In my old rental house, I had a large window air conditioner mounted in a window in my living room. It did a great job blowing cold air, but was ugly and noisy (as are all window air conditioners). When I moved to my new house, the need for air conditioning was a little lower since the new house has double-pane windows and the house is also shaded by trees. However, I play music with my band every week in one of the small bedrooms, and with six people in one room in the summer, the need for air conditioning is easy to grasp. I didn't want to mount the large window air conditioner in a window, since it is so big and ugly, so I mounted it in the house's crawl space and installed flexible ducting to connect it to the house.
I know it is very difficult to see anything in this picture; the crawlspace is very dark and cramped. The air conditioner is an LG 15,000 BTU/hr model. It's the largest LG model that runs on 120V. I modified it by adding standard register boots to its cold-side inlet and outlet. I also used a plastic storage bin to couple the inlet register boot to the evaporator coil. Everything is sealed with rubber foam gaskets. Currently, the hot-side inlet and outlet are not connected to anything and just vent into the crawlspace under the house. I know this is pretty dumb, so my plan is to connect the hot-side outlet (the back of the air conditioner) to some nearby crawlspace vents. I can use uninsulated flex duct for this.
I removed the unit's control panel and built some custom circuitry that allows me to control the device via the standard wall-mount thermostat in the house.
The inlet and outlet flex duct are 8" and travel up from the crawlspace in a utility closet that is open to the house's attic. In the attic, the return duct connects to a 14"x6" ceiling vent:
14"x6" return vent
The supply duct connects to an 8" inline duct fan that is mounted in the attic:
$130 shipped, new on eBay. This particular fan is very powerful and also very quiet. I was able to mount it to the ceiling joists, and it cannot be heard over the general air noise when the system is running. I originally tested the system without the duct fan, but the air conditioner's stock fan did not move enough air through the ducts to be useful.
10"x6" supply register
Another 10"x6" supply register
Yesterday, it was over 92*F according to this thermometer. It was 81*F inside my garage.
Ahhh, 70*F in the house!
I had just installed the new duct fan and was curious to see how well it worked, so I turned the thermostat way down and was pleased to see the system had plenty of cooling power. The house is only 1100 ft^2, so the 15,000 BTU/hr air conditioner is sized well. The supply air is just under 20*F cooler than the return air, which indicates a properly-sized system.
Today, I set the thermostat for 75*, and the system was able to hold that temperature while running between %25 and %50 (the thermostat uses "four cycles per hour"). It was about 92* today as well.