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Discussion Starter · #1 ·
I always hear the Hvac guys talk about system design based primarily on the sq footage of the house. But since some houses will have more vents or longer runs, do you calculate the volume of forced air needed per room. What about airflow loss over a long run with flex duct?

The reason I am asking is that our kids bedrooms were several degrees cooler than the other rooms. (New house,first winter in it)

First I think it's toys in the duct or a kink in the line. Nope. So I disconnect the flex from the trunk line and find this



Metal Circle



Eye Iris Organ Auto part Metal

So I opened up the holes, which increased the airflow by? Double,maybe? Not sure.

Product Auto part Tire Automotive tire Muffler

That one was a 12" secondary trunk line that fed 4 6" vents. I cut out about 2" all the way around the hole.

While the airflow is much better, It got me wondering if there isn't enough volume of air for the distance it's traveling. Maybe too much heat loss and friction loss?

Here is a real rough sketch of it.


Drawing Sketch Text Diagram Design

About 20' of 12" and then the 6" vents are 14'-25'.
 

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Shoddy workmanship is what you found at that joint/connection.

A room by room load calc should have been done to find out how many BTUs for heating and for cooling. Which then would also tell how many CFM each room needs.

The duct work should have been sized by the total equivalent length method. Which allows for the restriction on the duct length and all fittings including tees, ells, takeoffs, boots, etc.

A 12" flex duct with the equivalent length of 100 foot can move about 360 CFM at a pressure loss of .1", a 6" flex duct with an equivalent length of 100 foot can move about 60 CFM with a pressure loss of .1". neither of the above figures allows for improper stretching, or lack of proper stretching of the flex.

The 20 foot of 12 inch may only have an equivalent length of 80 foot depending how well it is stretched out. Same with the 6" runs you show.
 

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Discussion Starter · #3 ·
Thanks for the Info. That's a lot to digest, but I've been reading up on it. It makes more sense why some rooms have more airflow.

The 12" duct is connected right after an 90 in the main trunk and then dips down under a beam and back up. The same with a couple of the 6" lines. On the 6" it would add 60' equiv ft.

Thanks again.
 
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