Ductulator

[spam'n'rice]

Can someone please explain to me why .08 is the magic number for the friction rate?

[Ed Janowiak]

.08 is not the magic number.

The magic number (friction rate) is your available static pressure times 100 divided by your total equivalent length.

[nratom45-70]

There is no magic number

[spam'n'rice]

.08 is not the magic number.

The magic number (friction rate) is your available static pressure times 100 divided by your total equivalent length.

would you mind showing me an example? Ya see when I try to enquire about this im always told "just use .08". I say why .08 "cuz thats the magic number" and believe it or not ive been taught that in several classes as well they even pre-marked .08 on our ductulators before they handed them out.

Im just trying to learn here

[Milk man]

This was in the pro section, but I don't see anything in here a DIYer could use. I cut and pasted. This is a great explaination and thank you to coolerinfrederick for posting it.

coolerinfrederick
Professional Member Join Date: Apr 2006
Posts: 402

friction rating on ur ductulator?

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people this is not your static press., its the actual friction from 90's transitions etc., here's how to find it, i know it can be next to impossible on certain systems but here ya go:



Look at the blower performance chart for the selected piece of equipment, AH or furnace.

Find the static pressure at witch it will deliver the desired airflow.
Add up the pressure drops of all the air side devices at the desired airflow. Coil, filter, heat strips, registers and grills.

Subtract the sum of all the pressure drops from the static pressure needed for the blower to deliver the desired airflow.
This gives you the ASP, Available Static Pressure.

Next figure up the TEL, Total Equivalent Length, of the duct and fittings from the start of the longest return duct to the end of the longest supply duct.

Then plug the TEL and ASP into the formula: TEL/(ASP x 100)

This gives you the friction rate you need to set the ductulator to, be sure you have a ductulator for the type of ductwork you are dealing with.

After determining the minimum size duct needed to move the required CFM at the calculated friction rate, look at the part of the ductulator that shows the velocity.
If the velocity is above the recommended FPM for the application, move to the next larger size duct that will get the velocity down to where you need it. Dampers will likely need to be adjusted in any branch ducts that you have to upsize to get the velocity down in.

[dandyme]

your Dmanual should explain how to determine friction rate.

[Ed Janowiak]

This was in the pro section, but I don't see anything in here a DIYer could use. I cut and pasted. This is a great explaination and thank you to coolerinfrederick for posting it.

coolerinfrederick
Professional Member Join Date: Apr 2006
Posts: 402

friction rating on ur ductulator?

--------------------------------------------------------------------------------

people this is not your static press., its the actual friction from 90's transitions etc., here's how to find it, i know it can be next to impossible on certain systems but here ya go:



Look at the blower performance chart for the selected piece of equipment, AH or furnace.

Find the static pressure at witch it will deliver the desired airflow.
Add up the pressure drops of all the air side devices at the desired airflow. Coil, filter, heat strips, registers and grills.

Subtract the sum of all the pressure drops from the static pressure needed for the blower to deliver the desired airflow.
This gives you the ASP, Available Static Pressure.

Next figure up the TEL, Total Equivalent Length, of the duct and fittings from the start of the longest return duct to the end of the longest supply duct.

Then plug the TEL and ASP into the formula: TEL/(ASP x 100)

This gives you the friction rate you need to set the ductulator to, be sure you have a ductulator for the type of ductwork you are dealing with.

After determining the minimum size duct needed to move the required CFM at the calculated friction rate, look at the part of the ductulator that shows the velocity.
If the velocity is above the recommended FPM for the application, move to the next larger size duct that will get the velocity down to where you need it. Dampers will likely need to be adjusted in any branch ducts that you have to upsize to get the velocity down in.

It's really hard to condense how to determine friction rate, the poster above did a nice job of doing it. Only thing is the formula is backwards,
(ASP x 100) / TEL = FR is correct.

[270wsm]

Would it be ok to just use .05 all the time for sizing return duct? I know it may be overkill in some situations, but at least it wouldn't be undersized. I know people who use .06 on all returns.

[RoBoTeq]

Before manual D existed, or about the time it was being developed, I was designing commercial/industrial ducting systems. The standard at that time was SMACNA, which all of today's standards grew from. SMACNA air duct design manuals had different recommended static pressure design points, based on 100' of SP, for various applications.

For residential applications, .1 sp for supply air and .08 sp for return air was the suggested settings for ductulators (which were expensive to buy back then). You also want to pay attention to the FPM of the systems, again, the SMACNA recommendation for residential was 800 fpm for supply trunk, 500 fpm for supply branch ducts and 400 fpm for return ducting.

In over 25 years of designing residential systems, I have never had an issue using these recommended settings. Just keep in mind that the ductulators are basing the data on 100' of ducting, from the unit to the termination of the air. If your run is under that length, you have a buffer zone, if your run is over that length, you may want to think about increasing the size of that run accordingly.

[steve3871m]

Many folks may get the right FR, but loose it in the fitting selection or the installer makes changes from the original design with no thought of consequences.

Instead of using wyes, some guys will use triangular or square fiber board transition boxes and there goes the figured FR out the window. Some of those boxes have an EL of 125' compared to a wye of 30'.

[ktechnical]

I have always used .1 for supply unless any flex will run longer than I like (usually 12') because a trunk line is not installable then I use .08 for a larger cushion. One thing they need to teach in these duct design classes is "Nothing is Certain".
We have estimated calculations to get us to range, then sometimes common sense and experience takes over. Surprisingly though, many designs come out way within specs.
If your using flex on the whole job, I would use .08 for sure.
I've never had issues with this.

[bover]

I've always used .06ish for returns, and .08 for supply, makes the system a whole lot quieter too. This has always worked well for me, unless it's a green system that rhymes with dude, then all bets are off.

[coolwhip]

Duct design is not my forte, good cliff notes Milkman!

I have manual D and have read it, referenced it, used it in tests....and I still hate the damn thing!

[ACFIXR]

Can someone please explain to me why .08 is the magic number for the friction rate?

The magic number comes in "FPM"

and requires "DAMPERS"