What Size Register For 6 Inch Duct

Templeton Denard · Sunday, December 19, 2021

Branch size vs register size

Greetings.
Does the Manual D give register size with consideration to branch size?

Example would be if a room requires 100 cfm using a 6" branch, does that mean that the standard 6x12 register will work. Do registers tend to be oversized from the duct to compensate for two-way, three-way or even four-way openings?

My question stems from our system. The bedrooms are fed with 6" flex to a 6x12 three-way register with closing louvers. They were noisy. I removed them and it's very quiet. Well, less than with them installed. So maybe an 8x14 or 12x12 would allow the air to dump as well as the 6x12 with no register.

This isn't meant to sound like a DIY. Just looking for knowledge. I'm going to get a Manual J & D performed. Need to know what to expect from the D and if there is still room for improvement at a personal level.
Registers have nothing to do with branch size. The supply grille is sized for noise [mostly] and pressure drop. You want a NC of under 20.
Your reply is my point. If the room takes a 6" drop, the register is usually the standard 6x12. Does the Manual D address this?
What does NC stand for.
Originally Posted by hvacconsultant

Registers have nothing to do with branch size. The supply grille is sized for noise [mostly] and pressure drop. You want a NC of under 20.

Supply grilles are sized for throw and spread with a velocity that will meet acceptable noise levels. Manual T is the guidance document for air distribution. One of the problems with most duct systems is they are only sized for noise and pressure, and the ability of given registers to distribute the air correctly to meet comfort level are ignored.
When I have the Manual D performed for our house, will it address the register size? Sounds like there is yet an other manual, Manual T?
Originally Posted by regis101

When I have the Manual D performed for our house, will it address the register size? Sounds like there is yet an other manual, Manual T?

Diffusers are rated to a ( NC ) Noise Criteria by the manufacturer.
i.e.
http://www.titus-hvac.com/ecatalog/g...spx?fileid=898
... < 2 CFM / Sq In (300 CFM / Sq ft) should be quiet
IF installed properly.

Designer Dan __ It's Not Rocket Science, But It is SCIENCE with Some Art. _ _ K EEP I T S IMPLE & S INCERE ___ __ www.mysimplifiedhvac.com ___ __ Define the Building Envelope & Perform a Detailed Load Calc: It's ALL About Windows & Make-up Air Requirements. Know Your Equipment Capabilities
Nice link. Good info.
Does the Manual D give register size? Seems to be a vital part of the equation. Seems senseless to have it done if the registers are not incorporated. What about in the end, the register ends up noisy. Can I go bigger and still be happy?
It is a simple fact of life that there is a world of difference between a design built residential system and a mechanically engineered commercial/industrial job. With the latter, these is a blueprint that lays out the system including all duct sizes and supply outlets. CFM delivery is required and at some time during the commissioning process the entire system is flow hood balanced to design specifications. This attention to detail does of course, come at a price. Hence the substantially higher costs for an engineered job. But that doesn't leave the residential customer necessarily hurting.
A high quality company at the residential end will do a Manual 'J' load analysis, a Manual 'D' duct design and use the register/diffuser/grille manufactuer's engineering numbers to deliver the proper results of both noise, CFM delivery and balance. Any experienced HVAC company knows pretty closely what the static pressure will be in his designed system. That by default should dictate how many and what size supply outlets need to be installed in a room. As to branch duct size, that's a function of the velocity. Again, deferring to noise, a good company will design for low noise. All of this must take into consideration the actual installation. Our company issues it's installers a 'cheat sheet' that contains engineering data taken from our favorite diffuser manufacturer, using the sizes and CFM delivery closest to the anticipated static pressure we will see in the finished system. It's a relatively simply matter to adjust volume dampers in each supply branch to bring any errant balance problems under control but quite honestly, when done to the numbers, we have precious few trips out with the flow hood for balancing purposes.

If YOU want change, YOU have to first change.

If you are waiting for the 'other guy' to change first, just remember, you're the 'other guy's' other guy. To continue to expect real change when you keep acting the same way as always, is folly. Won't happen. Real change will only happen when a majority of the people change the way they vote!
Originally Posted by skippedover

It is a simple fact of life that there is a world of difference between a design built residential system and a mechanically engineered commercial/industrial job. With the latter, these is a blueprint that lays out the system including all duct sizes and supply outlets. CFM delivery is required and at some time during the commissioning process the entire system is flow hood balanced to design specifications. This attention to detail does of course, come at a price. Hence the substantially higher costs for an engineered job. But that doesn't leave the residential customer necessarily hurting.

A high quality company at the residential end will do a Manual 'J' load analysis, a Manual 'D' duct design and use the register/diffuser/grille manufactuer's engineering numbers to deliver the proper results of both noise, CFM delivery and balance. Any experienced HVAC company knows pretty closely what the static pressure will be in his designed system. That by default should dictate how many and what size supply outlets need to be installed in a room. As to branch duct size, that's a function of the velocity. Again, deferring to noise, a good company will design for low noise. All of this must take into consideration the actual installation. Our company issues it's installers a 'cheat sheet' that contains engineering data taken from our favorite diffuser manufacturer, using the sizes and CFM delivery closest to the anticipated static pressure we will see in the finished system. It's a relatively simply matter to adjust volume dampers in each supply branch to bring any errant balance problems under control but quite honestly, when done to the numbers, we have precious few trips out with the flow hood for balancing purposes.

Pretty much they WAY IT SHOULD BE.

Your grilles may be the wrong type,since they are noisey.

Last edited by dash; 12-23-2007 at 08:53 PM.
Well that was a big circle, huh. I see the hoods all the time at work. I bend an eye to see them calculating, closing or opening the damper and moving on to the next.
I just want and need to be assured that when I hire someone to plug in the numbers I give them that the human error factor is nil.
Most of the powers that be around here insist on having systems done using the Manuals. To this I agree. To this I will also have the duct designed. Hopefully the human factor is lessened due to the program. Hopefully the human factor is heightened to bring me, John Q Public, the results that we're paying for. I hate paying for stuff and still have questions.

You can read up on my dilema by searching in "Threads Started" in my profile
Programs don't eliminate humn error ,I think they actually can increase it.But don't let that be a reason not to have the calcs done!!!
I am having the calc's done. Sending off the paperwork tomorrow. I emailed the designer to ask about registers. He said that it'll be addressed. Also said most would just slap any 'ol thang on there.
Whew. Close one
Regarding dash's post #9, I agree that they are beasts of a different animal.
The commercial side, of high rises at least, use dampers at every drop. They get their supply from a common shaft. Unused air ends up back in the system. Correct me there where I'm wrong

Residential has the same air circulating in a closed loop. I would think the design for them would use no dampers. Each room has it's own requirement and open areas with multiple registers work in conjunction, It seems that the system should self-balance. Another wrench thrown in is when a second story is involved.

I'm hoping the Manual J & D with consideration of T that I'm paying for answers all my questions. Otherwise I'll have to buy a PC and do'er me dern self.
You still have dampers in your residential system, or at least you should. I do in my home and the ductwork was there before I was born. Sure all of the air that gets supplied will be returned but you need that air to go where it is supposed to. A lot of cheap builders will have just a single return grille located in the middle of the home without individual returns. The systems generally don't work that well.
In a commercial building you should have a damper in every branch of the ductwork so it can be balanced. Most aren't or not correctly balanced, but that is another argument. You don't have air circulating in a closed loop. Usually they are VAV with pressure sensors backing the main supply fan down as the VAV box dampers close. I have seen and used a closed loop supply duct, but it is intended to act as basically two supply runs with a point of equal pressure as the boundary between the two. I personally don't like that design since it doesn't usually behave as it is supposed to.
I'll have to wait and see what the Manuals work out.
My house is mainly 36x25 with a 10x17 room that would make the house shape as an "L". 1070 sq ft of inside living space. The 3 ton unit appears to be oversized and the duct work too small. Have to start all over, perhaps.
Originally Posted by regis101

I just want and need to be assured that when I hire someone to plug in the numbers I give them that the human error factor is nil.
.. done using the Manuals.
.. To this I will also have the duct designed.
.. I hate paying for stuff and still have questions.
You can read up on my dilema by searching in "Threads Started" in my profile

Send me (e-mail in my profile)
your house plans & orientation,
window sizes & specs,
r-value or walls and ceiling,
Thermostat set point,
Mfg and model number of condenser and air handler.
If YOU know these building envelope parameters,
the variation due Human Factor judgment is nil.

I let you know CFM required per room and diffuser sizes based on
Manual J 8th edition, Titus catalog for < NC 20.

Guesstimating
1,070 Sq. Ft.
2- ton .. 800 CFM
seems like there maybe 7 rooms > 100 Sq. Ft

i.e. 15 x 15 room = 225 Sq. Ft might need ~ 170 CFM
__. 10 x 17 room = 170 Sq. Ft might need ~ 140 CFM
__. 10 x 11 room = 110 Sq. Ft might need _~ 80 CFM

You're not likely to have more than one room requiring > 150 CFM,
unless there is a large window/ glass door area.
If you have > 160 Sq feet of windows and glass doors,
a 2-ton may unit be too small.

8" diameter duct will handle ~ 200 CFM.
So branch max size would likely be 8".

1,070 Sq. Ft should be anything BUT complex.

Merry Christmas

Designer Dan __ It's Not Rocket Science, But It is SCIENCE with Some Art. _ _ K EEP I T S IMPLE & S INCERE ___ __ www.mysimplifiedhvac.com ___ __ Define the Building Envelope & Perform a Detailed Load Calc: It's ALL About Windows & Make-up Air Requirements. Know Your Equipment Capabilities