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I found most of the calculations from the first link you provided in your previous post. I have a 40K BTUh output furnace which came out to 560 CFM. I also have a 2 ton AC unit that came out to 800 CFM. Based on that I have done my duct sizing based on the 800 CFM required by the AC unit.
From the links you provided I found that for 800 CFM my main supply duct could be 6" x 28". I have 8 rooms that I need to heat and cool. I assume that means my total branch duct CFM should add up to 800.
Currently I have these questions:
1. How do I determine what CFM is required for each room based on it's square footage?
2. Should the main supply trunk decrease in size as it gets closer to the end to maintain the air pressure in the ducting?
3. Based on what I've said, do you think I'm on the right track?
Thank you again for your insight.
From what I've read it appears I need to calculate the area of exterior walls, windows and doors in my house. Then determine what percentage of exterior surface area each room has relative to the whole house. Then, for example, if a room has 20% of the exterior surface area it would get 20% of the available CFM from the main supply duct. Since my house is older and has older windows & doors I have been told to multiply my window & door dimensions by 5 to account for their heat loss.
I assume this isn't the best scientific approach, but for a leaky, 950sq. ft., 75yr old house is this the right approach? Thanks for all your help.