When I started writing this I was using the acronym HVAC for Heating, Ventilating, and Cooling. Since none of the equipment ventilates the areas that it conditions I am going to just call it HaC for Heating and Cooling
I prompted my Wife that the home we now own was the right one to purchase partially because the heating and cooling equipment is within the building envelope. It was built in 1954. Is full masonry construction and has old leaky windows. Having the HaC equipment within this envelope is a situation that rarely happens in new construction. Something that I discovered later and to my dismay I had abandoned is that each room of the home had a supply and a return in it. Interestingly the supply is in the center of the home and the return was at the exterior wall. Both of these are at floor level which is not generally accepted these days but we often still feed from the floor and return under a staircase which is generally low in the wall. Or in the case of second floors is fed from the ceiling and returned from the ceiling.
Today we supply over or below exterior openings since these are usually cooler or warmer depending on the season and through a process called the stack effect the warmer air moves to the top or the cooler air moves to the bottom creating convection in this area that helps to mix the air. (I would assume this is fairly localized and does not mix all the air in the room well.) In my case we abandoned the floor returns and placed a sealed duct return in the attic to the HaC equipment in the mechanical room. This allows for the air to be moved from floor to ceiling thus helping to even the temperature throughout the space. New construction HaC equipment is relegated to crawl spaces and attics. Generally this unconditioned space is either extremely hot or relatively moist in relation to the surrounding environment.
In the former the air-conditioning has to cool that 140 degree attic air before it can cool the air in the home. On top of that cooling is a process of removing moisture from the air, so unless the system is sized right and allowed to run for extended periods of time it will not remove latent and occupant moisture from the air and therefore not cool. The crawl space system may have an easier time of conditioning the air in its environment but it has the unenviable task of keeping all that dirt, grime, and possibly mold associated with crawl spaces out of the air-stream. This in and of itself is the best reason to seal duct work in the crawl space.
Conditioning the crawl space helps with the ducting problem and goes a long way to mitigating moisture and other contaminants in the crawl space environment. This also will allow the equipment to generally only work with conditioned air and will have an easier, read equipment sustainability, time of maintaining the temperature set at the thermostat.