How to installed a return Air vent?

When using ACCA's Manual T for register selection, there are several basic rules that are followed regarding location. Most guidelines revolve around room air circulation and stagnant air, as well as the equipment application (heating only, cooling only, or combination). Application is the largest determining factor when it comes to return air locations.

For heating only applications, a low return duct is ideal to bring back the coldest air otherwise stagnant in the room. This is true regardless of the supply register location, floor or ceiling. I knew this at a very early age, lying next to the return (after fighting their dog for it!) at my grandparent's house was the coolest part in the home, even though they were just using the wood stove. As we know: "hot air rises", most house as a system folk can argue the true meaning of this, but either way in the heating heating season the cold air remains low and relatively stagnant. So, most of those floor registers in basement systems work well - in heating only applications.

In cooling only systems, we want to return that warm, stagnant air near the ceiling first. This works well for attic systems due to the ease of installation. Adding a return duct to every room keeps the temperature difference between conditioned spaces low. Remember, in the cooling season the warm stagnant air tends to remain high in rooms, so why do contractors still think under-cutting a door is an acceptable return air path? This method might have worked well in the heating only applications with low returns, not at all ideal for an air-conditioner. In New England, a considerable amount of installations I've seen continue to use central hallway returns for attic installations. Since this provides for poor air circulation in the surrounding rooms, both heating and cooling operation tends to see poor performance.