Friction Rate Calculation Woes

Hey all,

I’m learning more and more about ducting and system design every day, but one of the drawbacks is that I’m trying to teach myself out of Manual D and J by myself.  One thing I’ve gotten hung up on is calculating friction rates.

For practice, I’m designing a system based off this matchup.   Here’s the data I have so far:

Blower Data:

0.8″ WC total external static pressure

1200 CFM

Device Pressure Losses:

-0.30″ DX Coil (wet)

-0.16″ media filter (clean)

-0.03″ x 8 supply outlets = 0.24″

-0.03″ x 4 return outlets = -0.12″

-0.03″ x 8 balancing dampers = -0.24″

Total with dampers = -1.06″ WC

Total without dampers = -0.82″ WC

Here’s where I get lost: I’m already at a negative available static pressure before I even add in total equivalent length for ducting.

What am I doing wrong?  Should I not be multiplying 0.03″ pressure loss for every termination?   I’m using the example of 12 drops between the supply and return which seems pretty conservative for a residential home (I’ve seen many more on a single system.)   Even without the balancing dampers on the supplies, I’m over my total.  What gives?



Parts Changer Asked on March 9, 2018 in Design.
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