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Old 09-02-2009, 10:02 PM   #1
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Starving AC System & Flexible duct pricing


Hi, I'm currently listening to a 500mph suction wind being created at my return vent on my AC system. I can even put a thick envelope up to the grill and it will stick!
I'm thinking I should run some additional return vents from different locations of the house to ease the demand on that grill (whole system only has one 16” return… on 4 ton variable speed unit). Do you think that will increase the efficiency of the unit? My instincts tell me the system is starved for air. (It's a decent bit larger than the unit it replaced)

My main question is if anyone knows of an expensive online store which sells 10” flexible duct and maybe the material to make a junction box and return vents.

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Old 09-02-2009, 10:11 PM   #2
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Starving AC System & Flexible duct pricing


So if this is a 4 tn what was the size of the unit these replaced

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Old 09-02-2009, 10:17 PM   #3
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So if this is a 4 tn what was the size of the unit these replaced
Just a 3.5 ton.
I think the difference is that the air handler is a super mack-daddy variable speed thing. It looks twice as big as the old unit...
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Old 09-02-2009, 10:23 PM   #4
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Starving AC System & Flexible duct pricing


With a 16" return flex you are within specs for air flow. You need 1400-1800 cfm with 1600 being 400 cfm per ton.

16" flex will provide approx 1400 cmm at .01 static.

If I was going to add another return flex I'd go with 8" flex that will give you approx an additional 230 cfm.


The next thing you need to look at is your supply air sizing.

Just going bigger is not always good you really needed to have a load calculation done to see what you really needed
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Old 09-02-2009, 10:34 PM   #5
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Interesting Plumber101, I guess I'm still at an end-road because I'm not sure what to do at this point. The 16" return vent is right underneath the unit pulling in from my living room. The air handler is on the opposite side of the wall in the garage.
When the AC winds up we actually need to turn up the volume on the TV because the sucking sound is so loud. It seems to me (lamen) that the system should be cooling better than it is so I presumed it was starved for air.

I installed the larger unit to make up for a large Florida Room (10'x28') which was added many years ago. The Florida Room was tied in with two 8" supply's (no return) using the same system.
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Old 09-02-2009, 10:37 PM   #6
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Starving AC System & Flexible duct pricing


So how far from the return grill to the air handler is it...

and is it a straight shot
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Old 09-02-2009, 10:50 PM   #7
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Originally Posted by Plumber101 View Post
So how far from the return grill to the air handler is it...

and is it a straight shot
Yes, the current return is a large box attached to the bottom of the air handler. The top of the box is open to feed air handler and one of the sides of the box has a 16" hole cut out and married to the wall. So essentially the return is within inches from the unit (right through the wall and up into unit).

The additional return was going to be installed about 25 feed down the hall. the bedrooms down there don't get as cool as I'd like so I was either going to:
* Install larger return in the hallway and cut small vents above the doors
or
* put a box and the end of the hall (in attic of course) and run two smaller flex ducts to vents in the ceiling in those rooms.

Not sure what would be best there.
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Old 09-02-2009, 10:54 PM   #8
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There lies some your problem the return is feed into the Lvgrm. Sound travels in straight lines and that is why it gets louder. What you need to do is try to move the return distant to the Lvgrm.

Hallway
bedrooms
and some off the Lvgrm.

You need to get bends and turns in the duct work and that will help to reduce the noise.
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Old 09-02-2009, 11:01 PM   #9
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Oh wow... so what you're saying is that I should remove that grill, drywall over the area and install retures to different locations of the house? Generally the same return volume as the 16" duct now but not right there by my ear in the livingroom? Veeeery interesting.

I have one nice open side on that box in the garage that I can elbow right into.

(you've been a fantastic help I must add... thank you)
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Old 09-02-2009, 11:35 PM   #10
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(you've been a fantastic help I must add... thank you)
Darn right!
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Old 09-03-2009, 05:06 AM   #11
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Slighly disagree plumbler.

A 16" round return is small for a 4 ton(1600)CFM.
It creates too high of a velocity.
Roughly 1200 Foot Per Minute.

But, it may not b the only cause of your noise.

Some installers leave the blowers set to factory speed. Which could be 1800 CFM or more on your unit.
So the blower settings should be checked.
Next, most likely, your return grille is probabl too small. And creating much of the noise. Easly test. Open/remove grille, and see if noise is less.

Part of the problem now with those rooms not cooling right. Is your supply duct is undersized.
So the increase in pressure from the extra air that the new system wants to move is causing more air to be forced out the supplies closest to the air handler.

Check your blower settings first.
Then do the return grille test second, and post results.
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Old 09-03-2009, 05:57 AM   #12
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Starving AC System & Flexible duct pricing


I agree with Beenthere.

Check blower settings. Most likely it is set for 2000 cfm and not removing enough humidity by moving too much air over the evaporator coil, (not allowing coil to get cold enough to remove humidity).

The return grill is way too small for 4tons. Is there room to increase the grill size on this wall or add another on another wall that goes to return box? A rule of thumb I go by for return grill size is 1sq/ft per ton.
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Old 09-03-2009, 07:29 AM   #13
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Hey guys... so how would one check the blower setting? The unit is a variable speed unit so it starts slow and then after a few minutes it begins sucking a new dimensional black-hole in my living room. I'm waiting for it to snag one of my cats

The easiest way to get more air to the unit would be to install a grill in my hallway ceiling and run it back to the unit from there. I have just enough room to run as much as a 14" flex down and elbowed into the existing box. I thought about running 10" to the grill in the hallway but have a box in the attic above the unit for expandability... running a 14" down from it.
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Old 09-03-2009, 07:41 AM   #14
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In your install manual, will be instructions for blower speed settings. May also have a sticker/or instructions in the blower compartment.

If you add a 12" round. That would give you a velocity of roughly 715FPM for the 16" and 763FPM for the 12". Figuring that the 12" will have a slightly higher friction rate then the shorter 16" run. And the 16" is moving 1000CFM, and the 12" is moving 600 CFM.
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Old 09-03-2009, 07:56 AM   #15
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In your install manual, will be instructions for blower speed settings. May also have a sticker/or instructions in the blower compartment.

If you add a 12" round. That would give you a velocity of roughly 715FPM for the 16" and 763FPM for the 12". Figuring that the 12" will have a slightly higher friction rate then the shorter 16" run. And the 16" is moving 1000CFM, and the 12" is moving 600 CFM.
Hey beenthere... so is that good? Is that what I should aim for? I could run a 10" to the hallway location and a smaller one (not sure size) to my master bedroom. The reason I'm limiting myself to a 10" in the hallway is because I have to install it between the rafters in my attic. I could run a 12" back there and break it off to the actual bedroom locations with smaller flex and a box. BUT, when all is said and done my master bedroom on the other side of the house could probably use a return as well.

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