In my last house I had the same problem the AC fan overpowered the DH fan forcing the damper closed. I solved the problem by placing the duct start collar as near the evaporator coil in the supply plenum as possible. This is point of lowest static pressure because this is where velocity is highest. I further reduced this pressure by build a reverse scoop that basically made a curved wing with the top of the wing pointing into the airflow. Bernoulli did his job and the net result was very little static pressure, no damper shutting during ac fan operation and improved airflow into the system.
You don’t wan to shut the AC fan off during DH operations for two reasons:
1. The AC runs less than the DH when humidity is high during shoulder seasons.
2. When the AC is not running the DH air will try to flow backward thru the evaporation coil causing moisture to evaporate back into the air.

What would be really nice is if someone made some sort of properly designed venture or other device like a scoop that would allow DH air to be injected in the supply stream at a negative stick pressure (suction) point). The final part of the solution would be a computerized control that sequenced all the system AC, vent fan , ERV , DH in a smart (AI) based fashion that learned the optimum comfort strings and maximized efficiency.

Where should the supply and return registers for an independent ducting system be located, and why?

The AC blower is so powerful that it prevents the damper from opening when the dehumidifier also running. Should I (or can I) prevent the dehumidifier from running when the AC blower is running? Is there a way to reconnect the supplies to give the dehumidifier blower a fighting chance to open the damper? I’m wondering if a wye to the existing 12″ duct might be enough.

Living and working in the Mid-Atlantic region where there is more humidity than the heat load normally addresses, we strive to make cooling systems address latent heat more than sensible. Since doing this greatly reduces the efficiency “ratings”, doing so is not popular with those who dwell only on efficiency “ratings” while neglecting comfort at a reasonable cost of energy.
Utilizing variable speed ECM’s and physically larger indoor coils allows us to maintain the largest area of coil surface at the lowest temperatures to allow cooling systems to address latent heat removal more so while cooling the air. Since cooling system run times are less in lower heat gain areas, having those systems address latent heat as a primary focus greatly decreases RH, allowing us to be comfortable at higher temperatures. We need to focus less on efficiency “ratings” and a lot more on comfort at reasonable costs of energy usage. Robbing Peter (latent load) to pay Paul (sensible load) just to obtain efficient “ratings” is counter productive to obtaining overall comfort.

Even though anecdotal circumstances are usually considered somehow less important than charts, I will continue to design and set systems for my customers in a way they tell me they are the most comfortable while enjoying reasonable energy costs. We are in the business of comfort. No matter how much we dwell on energy efficiency ratings, we must achieve comfort to have satisfied customers.

I have to disagree with Robin. ASHRAE Std. 55 has a comfort envelope that shows for typical summer comfort conditions, a change in RH from 50 to 60% results in slightly less than a 1 F drop in dry-bulb temperature for the same comfort level. I am quite familiar with this chart and it was developed from extensive studies on human subjects in various environmental conditions. I seriously doubt that dropping the temperature setpoint 1 F in a humid climate would result in more additional energy consumption than the energy savings from an increase in RH from 50 to 60%.

When we consider that allowing humidity levels to rise from 50% to 60% as using less energy, we must consider the total human comfort factor. While maintaining 60% RH reduces energy usage simply by not requiring the refrigeration system to condense the water vapor from the air, 60% RH also causes us to “feel” warmer due to the lessening of the evaporation from our skin to the atmosphere. If one is comfortable at 76 degrees with 50% RH, one is not going to feel as comfortable at the same temperature when the RH has increased to 60%. In which case, one is going to decrease the temperature, wiping out any savings from reduced energy usage from allowing the RH to rise to 60%. We must always keep in mind that while we strive for energy efficiency, we still do what is necessary to make us comfortable. In my over five decades in the HVAC industry, until recently, the average comfort level for average humans is 72 degrees at 40% humidity. By changing the parameters to obtain better efficiency ratings, we only fool ourselves into believing we are achieving more efficient systems.

I gave that report a quick review and it didn’t appear to be very comprehensive or conclusive as to the best way to connect a whole-house dehumidifier to a central AC system. My conclusion based on this report and other considerations is still that one should just keep the whole-house dehumidifier separate from the central AC system and let it operate independently.