Small HVAC Should Have A Non-Plenum Returns only?

[calettso]

Hi,

I have a question. I recently had an hvac tech tell me that small HVACs should have non-plenum return(s) because plenum returns on small roof top units (example 6-tons) are virtually ineffective. Is this true?

[Sleuth]

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Have him put that in writing, signed and dated. Then find a new guy.
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[kdean1]

It appear he is saying to pull return air into the rooftop unit (package unit) from outside. That’s so wrong!

[calettso]

Hi Kdean1,

He is saying to pull return air into the rooftop unit from the occupied space.

[kdean1]

Hi Kdean1,

He is saying to pull return air into the rooftop unit from the occupied space.

Then there must be a trunk through the attic, or if commercial, space above the drop ceiling. If it is a drop ceiling used as a return, all wiring in that space must be plenum rated.

[calettso]

I was thinking it was a rather interesting response. I then started thinking he meant, given the facility we were discussing, it was virtually useless. So here is some background information:

- This relatively new (less than 1 year) commercial facility has a humidity problem that is obvious during the warmest most humid days of the summer.
- From floor to drop ceiling 10'.
- From floor to roof 20'.

I am not sure if he is saying given the vast amount of space above, it is virtually useless.

I believe the humidity problem is because the facility is not properly sealed to prevent external air from being sucked in from outside (cracks and small spaces in the walls and roof). I say that because the relative humidity in the facility on the day I was there was 66%.

[Sleuth]

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What region of the www is this building located?
Was it a quick build, was the concrete green when built?
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[calettso]

Hi Sleuth,

- The facility is located in metro Atlanta.
- Not aware of it being a quick build and concrete green when built. If I had to guess I would say no to both.

[Sleuth]

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This building has been occupied for one year? And has a suspended ceiling? And is in Hot Lanta.
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[BNME8EZ]

Is this unit sucking air free from between roof and ceiling? If so he is probably saying that putting a ducted return to below the ceiling would make no difference which would be wrong.

[Sleuth]

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You can ask for the manufacturer's install book or even look it up yourself. Just need model # and time to digest it.

We experimented with whole "house" plenums" in the 70s and that didn't go well....
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[calettso]

Hi BNME8EZ,

Thanks for responding. No he is saying the ducted return to the below ceiling would be better because the non-ducted (plenum) is useless.

[calettso]

Hi Sleuth,

Perhaps this is what the tech is saying. The whole "house" plenums in a hot environment, especially with the vast amount of space above the drop ceiling, is useless. If this is what he is saying, I can better understand his statement.

[Sleuth]

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In the 70s it was supply side...unfathomable on return side.
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[BNME8EZ]

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In the 70s it was supply side...unfathomable on return side.
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We did a couple houses in the late 70's early 80's when I was just starting in the trade that had web truss joists. They sheetrock finished all the ceiling in the basement and the space in the web truss was used as return. It seemed to have worked well until someone took some ceiling down and didn't put it back.

[firecontrol]

Hi BNME8EZ,

Thanks for responding. No he is saying the ducted return to the below ceiling would be better because the non-ducted (plenum) is useless.

That makes sense. With no direct connection to the conditioned space the rooftop unit would be pulling the hottest air from right under the roof deck 10' above the suspended ceiling.

As for a humidity problem a 6 ton unit should have an economizer on it and might be pulling in too much outside air when the humidity is high outside, then you add to that the return is pulling from above the ceiling and the AC might not be running enough to dehumidify the extra humidity from outside due to the economizer not being adjusted correctly OR someone didn't design/spec the job correctly.

[VTP99]

Okay since the OP has a current thread running I came across this one from his profile.
A open plenum is just a short drop down from the roof curb as opposed to a duct plenum which would come down to the ceiling grid.
One creates a condition space one doesn't.