Replacing plenum and some ducting, have a couple questions

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Old 03-15-19, 07:14 AM
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Replacing plenum and some ducting, have a couple questions

I'm redoing some of the ducting in my house along with a new plenum, because it's completely falling apart and leaking everywhere and doesn't seem to be very well designed (14" duct right near the end of the plenum leading to another plenum with three 12" runs and one 8" run, several extreme angles with far too much slack, etc).

Question 1: When measuring CFM, do Wyes add/detract? For example, there's an 8" duct coming off the plenum, that branches into two 6" ducts using a 8x6x6 Wye.
An 8" duct uses aprox 116-160 CFMs based on some charts I've seen. Do the extra two 6" ducts use more CFMs on top of that, or is the CFM measured by the 8" trunk only?

Question 2: I know that no ducts should be placed on the end cap of the plenum, and all ducts should be as far away from the end as possible (12" if not more). That being said, where on the plenum should I place ducts that will have longer runs? Does closer to the coil provide more airflow, or does closer to the end (still 12" away) provide more?

Thanks for any insight.
 
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Old 03-15-19, 09:39 AM
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8in duct is 200cfm at .08 6 in duct is 95cfm at .08 If you are pressurizing the main duct then really no change from on end or another. If you are using scoops then no real change either. Of course every branch should have a damper to adjust the airflow.
 
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Old 03-15-19, 12:57 PM
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Thanks Airman.

I've sketched up a duct diagram if you or someone doesn't mind taking a look at it. It's not to scale, but will give an idea, with duct sizes, etc.

Unfortunately, it'll have to be a bit of an 'octopus' because the unit is smack dab center of the attic.
It's a 2 story house, all bedrooms upstairs, right above the kitchen/dining room area.
The living room is vaulted ceilings, open to the upstairs via a balcony, so airflow is gonna suck there no matter what. Somewhat remediated with a ceiling fan.

Would it be better to have all runs go straight to the plenum, or are the wyes I've included better for pressurization, etc?

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