pende_tim
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join:2004-01-04
Andover, NJ
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| Slowing Fan Down on AHU
In my daughter's new condo, the return air plenum for the Goodman heat pump is in the Living room near the TV. ( Yea it is a Goodman, but that is what the builder installed a few years ago.) The annoyance we are having is that when the fan comes, on she needs to increase the volume on the TV to hear it clearly over the fan/air noise from the register. Typically we don't "blast" the TV, so any significant increase in noise makes it hard to hear. The noise does not seem really loud, just what I would characterize as normal "fan noise", no bad bearings etc.
It would seem there are several ways to solve this problem:
1. Slow down the fan a bit
2. Relocate the return air register to the hall behind the living room.
The least cost solution would be to slow down the fan, if that is physically possible.
However if I slow it down I have a few questions about the unit's performance. Since it is an air-conditioner and heater, what will happen to the cooling efficiency? will SEER take a big hit if the airflow is reduced lets say 10% ? I would assume the delta T across the coil would increase, which would decrease the RH in the room, correct? Currently I am seeing a 55-60% RH with a 75* dry bulb. Outside is in the 95* range. The current delta T is 19* from the home inspection ( 69* in/50* out )
The other option is to relocate the register to the hall on the other side of the AHU closet. This closet is about 3' deep, 6' wide and 9' tall. The closet houses the air handler (which pretty will fills up the area), an emergency drip pan and the disconnect for the strip heaters. The AHU is a vertical flow design. Can I just put a large hole in the intake duct below the evaporator coil, cover it with a filter, install some louvers in the closet wall to the hallway and use the closet as the "duct" between the hallway and the unit? I know I would have to seal up the cracks in the closet where the ductwork and refrigeration lines penetrate the walls and ceiling so I don't suck unconditioned air into the return.
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cdru
Go Colts
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join:2003-05-14
Fort Wayne, IN
| Are you hearing the fan, the air entering the plenum? Or the walls of the plenum (if they are sheet metal)?
Lowering the blower speed may have unintended side effects such as poor distribution of cooled (or heated) conditioned air as it doesn't have the same velocity. Slow it down too much and the coil could freeze over causing even more problems.
Have you tried installing any type of insulating material around the return plenum? Even just a few feet just by the actual vent may be enough to absorb sounds coming from the blower before it comes out the vent. Some quilt batting, stapled or stuck to the interior walls may be a quick and easy test to see if a better, more permanent solution could be devised. Just don't restrict the air passage too much. |
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pende_tim
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join:2004-01-04
Andover, NJ
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| I am hearing the fan running as well the rush/noise of the air as it passes through the grate.
No I have not tried putting anything inside the sheet metal plenum to act as a silencer.
Tim
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The difference between genius and stupidity is that genius has its limits. |
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HarryH3
join:2005-02-21
Georgetown, TX
·Verizon Online DSL
| reply to pende_tim
What size is the intake filter and what is the BTU rating of the unit? With that info you can look around on the net for intake sizing per ton of cooling. Our house has two 3.5-ton units which were each trying to suck air through a single 16x25 opening. By adding an additional 16x25 filter grill for each unit, in another room, fed via a 10-inch duct, three things happened. 1) The noise of the air passing through the intake has dropped tremendously, as it is shared between the filters now 2) The volume of air coming out of the output vents increased quite noticeably, which tells me that my system wasn't setup properly to begin with. 3) My a/c no longer runs continuously when the outside temp is over 100. It can actually get ahead of the Central Texas heat now, which is something it wasn't able to do for the first three years we lived in the house. 
So adding another intake on the other side of the closet may increase the efficiency and reduce the noise at the same time. If the noise in the living room is still too loud, just replace the filter in that room with a piece of duct board cut to fit the opening. |
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pende_tim
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Andover, NJ
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1 edit | HarryH3,
Great thought!
It is a 3 Ton unit. I will get the intake size later tonight when I call my Daughter.
Update: Filter is 20x20.
Is it also possible the fan noise will reduce when it breathes better?
Tim
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The difference between genius and stupidity is that genius has its limits. |
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tp0d
yabbazooie
Premium
join:2001-02-13
Carnegie, PA
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| Have you tried changing to a little thicker filter? I`m willing to bet theres a see-thru blue fiberglass one in there now, which would radiate noise quite well.
Definitely do not slow the fan, it could cause the coil to freeze.. not good.
-j
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if it aint broke, tweak it!!
currently on FiOS (kick aZZ!) |
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HarryH3
join:2005-02-21
Georgetown, TX
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| reply to pende_tim
I have several feet of 14-inch flex duct between the fans and the original filter grills, so I never had much fan noise to begin with. I don't know how much that would change. 
If you install another filter grille, get another 20x20 so both filters will be the same size. It's one less thing to remember...  |
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HarryH3
join:2005-02-21
Georgetown, TX
·Verizon Online DSL
| reply to pende_tim
Oh yeah, if you add another intake, be sure that you seal EVERY tiny nook and cranny in the new duct so that NO air can leak into the system after the filter. One of my systems didn't have the air box sealed to the ceiling and air had been leaking around it for years. The evap coil on that one was caked with dust.
A half gallon of duct mastic will seal a lot of leaks. Well worth the $11. |
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Tyreman
join:2002-10-08
4 edits | reply to pende_tim
19 degrees difference doesn't allow much room to slow down the fan.
15-20 degrees is the range generally.
around 17 degrees td air in out gives optimum humidity removal.
I have seen some in southern ontario running at 22 degrees but i don't like it
So you can see already where you are at and what you got to play with.
BUT you could judisciouly try slowing down the fan and measure the temps.
But the noise may not change that dramatically.
we have warm days and cool nights here, coil icing can occour on a minimal speed fan in our climate even with a proper charge and all clean.
Not enough air flow thru the coil ,hence low suction pressure.
house cools down a bit there goes the suction pressure even more.
Suction pressure is effected by indoor temperatures more.
You get the drift.
In the states you guys often have wamer nights.
you need 360-400 cfm of return air per ton of cooling.
BTW A lot of seer numbers while look good on paper don't always translate in real worlds installs.
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Southern Ontario,Canada |
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Tyreman
join:2002-10-08
1 edit | reply to pende_tim
say you needed "about" 1080cfm minimum for 3 tons
20x10 does about 1060 cfm
20x12 does about 1250 cfm
22x12 does about 1400 cfm
14" flex does about 700cfm
14" round metal does about 750cfm
16" flex does about 1000cfm
16" round metal does about 1200cfm
Sometimes a canflex connector in the duct between the air handler and the duct if its a short duct run direct off handler will make it quieter.
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Southern Ontario,Canada |
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pende_tim
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join:2004-01-04
Andover, NJ
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| Thanks for the information. It sounds like the intake is plenty big for the job. A am now wondering why it is so large? That size should be able to do 2000 cfm based on those numbers. It should be quite quiet with a 1080 CFM flow, Possibly the wind noise is being caused by the joint or transition where the 20x20 box meets the plenum? Or the air flow is a lot higher than 1080 CFM.
I really can't put a piece of flex in the duct work as the register is on the same wall as the return air plenum and the spacing between the wall and the plenum is only about 2" or so.
When they built the place, the merely put a hole in the wall, put a hole in the plenum and installed the return air box. The box is directly connected to the plenum, no duct work involved.
The more I think about it, the less I like the idea of slowing down the fan. Delta T is on the high side and air flow (without drafts) is essential to comfort and efficiency. The living room area has a cathedral ceiling so I think that we really need to keep the air flowing.
Since I am not near her to look at things, I have a local HVAC tech scheduled Wed AM to look things over. Unless he finds something wrong, it looks like a 12x22 in the hallway with 16" round/flex combination would solve the problem. This should be fairly easy to install. Will not have to modify any studs to get the RA grate in the wall.
Probably will leave the existing grate in place after we install the new RA grate to see if the reduced airflow with the second grate will help make it quieter. As pointed out, this can be closed off later.
Tim
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The difference between genius and stupidity is that genius has its limits. |
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builderbob
@cox.net
| reply to pende_tim
is it possible that the TV's sound system is part of the problem? some of the newer flat screen tv's have poor sound with the factory built in speakers. I've had two customers with media rooms with this problem. upgrading speakers took care of it. is it possible to move tv to see if problem is resolved? |
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pende_tim
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join:2004-01-04
Andover, NJ
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| Relocating the TV is a possibility however due to the way the room is designed, the only other place is in front of a window. The previous owner's had their set there but Daughter does not want to block window.
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The difference between genius and stupidity is that genius has its limits. |
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Tyreman
join:2002-10-08
1 edit | reply to pende_tim
Those numbers are averages if there was severe duct transitions, elbows and throat reductions or on site duct modifications due to buildings construction it might cause an one size up increase.
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Southern Ontario,Canada |
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neonhomer
Honoray Mythbuster
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join:2004-01-27
Edgewater, FL
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| reply to Tyreman
400 cfm x 3 tons would be 1200 cfm...
Anyway... best way to determine airflow would be to find someone who has a flow metering device.
The best way to see if you need the fan speed adjusted would be to have someone measure the amount of air that is being put out of your supply grilles, and being taken in by your return. Usually this is done with an air measurement device like a FlowHood. You could also use a VeloGrid. The unit pictured below is about $4000.... (I know... I have one...)
Shortridge FlowHood
It is quite possible that if your inlet and discharge statics are low, the unit is pushing too much air, and you would need to slow the fan down some.
The units I see most (Trane and Carrier) usually have three speed motors in them. You turn the unit off, and change the motor speed by changing the position of a wire. Turn it back on and you have changed your fan speed. (Of course you have to remove and reinstall the access doors...)
Like others have said, if you slow your fan down too much, you will cause the evaporator coil to freeze up.
Putting a more restrictive filter in would be doing the same thing as slowing the fan down.
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