Increase or Decrease Duct Diameter

JIGSAW2 · 04-30-2009, 11:18 PM

There is one room in the house that seems to have poor air flow from the register. I checked the duct and it's somewhat of a long stretch. There are no leaks or restrictions. Do I increase or decrease the duct diameter to get more air flow? Thanks for your help.

**beenthere** · 05-01-2009, 06:12 AM

Increase.

You need to lower the resistance to air flow.

Yoyizit · 05-01-2009, 09:20 AM

Quote:

Originally Posted by JIGSAW2

Do I increase or decrease the duct diameter to get more air flow?

Here's one link that may tell you by how much to increase the duct size.
http://www.engineeringtoolbox.com/ai...ram-d_328.html
but there must be other more userfriendly links on the Web.

I used a booster fan but they don't seem to be that popular.

**beenthere** · 05-01-2009, 11:39 AM

That site also has that chart in SAE.

But, it doesn't tell you what size. Unless you knwo what friction rate you should be using.

Yoyizit · 05-01-2009, 12:11 PM

Quote:

Originally Posted by beenthere

That site also has that chart in SAE.

But, it doesn't tell you what size. Unless you knwo what friction rate you should be using.

And Moody's equation has me going in circles. Can we make assumptions about air flow and get existing duct sizes and go from there?
Or it's just an upstream damper that's closed?

**beenthere** · 05-01-2009, 12:24 PM

Assumption are what causes most duct problems.

Most people don't know that flex duct and sheet metal have different air flow rates for the same FR.

EG:

6" Flex duct at .1" F is 85CFM for 100' of equivalent length.
6" round sheet metal at .1" FR is 110CFM of equivalent length.

End rooms on trunk systems can easily have a TEL of 300'.

Yoyizit · 05-01-2009, 02:01 PM

Moody's equation seems to be telling me that if you double the duct cross sectional area you'll halve the friction, for a clean, round, galvanized duct.
I assumed a 10' duct, 100 cfm, 100 fpm, 1 sq. ft. duct area, 0.017 in. of W.C. dropping to 0.0085" by going to a 2 sq. ft. area duct.
Grainger probably has a plastic airflow meter for $20. For the labor & materials saved by the OP by making some measurements, I'd recommend he/she buy one.

**beenthere** · 05-01-2009, 03:11 PM

Thats the problem.

So many people don't know how to measure and determine Total Equivalent length.

There is no such thing as a 10' Total Equivalent Lenght duct, from the return to the supply register.
1-90° ell is 15' of TEL. A supply boot is 30', a start collar can be 15 to 35' depending on its position on the duct, and how close any other take offs/start collars are to it.

I know you used 100FPM for example only.

While 1 sq ft at 100 FPM is 100 CFM. The reason its a useless velocity.
Is it too slow. It allows the pipe/duct to gain or lose too much heat.

A 6" round needs a velocity of 510FPM to move 100CFM.

Checking static pressure would tell the OP more then a flow meter.

Just looking at a friction chart, and guessing what FR to use is how many installers determine what size duct to use. And why so many undersized duct systems are installed.
Without determining TEL, its a WAG.

Using .1" FR can easily give a system static pressure of .8" and more.

Most systems only have .3" or less, Available Static Pressure to work with. If you want to stay at air handler/furnace ESP rating.

The OP may be able to safely go up one size on that run.
But increasing by 2 sizes could create other troubles.

6" round 510FPM=100CFM
7" round 375FPM=100CFM
8" round 286FPM=100CFM

6" round 615FPM=120CFM
7" round 450FPM=120CFM
8" round 345FPM=120CFM

While halfing the static or that run may sound good. It can also be the worst thing to do.

Yoyizit · 05-01-2009, 05:05 PM

Quote:

Originally Posted by beenthere

1-90° ell is 15' of TEL. A supply boot is 30', a start collar can be 15 to 35'

I couldn't find this equivalent length method online for fittings for free, only links using the loss coefficient method.

What I did find is that while most bookstores and libraries do not have ASHRAE and ACCA books, they do have books like "Mech. Eng. reference manual for the PE exam" by Lindeburg, and it has chapters on HVAC stuff.

Maybe I should limit myself to bathroom fans/ducts

**beenthere** · 05-01-2009, 05:19 PM

Some libraries have Manual J, but few have Manual D.

College libraries would have Both J and D.

Fan/air laws are abundant. And can be confusing.

qbert · 05-01-2009, 09:10 PM

FR=PD x 100/ T.E.L ?

**beenthere** · 05-01-2009, 09:24 PM

FR=ASP*100/TEL

You need to subtract all yoour device PDs from the units rated esp.
To find ASP.

.06 will usually cover the supply and return grilles/registers.
Then you also need to suntract the air filters PD.
If its a furnace, you also need to subtract teh evap coil.
Air handlers, you need to subtract the strip heaters PD, if ti has any.

For a gas furnace.
Grilles and registers=.06
Air filter=.14
Evap coil=.16
Total PD = .36
.5ESP-.36=.14ASP.

.14 ASP*100/300' TEL =.046 FR.
.14 ASP*100/250' TEL =.056 FR.
.14 ASP*100/200' TEL =.07 FR.