Exhaust flue through return plenum

Does anyone know what this would be used for? Looks to be an exhaust flue exiting from the plenum but doesn’t seem to serve any function. It’s not attached and the flue doesn’t discharge through the roof. It’s capped off at the ends as well.

It was probably supposed to be a fresh air intake/ dehumidifier for the attic space since it was spray foam.

Were there any supply registers in the attic?


No, there were no supply register. Just a flue exiting through the plenum where it is cut off and capped before the next section, and that next sections extends up to the spray foam before stopping there, no roof penetration.

Nothing can pass through HVAC duct.

What makes you think it is an exhaust flue?

Did you open the equipment panel and look into the plenum to see what it was for sure? Only 4 screws.

It is capped off, so it really doesn’t matter. They are just trying to save energy cost, at the expense of Indoor Air Quality.

As Daniel said, it’s a fresh air duct (99.99%).

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Appears to be abandoned duct work for intake to condition air. Not in use.
Should be caped better.
Is that a condensate fitting at the rear?

No, that is the return.

To the roof?

This (if it were open) would improve airflow to the supply registers.

The filter is inside the HVAC equipment.

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Hard to see from the pics…could it be part of the make up air vent?

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For the furnace? No.

I see these things all the time here. They control negative building pressure from leaking air ducts and other ventilators etc. Pulling the OA through the back side of the HVAC is the best practice.

Encapsulated attic space indicates a .high efficiency build which may reduce the required ventilation requirement amount, resulting in the need for this type thing to be installed.

Insufficient information on this install…

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Well i wasnt sure but if its on the return side then yes return. Extra make up air would be the answer. I was asking if it was supply in 1st comment. So i didnt need correction, you act like a know it all. Good thing youre here to set everyone straight.

When it involves HVAC he (David A) pretty much is.


Really? Where??

Appearently, you did.

Yes, Thank G-d there’s someone here that knows his shit, and is more than willing to share his extensive knowledge!

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Oh! I thought it was meant to be something like this…

That’s the way it’s done Ray. But in this thread, we have a Package HVAC unit. Same principle, but this one was not hooked up yet.

David, are you saying what is in the OP pics is a packaged HVAC unit? I’m confused.

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He misspoke…a package unit sits outside and the air is transferred inside through attached ductwork…I think what happened is somebody disconnected the make up air vent because they didn’t want it sucking outside air…maybe they couldn’t calibrate it or maybe they just didn’t understand what it was for. Now an encapsulated attic can suffer from high humidity especially here in the south so there really should be a method to handle that. I’ve seen that some people will install a dehumidifier and some will leave an AC supply vent open in the attic in the hopes that it will be enough to dehumidify that space.

Yes, my fingers had a brain fart! I had a phone call in the middle of typing that reply. :face_with_spiral_eyes:I meant it’s a horizontal vs. vertical flow.

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