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Old 08-13-2020, 05:13 PM   #31
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Re: Can Central AC return duct sizing be too big?


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Originally Posted by 57TinkerMan View Post
Is the 6" return from the bedroom in the wall or ceiling?
The 6" return was run over the ceiling and into the wall near the whole house humidifier. It was "sealed" with some metal tape, for some reason. I figure I can just reuse that return and put it in a register with a filter to add additional air to the return. So making the straw a little bigger, so to speak.
So it's free now, and I can do anything I want with it.
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Old 08-13-2020, 05:16 PM   #32
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Re: Can Central AC return duct sizing be too big?


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You won't get a higher velocity from an air filter being in. You got a bogus reading.


The small pipe may be a fresh air intake.
Interesting. so if I am trying to pump in the same amount of volume with a smaller opening, that won't increase velocity? so you're saying that the amount of air going in will be reduced?
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Old 08-13-2020, 05:20 PM   #33
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This is the duct in the ceiling and it was attached to the piece of duct in the wall previously. I disconnected it and want to reuse the duct in the ceiling as a return in this closet.
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Old 08-13-2020, 07:02 PM   #34
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Re: Can Central AC return duct sizing be too big?


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You won't get a higher velocity from an air filter being in. You got a bogus reading.

If I understand the OP's post on this, I think he got his anemometer into the 24x14 return duct and reinstalled the filter. If so, he would see an increase in velocity due to the smaller area of the duct.


A1V1 = A2V2


His methodology for measuring is definitely questionable. As far as the 6" pipe goes, he may have had a bypass humidifier installed at one point.
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Old 08-13-2020, 08:06 PM   #35
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Re: Can Central AC return duct sizing be too big?


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Originally Posted by 57TinkerMan View Post
If I understand the OP's post on this, I think he got his anemometer into the 24x14 return duct and reinstalled the filter. If so, he would see an increase in velocity due to the smaller area of the duct.


A1V1 = A2V2


His methodology for measuring is definitely questionable. As far as the 6" pipe goes, he may have had a bypass humidifier installed at one point.

I thought he was saying that he took a velocity reading without the air filter, and then one with the air filter in. But read at the same opening.
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Old 08-13-2020, 08:23 PM   #36
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Re: Can Central AC return duct sizing be too big?


You're probably right. I was focused on the higher velocity readings, after reading the post again, it's still not clear to me.
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Old 08-14-2020, 10:39 AM   #37
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Re: Can Central AC return duct sizing be too big?


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Originally Posted by beenthere View Post
I thought he was saying that he took a velocity reading without the air filter, and then one with the air filter in. But read at the same opening.
Hi BT, that's exactly what I did. I just retested and got similar results. Im sure my testing methodology isn't precise, but im not looking for it to be precise at this time, and it's more for understanding how the filter type will affect my noise(dB) and performance (airflow).

That said, what I did this time around was to place the anemometer inline, ontop of the filter, wait about 15 secs, and then take the "max" reading.
I then tested it out without the filter and placed the meter in the same approximate location and got "lower" CFM readings. If this is the wrong methodology, can you let me know how I should test it? Note, I just want a general trend, and don't need to down to the exact CFM readings.
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Old 08-14-2020, 10:41 AM   #38
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Re: Can Central AC return duct sizing be too big?


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I thought he was saying that he took a velocity reading without the air filter, and then one with the air filter in. But read at the same opening.
Just to clarify, the meter was placed in the stream of airflow AFTER the filter, not before.I basically attached a string to the anemometer so that it would not get sucked up the return after I closed off the opening with the filter.
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Old 08-14-2020, 10:49 AM   #39
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Re: Can Central AC return duct sizing be too big?


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Originally Posted by 57TinkerMan View Post
Just noticed this...

Attachment 608295
Apparently this return was going into the walk in closet ands "sealed" in the wall so it wasn't effectively working anyway. It also has a wire coming from what looks like the humidifier controller, but using the interior of the return as a "raceway" to get to the actual humidifier.
Was this lazy work on the part of the installer, or is there a reason for a wire from the AC unit to the humidifier through this return pipe?
Finally, do you see any problem if I "open" up this return duct and place it in my closet ceiling for additional return air? With a filter of course. I would just need to rig one up for a 6" flex duct, unless you know of any adapters that I can use a normal 1" filter with. thanks.

-T
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Old 08-14-2020, 05:45 PM   #40
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Re: Can Central AC return duct sizing be too big?


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Originally Posted by ilumo View Post
Just to clarify, the meter was placed in the stream of airflow AFTER the filter, not before.I basically attached a string to the anemometer so that it would not get sucked up the return after I closed off the opening with the filter.

The air flow with the filter in is less than with it out. So you should see lower CFM, and velocity.


The filter itself can cause a turbulence at certain areas. An anometer needs to be placed at several locations and then the readings averaged.
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Old 08-14-2020, 09:47 PM   #41
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Re: Can Central AC return duct sizing be too big?


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Originally Posted by ilumo View Post
It also has a wire coming from what looks like the humidifier controller, but using the interior of the return as a "raceway" to get to the actual humidifier. Was this lazy work on the part of the installer, or is there a reason for a wire from the AC unit to the humidifier through this return pipe?

Lazy, maybe? Definitely cost effective as it probably was the easiest way to route wiring for the humidifier. A wire from the humidifier to the a/c unit more than likely allowed the humidifier to operate in the heating mode only.
Use your anemometer to check the velocity of the airflow in the 6" pipe near the humidifier. The tape should be removed and a filter should be installed in the grille.
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