HVAC register ductwork

[bcossa2020]

I’m investigating my home HVAC system and have low airflow in a register ( 90 cfm vs 130 cfm). It’s fed with a 6 inch round duct off the trunk line to a 6 inch 90 degree boot in the wall. The wall is 2x4 studs. The damper at the 6“ duct by the trunk line is wide open.
I would like to change the entire run to 8 inch round pipe but the only available boots I can find are 14 x 6….too large for my wall.
Would it make any noticeable difference if I ran an 8 inch duct from the trunk line and then installed an 8 to 6 reducer at the register?
Overall static is .5” but my airflow is low, especially on the return side and the master plan is to add more returns, but right now I am trying to make a plan for the supply.
The existing trunk is under sized so it will be replaced also, but if I can’t enlarge the wall registers noted above, and increasing the duct size that feeds the register (from 6” to 8” and adding the reducer) doesn’t increase air volume then I’ll need to look at maybe adding more supply registers….ugh.
Thanks in advance….

 

[JohnnyG]

I understand that oval piping flows bett that square. You might want to try this:
https://www.imperialgroup.ca/product/galvanized-duct-pipe-fittings/oval/pipe-elbows/oval-pipe/

 

[doitmyself]

the only available boots I can find are 14 x 6….too large for my wall.

Not sure why 14 x 6 is too large. Which dimension is too big, the 14 or the 6??

Here's a 14 x 4 x 8 inch: https://www.southernpipe.com/2348594/PRODUCT/N/Gray Metal South-243-4148?gclid=EAIaIQobChMI1Iy44-rV-QIVNSCtBh0LbwIUEAQYAyABEgIXZfD_BwE

Here's a 12 x 4 x 8 inch: https://www.winsupplyinc.com/southwark-metal/_/A-01414385

And, 12 x 6 x 8 inch: https://www.winsupplyinc.com/gray-metal-south/_/A-01371702

 

[PimTac]

I’m not a HVAC expert but increasing duct size will not increase air flow. The blower will put out X amount of CFM per minute of air regardless of the duct size.

Is this particular register at the far end of the line perhaps?

I would measure the entire space that you want cooled and see if the current system is sized properly.

 

[PWMDMD]

I’m not a HVAC expert but increasing duct size will not increase air flow. The blower will put out X amount of CFM per minute of air regardless of the duct size.

Is this particular register at the far end of the line perhaps?

I would measure the entire space that you want cooled and see if the current system is sized properly.

Doesn't static pressure play into it? In other words, if the duct size is too small it will increase static pressure at the blower and reduce CFM. Like trying to blow out through a straw that is too small.

 

[PimTac]

Doesn't static pressure play into it? In other words, if the duct size is too small it will increase static pressure at the blower and reduce CFM. Like trying to blow out through a straw that is too small.

My guess is that the air will find a way. Since there are multiple registers air will be forced into other directions.

Obviously my knowledge at this point is lacking.


They do make booster fans. I have no experience with them but I wonder if that is a option?

 

[Gyro Gearloose]

The blower will put out X amount of CFM per minute of air regardless of the duct size.

The blower is a form of a pump and every pump has a discharge pressure vs flow curve. The lower the discharge pressure a pump works against (big ducts) the more volume it puts out but the higher the discharge pressure the less volume it puts out. Pumps/fans are rated for a given output which is usually their point of best efficiency, but they will actually operate across a wide range of flows and discharge pressures up to certain limits.

Upsizing the duct from 6 to 8 should result in some flow increase just because mathematically an 8 inch duct has almost double the flow area of a 6 inch duct and therefore the pressure drop of an 8 inch duct should be only about 1/4 the pressure drop of the 6 inch duct. But it's going to depend on the length of the run as to how much benefit that will be. Replacing a 20 or 30 foot run might be just what you need but replacing just a 6 foot duct run may not make a significant difference.

 

[doitmyself]

The OP also states that his return side airflow is low, so maybe this is a bit more complicated than we can armchair quarterback.

 

[bcossa2020]

Not sure why 14 x 6 is too large. Which dimension is too big, the 14 or the 6??

Here's a 14 x 4 x 8 inch: https://www.southernpipe.com/2348594/PRODUCT/N/Gray Metal South-243-4148?gclid=EAIaIQobChMI1Iy44-rV-QIVNSCtBh0LbwIUEAQYAyABEgIXZfD_BwE

Here's a 12 x 4 x 8 inch: https://www.winsupplyinc.com/southwark-metal/_/A-01414385

Not sure why 14 x 6 is too large. Which dimension is too big, the 14 or the 6??

Here's a 14 x 4 x 8 inch: https://www.southernpipe.com/2348594/PRODUCT/N/Gray Metal South-243-4148?gclid=EAIaIQobChMI1Iy44-rV-QIVNSCtBh0LbwIUEAQYAyABEgIXZfD_BwE

Here's a 12 x 4 x 8 inch: https://www.winsupplyinc.com/southwark-metal/_/A-01414385

And, 12 x 6 x 8 inch: https://www.winsupplyinc.com/gray-metal-south/_/A-01371

And, 12 x 6 x 8 inch: https://www.winsupplyinc.com/gray-metal-south/_/A-01371702

Not sure why 14 x 6 is too large. Which dimension is too big, the 14 or the 6??

Here's a 14 x 4 x 8 inch: https://www.southernpipe.com/2348594/PRODUCT/N/Gray Metal South-243-4148?gclid=EAIaIQobChMI1Iy44-rV-QIVNSCtBh0LbwIUEAQYAyABEgIXZfD_BwE

Here's a 12 x 4 x 8 inch: https://www.winsupplyinc.com/southwark-metal/_/A-01414385

And, 12 x 6 x 8 inch: https://www.winsupplyinc.com/gray-metal-south/_/A-01371702

The front to back dimension is too large for a 2x4 wall. Thanks for the links you sent, but those are all 4” front to back which are too “deep” for a 2x4 wall. I’ll search in the links for one 3-1/2” deep.

 

[bcossa2020]

The blower is a form of a pump and every pump has a discharge pressure vs flow curve. The lower the discharge pressure a pump works against (big ducts) the more volume it puts out but the higher the discharge pressure the less volume it puts out. Pumps/fans are rated for a given output which is usually their point of best efficiency, but they will actually operate across a wide range of flows and discharge pressures up to certain limits.

Upsizing the duct from 6 to 8 should result in some flow increase just because mathematically an 8 inch duct has almost double the flow area of a 6 inch duct and therefore the pressure drop of an 8 inch duct should be only about 1/4 the pressure drop of the 6 inch duct. But it's going to depend on the length of the run as to how much benefit that will be. Replacing a 20 or 30 foot run might be just what you need but replacing just a 6 foot duct run may not make a significant difference.

by odd coincidence I am replacing a 25 ft run with a 14 ft run…..

 

[bcossa2020]

I’m not a HVAC expert but increasing duct size will not increase air flow. The blower will put out X amount of CFM per minute of air regardless of the duct size.

Is this particular register at the far end of the line perhaps?

I would measure the entire space that you want cooled and see if the current system is sized properly.

thanks…I’ve spent a few weekends investigating and the numbers for the current system should work, so I’m looking at duct sizes and routing…

 

[brianl703]

I’m not a HVAC expert but increasing duct size will not increase air flow. The blower will put out X amount of CFM per minute of air regardless of the duct size.

That is true IF the furnace/air handler is equipped with a constant airflow ECM motor, which is found on higher end, newer systems. If it has dipswitch settings to set the CFM, then it's one of those.

 

[bcossa2020]

Update, kind of…………
Im working on the return side of the system. Cut an 8” hole in the trunk duct and measured 191 cfm. Then installed an 8” to 6” reducer and measured 121 cfm. These numbers are in line with my ductulator.

I think I’m learning that for a given size of air handler, if you use wall registers in a typical 2x4 stud wall you’re limited to 6” round ductwork based on the duct size that will fit inside your wall. So if you have insufficient air flow it can’t be fixed by upsizing the ductwork.
If you have floor or ceiling registers you can upsize the ductwork and register to increase air flow.
So you may end up with a greater number of wall registers than floor/ceiling registers due to the duct size limitations.

 

[Lubener]

Often the duct work is just slapped together and leaks are present at the joints.

 

[dlundblad]

I have a few runs that I am guessing are too long. Ive thought about getting some duct fans and putting them between the furnace and the vent.

 

[Fitter30]

Most systems returns are to small. Properly sized 144 sq in per ton of cooling and have to look at the free air of the grill most are 80% of the size.

 

[John105]

I’m not a HVAC expert but increasing duct size will not increase air flow. The blower will put out X amount of CFM per minute of air regardless of the duct size.

Is this particular register at the far end of the line perhaps?

I would measure the entire space that you want cooled and see if the current system is sized properly.

I spent what I consider a fortune on a Trane system in 2020 (old Bryant broke after 19 years, 16 of which I benefited from as previous owner had it installed). Anyway sitting in my dining room, I was freezing. Measured the temp at the vent and it was 43F. When I brought this up the salesman said oh that's a brand new system 31F drop is not unheard of. Googling seemed to lead to poor airflow. Sure enough, about 8 days later the system completely froze up, and the AC was running 19 hours a day (luckily where I live there are 40 hours in a day so about right). The installers had set the AC to blow around 600 CFM, not the 1200+ recommended.

Using logic, if the ducts were ok with the last system, why would they be inadequate with the new one, since the tonnage was unchanged...took me a while to think of that. But until the tech came out and found the wrong seting in place, I was concerned about the ducts...we are very careful not to block registers...

Also I wonder if someday a repair is done under warranty, how do we know the tech doesn't switch out the ECM motor for a PSC? I saw a YouTube where the tech said I don't have a ECM so I'm using a PSC and the customer doesn't know anyway....he tried to justify saying the didn't have the ECM available and needs to get the system running...

 

[brianl703]

Also I wonder if someday a repair is done under warranty, how do we know the tech doesn't switch out the ECM motor for a PSC? I saw a YouTube where the tech said I don't have a ECM so I'm using a PSC and the customer doesn't know anyway....he tried to justify saying the didn't have the ECM available and needs to get the system running...

You'll know because, with a PSC motor, there will be no difference in airflow between "fan only" and when the AC is on.

With an ECM motor, fan only is much lower airflow.

Also with an ECM motor, the fan speed ramps up when the fan comes on. PSC motors cannot do that.

As far as Trane, the dip switch settings they use to set airflow seem to utterly confound some HVAC "technicians".

 

[cmarshall78]

Dumb question, but how are you measuring cfm, and why do you believe it should be 130 cfm? Is this room not maintaining temp like the rest of the house? I'm a sheet metal worker, and have been doing test and balance for the past 20 years. 130 cfm is pushing it for 6" pipe, but if there's other dampers then you need to balance the other registers to it, thus increasing the cfm. I wouldn't change duct sizing over 40 cfm.

 

[bcossa2020]

Dumb question, but how are you measuring cfm, and why do you believe it should be 130 cfm? Is this room not maintaining temp like the rest of the house? I'm a sheet metal worker, and have been doing test and balance for the past 20 years. 130 cfm is pushing it for 6" pipe, but if there's other dampers then you need to balance the other registers to it, thus increasing the cfm. I wouldn't change duct sizing over 40 cfm.

I appreciate hearing from an experienced T&B professional. I bought a small hand held anemometer and am measuring flow at the center of the grilles. I‘m basing the design flow on 1 cfm per sf of room -admittedly not a detailed analysis. This room is warmer than the other rooms on the floor. After looking at the typical duct fittings available I don’t think I will be upsizing any of the duct for the current wall registers. All of my current balancing dampers are wide open.

My current plan is to finish the investigation and sealing of the current return ductwork and then replace the accessible undersized trunk duct along with sealing the existing runouts and minor rearrangement of the taps to a better layout. I will keep track of the supply air and with any luck this will make the desired changes in comfort. If not I’ll start looking at adding some runouts….6” for wall registers and 6 or 8” for floor registers.

I‘ve “fixed” something so far because the unfinished garage is no longer cool .