RE: 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?



oldthinker Parts Changer Asked on March 9, 2018 in Design.
Add Comment
1 Answers
Best answer

Here is a response from Jack Rise – Author of the companion to manual D

Manual D teaches a design methodology called Equal Friction. The procedure is to find the supply outlet with the greatest resistance to air flow and the return inlet with the greatest resistance to air flow. A line is then drawn between the inlet and the outlet to create the “critical path”. So, there is only one supply register, only one return grille and only one volume damper.

That friction rate is applied to every duct run, both supply and return. The assumption is that if you can get air from the return inlet with the greatest resistance to the outlet with the greatest resistance to air flow, then all the other inlets and outlets are cake.

In the field, the damper in the critical path will be wide open. The others will have to be adjusted.

Hope this helps some,

Pretend Internet Professor Answered on March 9, 2018.

That is so easy to understand, the calculation makes a lot more sense now.

You’ve got the connects, man.  A little “star-struck”!

on March 9, 2018.

Haha! Jack is really a good dude. All the high profile educators in this trade are in my experience.

on March 9, 2018.
Add Comment

Your Answer

By posting your answer, you agree to the privacy policy and terms of service.