Q: Is an outlet above the HVAC air return considered within the environmental air space?

We need to move the HVAC air return in one of our rooms, and there is an existing electrical outlet in the most convenient spot. Rather than remove the electrical outlet, I was thinking about just moving it up a foot or so to make room for the new air return grill, as shown here: enter image description here

My understanding based on NEC is that the space between the studs through which the air will be flowing from the grill to the furnace is covered by 300.22(C) "Other Space Used for Environmental Air". What I can't figure out is whether the same rules apply if the outlet is located above the HVAC air return. Theoretically, the air flow would be downward from the grill, so it would not flow past the outlet; however, there is no barrier within the space between the studs to separate the outlet from the HVAC air flow.

Clarifying Edit: The return air is not enclosed in a duct; this is an old (1920s) house, and it just uses the stud bays for the air return.

  • 1
    I take it the return air is completely enclosed in a duct, no? Commented Sep 5, 2020 at 18:36
  • No - thanks for the clarifying question. This is an old house, and just uses the stud bays for the air return. I will edit to include this detail
    – Sam Zipper
    Commented Sep 5, 2020 at 18:42
  • What wiring method was used for the run to the existing outlet, and is replacing said wiring with new wiring an option? Commented Sep 5, 2020 at 19:17
  • @ThreePhaseEel - good question, I haven't opened the wall up yet so I'm not sure what's back there. I'd like to minimize drywall work since we're worried about being able to match the paint...
    – Sam Zipper
    Commented Sep 6, 2020 at 17:06

2 Answers 2


The air space is just that if the wires are within the air space in any way they need to be plenum rated power wiring even low voltage requires the plenum rating.

the only way around this Is to enclose the wiring or isolate from the environmental air source OR use plenum rated cables.

In your case if I understand it correctly the vent would be above the receptacle but in the same air space no isolation.

A electrical fire in this case could spread more easily without plenum rated cables and the toxicity, burn rate of standard cables vs plenum rated is the reason air spaces need to be isolated or plenum cable’s used.


Just add fire blocking above the air return and below the receptacle.

Now they are separate spaces.

Problem solved...

  • 1
    If the wires are feeding from the bottom not quite that easy. If the feed is from the top move the receptacle up block it off and add the vent below the receptacle would work.
    – Ed Beal
    Commented Sep 6, 2020 at 3:47
  • Thanks. I haven't opened the walls up yet to see which way the wires are running, so hopefully this is a nice and easy solution.
    – Sam Zipper
    Commented Sep 6, 2020 at 17:06

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