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.

For combination heating and cooling duct systems, we have to decide a location knowing that the performance of heat or cool will suffer. That is, unless you install a high/low combination return duct in a wall. The problem with this configuration is that contractors were not ducting in the return, just using the open bay in walls and floors. Since this is not a sealed return air path, it is likely that very little air came from the conditioned space. So, most make a choice of either a high or low return. Is it any wonder why you get the uncomfortable customer calling you the following season?

Since new replacement furnaces tend to have higher efficiencies, and in turn require higher air flows, I have found the stagnant zone in most rooms tend to shrink during the winter. The lower tempered, higher volume of air tends to create more velocity at registers and mixes well in the rooms. This results in a more comfortable customer in the heating season. But, what happens when we switch to cooling, try to force the air through poorly designed airways, and cannot get the stagnant humid air back to the evaporator? An unhappy, frustrated customer that will always insist the system never worked correctly from the start. Technicians need to open their eyes sometimes, use the information the customers gives you, and "think outside the box" - look at the distribution system for once!