The whole principle, in one paragraph

Warm air is less dense than cool air, so it rises. Cool air is denser, so it falls. Every argument about register placement is downstream of that one sentence.

Good placement works with that movement, so conditioned air travels through the part of the room people actually occupy before it reaches the return. Bad placement lets it take a short cut, which is how a room reads the right temperature at the thermostat and the wrong one in the armchair.

Cooling wants to come from above

Air leaving a supply register in cooling mode is denser than the room. Introduce it at the ceiling and it falls naturally, mixing as it descends through the occupied zone. The physics does the distribution for you.

It also lines up neatly with how those buildings are built. Slab-on-grade leaves nowhere to run ducts underfoot, so the air handler goes in the attic and everything is distributed from above. Climate and construction point the same way, which is why the pattern is so consistent across the southern states.

Heating wants to come from below

Reverse the season and the logic reverses with it. Warm supply air is buoyant. Introduce it at the ceiling in heating mode and its instinct is to stay there, stratifying against the ceiling while the floor stays cold.

Introduce the same air at floor level and it rises through the occupied zone on its own. This is why heating-dominant construction puts supply in the floor, and why those houses tend to have basements or crawlspaces, which is also where the ducts conveniently go.

There is a second reason that has nothing to do with buoyancy. Floor registers are usually placed at the perimeter under windows, where they counter the cold downdraft coming off the glass before it reaches the room. A ceiling register cannot do that job.

Returns are a separate question

This is where most discussions get muddled, because people reason about supply placement and then assume the return follows the same rule. It follows the opposite one.

The return should draw from wherever the air you least want is collecting. In cooling that is the warm layer at the ceiling. In heating it is the cool layer at the floor.

Almost every residential system has to do both through one set of ducts, so the answer is a compromise rather than an optimum. Bias toward whichever season is harder where the building sits, and accept that the other one runs slightly less efficiently.

Ceiling registers are not a hot climate product

The assumption follows naturally from everything above. Ceiling supply suits cooling, cooling means the south, so ceiling registers must be a Sun Belt product.

Our own order data says otherwise. Ranked by ceiling register sales, the largest market is not in the south at all.

New York leads, comfortably, in a heating-dominant climate where the physics above would predict floor supply.

The likeliest explanation is that building type overrides climate. Apartments, condominiums and converted buildings frequently have no basement, no crawlspace and no floor cavity available for distribution. What they do have is a dropped ceiling or a soffit. In multi-family and high-rise construction, air comes from above because there is nowhere else for it to come from, and that holds in Buffalo exactly as it does in Houston.

In a renovation you inherit the plan

Everything above matters most on new construction, where the duct layout is still a decision.

On a renovation it is usually already made. The boots are where they are, the plenum is where it is, and relocating a ceiling supply means attic access, cutting drywall, patching, and then painting. Not patch-painting either, because a patch on a ceiling reads as a patch under any light, so it becomes the whole ceiling.

So the honest version of the question, for most people reading this, is not whether ceiling or wall is theoretically better. It is which cover to put in the opening that already exists. Choose the cover, not the plan, unless something is genuinely failing.

It is worth knowing the theory anyway, because it tells you what to expect. If you have inherited ceiling supply in a heating-dominant house and the floor feels cold in winter, that is the system behaving exactly as designed rather than a fault to chase.

What actually changes in the cover

Once placement is settled, the cover itself has different work to do in each position.

• Floor. Takes foot traffic, so it has to carry weight without deflecting and sit flush enough that nothing catches on it.
• Wall. No traffic, but it is at eye level, so the face and the fixings are seen properly. It also gets knocked by furniture and vacuum cleaners.
• Ceiling. No traffic at all, which tempts manufacturers toward thin material. The catch is that a ceiling register hangs. It is held at its edges and loaded by its own weight across the middle, and a thin face will bow and then buzz against the frame every time the blower starts.

A ceiling register does not need to be strong the way a floor register does. It needs to be stiff, which is a different property. TRM builds ceiling registers from the same 18 gauge steel as the rest of the range, which is more than adequate for the load and enough to keep a large face flat and quiet.

Then size it properly

Placement decided, sizing is the next question, and it is the one that actually determines whether the installation is quiet. The face size of a grille is not the same as the area air can pass through, and the gap between the two is larger than most people expect.

That is covered properly in how to size a return air grille, and the reasoning applies to supply registers too.

TRM stocks ceiling registers in 6 x 6, 4 x 8, 6 x 10, 10 x 10, 12 x 12 and 20 x 20, and makes them to any size up to 24 by 72 inches, which matters in exactly the buildings that need ceiling distribution most, because converted and older multi-family stock rarely has openings that match a catalogue.