FHA main trunk replacement: Follow-up


Took your advice from your responses to yesterdays question.
22"x8" is a standard rectangular size I could get at a supplier or on-line. I've currently got 6 flex-ducts going to the room registers. It's a one-zone system. No branches. I think I could do the gig myself, but I am unsure of sizing the duct.
Currently, the trunk size becomes smaller as you travel the length of the trunk. There are three places where a reducer fitting is in place. The quote from the sheet metal company only stated 22" x 8". Not knowing anything about duct sizing, how would I know what size to get and where to put the reducer fittings? Otherwise, it looks like a job I could do using standard prefabricated parts.
Thanks again for your consideration.
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Just copy the layout of the existing duct. The replacement duct should have the same area as the existing duct at each point. If the calculation falls in between avilable duct sizes, go with the next larger one, as there is slightly less flow in a 'flatter' rectangular duct than a square one. If it starts out as 8x22 then at some point it will neck down to 8x20 or whatever and then 8x16, etc. The reducers are available where you buy the ducts.
So if the existing duct is 12x12 at some point, an 8x18 would have the same area. Ducts are made in 2" increments, but the supply house may only stock the more common sizes and you will need to order them.
--
Dennis


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Use graph paper to draw the ductwork to scale, making note of all of the sizes, reducers, etc., then fax it over to a local HVAC supply house and ask for a quote on the materials as is, and another quote for reducing the height of the ductwork to the desired height. When estimating for a DIYer project start with the materials cost as that is relatively fixed. If the materials cost is somewhere between 1/3 and 1/2 of the contractor's quote, then the price you got is probably a reasonable one.
Here's a thread from another forum that might be of interest to you: http://www.thesheetmetalshop.com/PNphpBB2-printview-t-352-start-0.html
R
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You can use the same cross-section area to keep approximately the same air flow only if it is relatively square. In the extreme case you could go to 88 x 2 or even 196 x 1, but obviously the air flow will be greatly reduced due to friction with the walls of the duct. So, if you go much lower than 8 x 22 you will have less airflow. I haven't seen your earlier posts so don't know what duct size you currently have.
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