How do dampers affect velocity and flow?

[jackmorgan]

Hopefully someone can enlighten me ...

Dampers are used to control the volume of air that enters a branch, but how do they affect velocity? I would assume the air speeds up as it flows through the damper opening, but would then slow back down after continuing down the branch.

Here's the problem ...

I have a room that requires 57CFM of cooling according to manual j. It calls for a 4 inch duct at 600fpm. Everyone I talk to recommends using a 6 inch and installing a damper, but with 57cfm in a 6 inch duct; velocity will drop to 300fpm which will not allow proper room mixing. Should I just resize the register to increase velocity, use the 6 inch duct or go with the manual d recommended 4 inch???

I'm having the same issues on other outlets calling for 5 inch, 3 inch, etc. Should I go with the so-called experts or the manual d/j????

Thanks.

Jack

[tinmarine]

Who are the "experts" you are refering to? 57 cfm with a 6"? A little overkill.

[cem-bsee]

one would be hard pressed to measure the effects from the normal designed damper for HVAC residential ducts -- so far as causing flow problems --

are you still in the design stage?

many installers will just use 1 size duct, then add dampers --

[beenthere]

You'll have some turbulance right at and after the damper. But if your register isn't within 2' or so of the damper no problem.

For a room requiring 57 CFM, I would use a 5" and damper. Depending on length of the run. You'll be hard pressed to find 3" pipe. I don't use anything less then 5". On jobs that only have one supply that would size out less the 6" inch, quite often we use just 6" and a damper for that one run. If all supplies are sized to the FR that the Manual D said to use for the duct system, very few supplies come to less then 5".
Select registers and grilles for the throw you need.

[jackmorgan]

You'll have some turbulance right at and after the damper. But if your register isn't within 2' or so of the damper no problem.

For a room requiring 57 CFM, I would use a 5" and damper. Depending on length of the run. You'll be hard pressed to find 3" pipe. I don't use anything less then 5". On jobs that only have one supply that would size out less the 6" inch, quite often we use just 6" and a damper for that one run. If all supplies are sized to the FR that the Manual D said to use for the duct system, very few supplies come to less then 5".
Select registers and grilles for the throw you need.

So you would use a 5 inch duct for a 25 square foot powder room?

[jrbenny]

So you would use a 5 inch duct for a 25 square foot powder room?

I wouldn't waste money on a 25 sf room.

Just keep the door open. Tie the fart fan to the light. When door is closed, fart fan runs and pulls air under the door. Otherwise, door is open and room is conditioned by adjoining space.

[beenthere]

So you would use a 5 inch duct for a 25 square foot powder room?

Yes I would. If the room really needs to have a supply.
The register selection will determine throw.

[jackmorgan]

one would be hard pressed to measure the effects from the normal designed damper for HVAC residential ducts -- so far as causing flow problems --

are you still in the design stage?

many installers will just use 1 size duct, then add dampers --

Still in the design stage. I'm aware many installers will just use one size, but my question is "is this the correct way of doing things?". What would you do for your own house? I want to make sure I do the job correctly and get proper airflow to each room.

[jackmorgan]

Who are the "experts" you are refering to? 57 cfm with a 6"? A little overkill.

The "experts" are just some colleagues that have been in the business for 20+ years. Perhaps they are just taking the easy way out by using 6 inch branches for every room.