Why does the vent need to be on the floor? If the main runs are in the basement, rectangular duct can run through an interior stud bay to up higher in the room if you need the floor storage. But, it will have to exit the stud bay somewhere, so figure out where that might be best accomplished. It's the same with a return. Ideally, it should be located across the room from the supply, but there is some leeway in placing it. Talk to your HVAC professional here about your needs. However, ventilation of a pantry is a MUST, so don't skip it.

Sue36--You're not understanding Buehl's duct. The opening is in the floor, next to the wall. It's not spaced out from the wall where she could use a floor duct. The duct is not coming up into the stud cavity. This style of register is quite common here in the upper Midwest though they were new to me when I moved here from the south central part of the United States. My house has them & is 30+ years old. And it is the overwhelming choice for forced air systems in new builds. They looked weird to me for a while, until I got used to them. Now, I don't notice them because they're 'normal'. We've owned houses with both styles of registers. I prefer these because less dirt & stuff gets into the duct than what does with floor registers. And, there's no need to attach a deflector to them, either. Moving that opening out from the wall is a big deal if she has rigid metal ductwork. I can guarantee blood loss & depletion of one's French vocabulary. :) And for no good reason other than, to a few people, a minor aesthetic one. No one is going to notice that duct except for Buehl and only for a few days and that's because she is admiring her beautiful new floors. Everyone else is going to see the gorgeous floor, the pretty windows & what's outside the windows and the duct is going to disappear in the same way that outlets & switches do.

The ceiling above the crawlspace is the bottom of the first floor. The warm moist air in the crawl space will rise up into to house if there is no vapor barrier to stop it. I would expect a three year old house to have tight windows and doors. The crawl space might be a source of the excess moisture in the house. What size is the AC? I would think a 2.5 ton single stage, or 3 ton 2-stage AC would be sufficient for a newly constructed 1749 sq. foot single story house. Did the builder provide a load calculation? A 6-10 minute run time might be considered short cycling. How often does this happen in an hour and what is the outside temperature?

Ray, Less than ideal doesn't imply something won't work, it simply means there are better options if you want a zoned system. I had a single-stage heat pump and fixed-speed air handler for HVAC on the 2nd floor of my previous home. It was a zoning retrofit that provided two zones of control. It worked for 15 years until the heat pump gave out and then for another three after the heat pump and coil were replaced. It was less than ideal. The variable speed two-zone control system on the 1st floor was better. That system wasn't a retrofit; it was a brand new system with all new duct work. I'll make an attempt to answer the air flow question for you one more time, Ray because I don't want you attempting to design or sell someone a system without knowing the basics. A single-speed air handler isn't a variable speed air handler like your beloved Bosch inverter. It produces air at a single flow rate depending on the blower's fan curve. The more pressure drop, the less flow. Consider a two-zone system where the cooling load on each zone is equal (i.e. 50% of the total load.) When only a single zone is calling for cooling, a fixed-speed air handler will direct all the air flow it can produce into that zone. In this case, 100% of fan capacity for only 50% of the cooling load. The amount of air flow won't be fully 100% of the system's normal air flow due to the additional pressure drop compared with both zones open and calling for cooling. So there's an excess air issue. That means too much air, Ray. To deal with the "too much air" issue a bypass is typically installed. It recirculates a portion of the air leaving the coil back to the upstream side of the coil in a continuous loop. The temperature of the air entering the coil gets reduced, and continues to get reduced as the zone approaches set point temperature. There's not enough load on the coil and it's at risk of freeze up because the air flow in a single zone is less than the full air flow rate the single-stage compressor and coil are designed for. That's the too little air flow part. Variable-speed and multi-stage compressor systems are much better for zoning applications than single-stage, fixed-speed systems because they can adjust the air flow and the cooling to match the actual load. By contrast, single-stage compressors and fixed-speed systems are less than ideal.