I believe you have a zone
I believe you have a zone damper stuck in the middle. It would normally be a higher velocity when conditioning either upper or lower levels but when trying to do both it is inadequate.

Also check for blockages at the evap coil . It could be heavily impacted.

Well said Dan !
Well said Dan !

@Darin, you seem to be
@Darin, you seem to be conflating coil face velocity and duct velocity when the two are (or at least should be) independently determined by design. Supply air temperature is a function of the volume (CFM) of air that moves across the coil. For a given system, airflow, and operating conditions, coil geometry (in particular, face area) determines supply air temperature, which is unaffected by downstream duct & diffuser geometry. Thus, duct velocity has no bearing on system capacity (notwithstanding any impact on conducted heat transfer through duct surfaces).

I have reviewed your scholarship and watched the videos on the bypass damper being used (and usually abused) in zoning systems. AND, I dont think anyone can argue that there is an energy penalty for dumping unrestricted conditioned air back into the return of a cooling system.

However, what other negatives do you think are applicable to using a properly designed bypass damper as prescribed in ACCA Manual ZR. ZR limits the amount of bypass air based on the OEM’s Leaving Dry Bulb (LDB) limit? Worst case scenarios I have seen even with close to 40% bypass, the LDB still hovers around 44 degrees F. Of course that is in my climate at 90 degree F outside, 75 degree F inside with 50% RH. Of course all bets are off with the surgeon who insists on 68 degree F indoors. Contrary to his belief, the laws of physics still control.

Low duct velocity, in of
Low duct velocity, in of itself, has no impact on latent performance. In particular, low velocity may be indicative of low airflow across the evaporator coil, which would actually *increase* moisture removal capacity.

The Manual D velocity references you cite are from Table A1-1, Air Velocities for Noise Control. So it should be obvious from the title that these values are intended as upper limits (“conservative” and “maximum”). There is no lower limit for return-side velocity, and I would argue there’s no lower limit on supply-side velocity, at least not within the scope of Manual D. (That’s not to say there’s never a downside associated with low supply-side velocity, as noted in the article and some of the comments).

The other issues you mention seem like potential contributors to high RH, but these topics are beyond the scope of this article. I suggest that you seek advice at the Home Energy Pros Forum.

Richard, I still do not
Richard, I still do not believe low air velocity is a significant part of your issues. It sounds like oversized cooling is definitely an issue, but reduced air volume will actually allow the dehumidification to be better as the indoor coil becomes colder, faster, and stays below dewpoint.
It sounds like you already have control between levels through zoning dampers.

Good morning all, thanks for
Good morning all, thanks for the responses to my post. Just to put the information into perspective as I have noticed people mentioning North Carolina and the codes in North Carolina. I am actually in North Carolina and have lived/worked here my whole career.
The homes I am having these issues with are built to Energy Star type standards.
The attics are atrocious, minimal ridge venting and soffit venting is not that great either. I have addressed this with builder and upper management. Attics in these homes get very hot and humid, not to mention stagnant.
Also, these homes are slab homes that sit right on grade, and I am aware and have pointed out that drainage is an issue. I am not that great with building code, but understand and follow mechanical codes pretty closely. I was always of the understanding that these slab homes were supposed to be built in a fashion to promote drainage, either graded up to slab or grade yard down. These yards are as flat as a piece of paper, I even have pictures of me standing in 1-2” of standing condensate water to assess one outdoor unit.
These are 1 system 2 zone homes, which is permitted in NC. Indoor units are located in the attic along with most of the duct work that is R-8 in attic and switches over to the permitted R-4.2 once it goes inside the envelope.
Manual D states they want to see 700-900 FPM in supply trunks, and 600-700 FPM in return trunks. I classify my trunks as the plenums and the larger/zoned ducts feeding the distribution boxes for each zone. Yes, this is a box and flex system, that is how new construction is done, cheap, cheap, cheap…. Not that I agree with it.
In my supply plenums I am seeing anywhere from 794.5 on the high end down to 620 on the low end. Once the air leaves the plenums and enters the larger flex trunks, the velocity averages 483.3 on the high end to 235.5 on the low end.
In my return plenums I am seeing anywhere from 544.3 on the high end down to 281.0 on the low end. Before the air enters the plenums from the flexes, I am seeing 559.5 on the high end down to 198.3 on the low end.
My velocity measurements were conducted by traversing each duct with a hot wire anemometer with the correct duct dimensions put in before each test, just in case anyone is questioning testing practice.
The person responsible for sizing/designing these systems and I have spoken at length. According to his designs, the systems are within the ACCA Manual S sizing restrictions of 90-130% I believe it is. The refrigeration sides are running in the 115% range of load, to allow for less strip heating in the winter. I struggle to agree with this sizing as per customer discussions, they were maintaining 70-72* indoor temp with high humidity during our 96-100* this summer. Maintaining temperature but having high humidity are the classic signs of oversized equipment.
I do not think that low low velocity is the only problem I am dealing with. But it is a compounding factor to go with the permitted oversizing, lack of grade and water getting back under the slabs, wicking up back into house, shoddy craftsmanship on attic ventilation, over sizing of equipment, home construction in general… But low velocity with over sized duct work in extremely hot attics increasing the conduction surface, in my mind is something we can control. No need to add to the issues..
Oh and the systems range from 2.5 ton to 4 ton…
I hope I was able to give more of the pertinents that people were questioning in this response. Any ideas/input is greatly appreciated. Thanks

David, I agree 100% that
David, I agree 100% that multi-level buildings, whether residential or light commercial, should have levels controlled independently. More than any other aspect, different levels have completely different heat gain and heat loss factors that can vary within hours depending on whether the Sun is shining or not. North Carolina does have some of the most stringent HVAC codes, so that doesn’t surprise me.
I also agree with both you and Richard that mechanical zoning is much preferred over air zoning from a central system. I don’t do many installations these days, but my son and I just conditioned a friend of his 1850s brick construction mill house with three levels. We opted for using 2, 4 zoned ductless systems with the interior units zoned by levels more so than by geographical positioning of the house. In this way, we were able to keep the capacity of the equipment lower because the upper level uses most of the unit capacity in the cooling mode and the lower levels use more capacity in the heating mode. This type of system also controls the high humidity we have in the Mid-Atlantic area when there is not enough heat load to keep systems operating for long periods of time.

@John, thanks for mentioning
@John, thanks for mentioning the ‘infamous’ bypass issue. Poor humidity control is but one of several detrimental consequences of routing supply air directly back into the return side.

And of course, zoned systems can be designed without a bypass. In fact, at least one manufacturer, Carrier, prohibits the use of bypass with its Infinity Zone Control System.

The problem I’m running into with beyond-code homes is that the loads may be too small, even for mini-splits, to use equipment zoning and, as you know, the only way to maintain proper temperature control per level from winter to summer is to have thermostatic control on each level. Seasonal manually operated balancing dampers are a poor substitute, IMO.