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Old 07-14-2011, 09:07 AM   #1
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noisy blower / filter box redo


Right now my furnace sounds like a train engine... when ever it speeds up to high speed that is (claims at .5" WC it should be doing 2028 CFM, if I change the jumpers to run at low speed which at .5" WC it should be doing 1287 CFM the noise goes away... which leads me to believe that either the return is undersized, or the blower is just loud as heck.... now other furnaces I've seen when running full speed you barely notice them, this one you can hear running through walls...

the total conditioned Square footage of the house is ~3,800 sq.ft.
Basement: 1500
1st Floor: 1500
2nd Floor: 830

the furnace install documents say this about the return:
Return opening 19 x 21 5/8
minimum filter size 15 1/2 x 25


right now the return has a 16x24x1 filter... the return duct coming from the boot is 24x8, which turns into 2 separate 12x8 ducts (that is what about 1,200 CFM? not sure since I dont know the velocity and true WC of the design)

Furnace Info:
* has a 4 ton condenser unit on it (R-22 refrigerant)
* 125,000 BTU/h
* 92.1% AFUE
* 3/4 HP blower motor
* 12 x 12 blower wheel size


what I would love to do is get a media filter on here, but not sure what size would fit the furnace..
Was looking at the Honeywell F200E1037

CFM: 2,000
WC drop: 0.28"
Opening Dimensions: 23.25W x 18.44H inches
Dimensions: 25.5W x 20.18H x 6.75D inches
wasn't sure if that was too big, the wrong size, etc... it's the closest I could find size wise to the return opening on the furnace, and the filter size is 20x24x4 which is a little more interns of sq inch for the opening area.

I know you don't want the return velocity too high or stuff will be sucked right past the filter or not filtered properly, and it seems like its trying to pull more then it can get through right now which might be the cause of the noise... if you ever had a oh say 4' x 4' sheet of sheet metal and moved it around to make a thunder like noise, its close to that sound when running just a little bit quieter. So I am thinking this return needs to be taken up to the following:

* Larger filter area, hoping media filters will help since they have a ton more surface area then a flat filter

* Larger return boot and stack, right now the stack off the boot is 24x8, should be at least 24x10?

* increase the size of the return trunks, right now they are 12x8, maybe 16x8 each way?
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Old 07-14-2011, 01:33 PM   #2
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noisy blower / filter box redo


If the blower motor is making noise the bearing is going bad.
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Old 07-14-2011, 02:21 PM   #3
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If the blower motor is making noise the bearing is going bad.
it's not the motor though as far as I can tell, the noise sounds like never ending thunder like if you pushed in on a plenum then let it push itself back out

if you take the blower out and run it by itself its pretty quiet minus the air disruption caused by it

I looked up the original fact sheet that aire-flo (lennox) gave out on this furnace and it says it should be "extremely quiet", definitely not what I am experiencing
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Old 07-14-2011, 02:25 PM   #4
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noisy blower / filter box redo


Remove the return grid and check if the noise will get strong.
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Old 07-14-2011, 02:30 PM   #5
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Remove the return grid and check if the noise will get strong.
that's what I am hoping on doing tonight to test.... that and trying the other speeds the blower runs at to see if its any different...

AC wants it to be on High though to work efficiently apparently, and thats where all the noise is at when its trying to draw 1600 to 2000 CFM (not sure which one it is for this model with the current duct design)
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Old 07-14-2011, 02:36 PM   #6
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noisy blower / filter box redo


In cooling mode the the blower motor should be set in high speed. Check if this blower motor is the correct.
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Old 07-14-2011, 02:40 PM   #7
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In cooling mode the the blower motor should be set in high speed. Check if this blower motor is the correct.
right now heat call and cool calls are running in high speed, not sure if heat should be there or not, thats where it was when I bought the house

the blower motor is the original part number, so I think its the right one. the furnace is only 4yrs old
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Old 07-14-2011, 03:09 PM   #8
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noisy blower / filter box redo


To be able to check static pressure you'll need a Manometer. But you can focus on the noise issue since cooling speed is set to high speed.
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Old 07-14-2011, 04:08 PM   #9
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noisy blower / filter box redo


4 ton AC x 400 cfm/ton = 1600 cfm max. No more required. Carrier is now using 350 cfm/ton on the newer units so that = 1400 cfm. With a TX valve and ECM motor you can safely run it at 1400 cfm . De-humidifies a lot better at the lower cfm but you should get the subcooling checked to be safe.
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Last edited by yuri; 07-14-2011 at 05:32 PM.
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Old 07-14-2011, 05:28 PM   #10
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8x24 isn't anywhere near big enough blue. 1600cfm and you want 400fpm max at the drop so the duct needs to be equal to 4 square feet. Usually the big 4 and 5 ton units are better off with half coming in from each side. I bet you have more then a half inch of static just on the return side.
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Old 07-14-2011, 08:30 PM   #11
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8x24 isn't anywhere near big enough blue. 1600cfm and you want 400fpm max at the drop so the duct needs to be equal to 4 square feet. Usually the big 4 and 5 ton units are better off with half coming in from each side. I bet you have more then a half inch of static just on the return side.

4 sq ft... humm, this will be interesting to figure out... might just have to call a pro in to do some calcs....

I knew it was undersized with the 24x8 return, my parents house is half the size of mine and their return is 24x16! and their furnace is extreamly quiet running at high speed
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Old 07-14-2011, 08:45 PM   #12
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noisy blower / filter box redo


CFM/2 = sq.inches of area.

2 ton 800/2 = 400 sq.inches
3 ton 1200/2 = 600 sq. inches
4 ton 1600/2 = 800 sq. inches
5 ton 2000/2 = 1000 sq. inches
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Old 07-14-2011, 09:28 PM   #13
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Furnace seems way oversized for the house. Where do you live?
Most furnace manufactures want you to come in on both sides for 2000+ cfm. As said you don't need that much air for the air conditioner.
Sounds like your whole return air duct system needs to be redone. What's the make and model of the furnace?
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Old 07-14-2011, 10:15 PM   #14
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Originally Posted by hvac122 View Post
Furnace seems way oversized for the house. Where do you live?
Most furnace manufactures want you to come in on both sides for 2000+ cfm. As said you don't need that much air for the air conditioner.
Sounds like your whole return air duct system needs to be redone. What's the make and model of the furnace?

Aire-Flo Model: AF92MPE125U5B I think its a 1,600 CFM furnace
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