04:09PM | 04/06/03
Member Since: 03/09/03
26 lifetime posts
I have a one-story home that was built in 1926. It has a forced air furnace from 1994, but it still has old 10" furnace ducts in the basement. They are constantly in the way and take up a lot of headroom. I would like to replace these with smaller ducts, but from what I've read it's not as simple as just replacing them. I've read that I should be concerned about the size of the ducts, the rooms they go to, the location in the room, etc. Can anybody suggest a website or book that I can use to thoroughly research replacing ductwork before I get started? Thanks in advance.


05:09PM | 04/06/03
Replacing duct work is typically not a DIY project. The variables you mentioned are just the tip of the iceburg in sizing and configuring duct design. There are numerous variables such as aspect ratios to be taken into account of when sizing duct. You want to get the best effiecency out of your system yet still move air where you need it and in the proper quantity. This may be a job best left to a reputable company.


07:02PM | 04/08/03
Member Since: 03/09/03
26 lifetime posts
Thank you HK for your very helpful response. After researching the links you provided I agree that air duct sizing is not a simple matter. However, I would like to continue to research the possibility of doing this myself. I'm not ready to start buying material yet. Instead I was hoping I could run my numbers by you.

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.


11:53AM | 06/14/03
Member Since: 03/09/03
26 lifetime posts
I've done some additional research on calculating the branch duct sizes to each of the rooms.

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.



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