Supply and Return Short Circuit

I’m looking for a code reference that addresses the minimum distance allowed between a supply and return to prevent short circuiting (when the return grille is too close to the supply diffuser, air by-passes the room and flows directly back into the return duct). We see this alot, and need something clear and definitive to show this installation is not proper and needs to be corrected. I appreciate any help and guidance on this.

It helps to know where you’re located and what code your jurisdiction follows.

There is none.
They just expect the installer to not be stupid!

Yes, it is a problem.
I’ve made good $$$ on this one over the years.

In actually, the supply and return should not be at the same elevation in the room!
The location is determined by the primary load on the building within a region. Some areas require heat over cooling, and vice versa.

For HVAC Standards, look up ASHRAE Standards. (Note; “Standards” ).

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We’re a national company, but the last example we saw this condition was in North Carolina. The buildings fall under IRC code, inspected by local jurisdiction (i.e. Charlotte in this example).

It is a common issue in all homes, regardless of region, with slab foundations.

New codes prevent air duct in exterior walls and don’t want to take up closet or floor space, so now they just stick everything up in the ceiling.

I do not know if this will help but here is a link Where Should Return And Supply Vents Be Located?

Good to know.
Is there a code or standard for distance of the return from the furnace? Many years ago when I researched this, I came up with two standards. One, don’t put the return in the same room as the furnace, unless it is a big room and then if in a big room, it must be more than ten feet from the furnace. Is that correct?

In a place like here in Colorado where our warm season is almost the same length as our cold season, I see supply on the ceiling and return on the floor in one new subdivision and the new subdivision next to it, puts the supplies on the floor and the returns near the ceiling. It’s a real mix.
Here where historically, folks were more concerned with heating and until 30 years ago, AC was a rarity, systems were designed to heat a house. But now, AC has been added to the existing forced air system in these older homes. The constant complaint from homeowners is about poor cooling in these older homes, particularly on the 2nd floors.


Operates best in cooling mode.

You can not heat or cool effectively from one unit servicing two floors without a “balanced” return design.
Around here you see two return ducts on each floor with a damper at the return plenum that controls return air volume from each floor. Often marked Winter - Summer. An actuated damper system can do this automatically, but $$$.
We have very hot and very cold here.

Also, concerning 2nd floor supply; A/C supply is heavier and will not go upstairs, rather mostly to the 1st flr. What a/c air that makes it to the second floor, it just flows across the floor to the stairs and cascades like a waterfall back downstairs, where you don’t need it. Cold feet, hot head…

Think of air as water (they are both fluids and act the same). Consider pressure and volume.

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Yep, I have some version, usually simplified to “the house was originally designed to heat and not cool”, multiple times a year. Particularly in the summer when we are standing on that very warm 2nd floor.

Are you sure they aren’t just “jumpers” to equalize air pressure? Or are they actual returns?

It’s a ceiling supply and return that are 2’-3’ away from each other. I’ve attached an example here.