and hottest in winter. No return air.
3) Walk-In-Pantry just off kitchen with no door. 6" round duct is 7x6x8 or 336 cubic feet. Using formula for kitchen as it's connected. Says it needs 39.2cfm of air and the run length is 23' with about 3 90's. Obviously gets way more cfm's than required, but might help make up for kitchen!? as vent is right near door opening. Standard 4x10 floor register.
4) Kitchen 11x13x9 or 1287cubic feet. Calculation specifies 171.60cfm requirement. Obviously the 6" round duct supplying it is a little under sized but possibly offset by Pantry? The total run is 26' and has about 4 90's and a 4x10 floor register.
Being as the kitchen and pantry require 210cfm and they are being supplied with about 240cfm, do I stay with 6" duct or jump up to 7" for most of the run? I could downsize the pantry to 5" if that would be better and might be able to run 7" all the way to the kitchen.
Also, like was posted above I see that 14x8 trunk is 500cfm, that seems just about right for my current setup, which would appear to require 453.8cfm, if you don't adjust for friction loss, which I'm not certain how to figure or implement. I will also say at one point, it appears, there was a 2nd 7" line going to the kitchen but it was closed off. I don't notice the kitchen being hot or cold, so I'm guessing all is good there.
The basement area in question is 28x16x7 so 3136 cubic feet and thus should require about 209.07cfm. Looks like if I run an 8-9" duct off furnace I should be good for that room. If this is all correct, it'll help me check the rest of the house, especially since there is a trunk feeding the other side of the house.
Please let me know if I'm thinking too hard on this and over-complicating it. I'm also trying to figure out how the return air fits into all of this. Being as it's an old house there are things that have been changed over the years such as some registers were probably return air registers at one point and vice versa. Am I correct that I just want to match what comes out to what goes in?
Also if it matters at all the furnace is a Goodman GMP 100-3 installed in the early to mid 90's by my best guess as I didn't own the home at that time.