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!


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Comment by Steve Waclo on January 18, 2013 at 9:33pm

Thanks for making time to write your article. Some additional thoughts on return air and grill locations.

As Ed points out below, I don't believe it's possible to have too much return capacity but from a purely anecdotal perspective (my own single story home with attic ducts and one central hallway return in the 9' ceiling), I believe that in some situations, return location does matter. Warming to the occupied set point during the heating season was interminable as the system worked from the top down and constantly pulled warm supply air off the ceiling, back to the furnace. My solution was to drop an 8" return duct from the other side of the ceiling return in the attic, down through a closet, add a 90 degree adapter (for a small filter) and add relief grills to the closet door. Cold air off the floor probably helps maximize Delta T across the condensing furnace heat exchanger and crawl space encapsulation a few years later (including 4" supply to the CP) really warmed up the floors and improved morning pick-up (again, anecdotal :-).

Apologies to Ed for the clown who always manages to come up with an exception to what he has learned over many years and hundreds of homes.

"Don't even get me started on why we have ducts in the attic to begin with."

Think it had something to do with money? :-)

Best wishes.
Comment by Ed Minch on November 21, 2012 at 8:55am

There are plenty who will chime in here about the need for a duct system at all, but I tend to disagree with many of your assumptions.  When we design a system, we make sure there is 110% of the return area versus supply area, because we want as little static as possible on the return side - check that chart in Manual D that states that 50% of the horsepower of the fan can be eaten up on the return side by poorly designed returns.

In a properly designed system, there is enough air movement at the supply grills to mix air thoroughly in the room.  It should not matter where the supply is in either heating or cooling mode if they are mixing properly.

If you think of a return as nothing more than pressure relief from the supplies, you will see that it does not matter where it is located - high, low, upstairs, downstairs - as long as the supply air can get to it.  Energy Star V 3.0 allows for an engineered return system, which would be one that does not allow a supply to build more than 3 pa of pressure behind a closed door.

We have designed houses with single returns, down low on the first of two floors and have had no problems.  You have to be sure the doors are undercut enough to get the air in each room out, and this means that there must be a transfer grill in the master bedroom where there are multiple supplies, but we find it to be cheaper and better - a combination hard to beat.

In our mid-Atlantic climate, here is how things evolved:  We only had heat in houses until about 1975 when A/C started to become standard.  Duct systems didn't change even though the A/C moved more air, resulting in the return being too small as the supply pushed 10 pounds of air through a 5 pound duct system.  The HVAC contractor was called in to fix the problem and he installed a return upstairs that solved the bulk of the problem.  He didn't realize it, but he could have put that same return downstairs and done the same thing (provided air could get out of the rooms).  But he was convinced it was needed upstairs and that worked, so it became "common knowledge".

This does not address the need to change air flow amounts seasonally between vertical floors.

Don't even get me started on why we have ducts in the attic to begin with.

Comment by Curt Kinder on November 17, 2012 at 1:33pm

We (Florida) are required by code to provide returns in most rooms with doors (baths, kitchen and utility rooms are exceptions). It's a good code, because in addition to reasons above, closed rooms become pressurized and leak outward, and the central return area goes negative and leaks inward.

Although higher efficiency furnaces (and heat pumps) need more airflow, this is often countered by tighter houses needing much less capacity and therefore airflow. We sometimes go as high as 1500 SF per ton, and that requires fairly careful attention to supply duct placement.

I have a pretty good business eliminating wall cavity returns so as to get duct leakage down to reasonable percentages.

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