Whining from blower only with A/C - Troubleshooting Advice

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Old 04-11-14, 07:46 AM
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Whining from blower only with A/C - Troubleshooting Advice

Hello,

I'm fairly handy, but need insight on troubleshooting whining noise recently permeating from what appears to be the blower/motor on my unit. The continuous whining noise is only evident when the fan is running at high speed under cooling operation; no noise is evident with heat, assuming differential fan speed. Moreover, the noise is not evident with only the fan on and the unit being turned off (fan appears to be running at low speed with the heating/cooling unit off).

After limited research, appears the issue could be related to motor bearings, possible failure/inbalance of the fan or loose mounting hardware.

I am going to attempt to diagnose the issue this evening. I am writing for your feedback and am open to suggestions.

Thank you for your time.
 
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Old 04-11-14, 05:31 PM
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It can be tough to diagnose a noise that we cannot hear but I have found loose duct tape allowing air to pull into the return air duct associated with whining noises before.

You might also remove power and check the blower wheel for paper or plastic debris.

Either of these would be an easy and inexpensive fix.
 
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Old 04-12-14, 07:16 PM
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On most furnaces, when the stat is switched to "fan-on", the fan runs on the cooling speed. Houston's ideas are certainly worth checking out but I'm suspecting a refrigerant noise. Try using a screwdriver against the refrigerant lines as a stethoscope. You might be able to pick up the noise.
 
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Old 04-13-14, 06:52 AM
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with the stat in FAN ONLY no system running...restrict the large return grill with the noise going on see if it changes ,and or run the cooling see if it changes starving the unit.. if nothing changes then it could be the liquid going thru the metering device..restrict the top of the condenser with a garbage can top and a brick or two...to raise the head pressure into the evap section.... see if he noise changes...
 
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Old 04-14-14, 05:16 AM
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FYI,

Broke it down this weekend. Squirrel cage was filthy, caked with dirt; cleaned it the best I could. No broken, bent or damaged blades were evident. The motor shaft has about 1/4" lateral play, but there is also slight vertical play (~1/8"). Still spins freely and appear to be balanced, but I suspect motor bearings are failing causing slight vibration in the wheel.

Will be ordering a new motor, cap. and wheel today.

Thanks all. Will update after the install.
 
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Old 04-14-14, 09:31 PM
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Grady I believe Carrier and Trane gas furnaces run the heating speed with Fan only, the fan speed increases when the Y terminal is also energized, but that was a great idea. I might try that sometime.
 
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Old 04-15-14, 04:15 PM
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Interesting, Houston. I'll have to check a Rheem. I wonder if it makes a difference if the stat is set to heat, off, or cool with the fan set to on.
 
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Old 04-15-14, 04:59 PM
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Most modern Rheem furnaces have a third terminal on the control board for Fan only. This can be bridged to the cooling fan speed but I usually connect an unused fan speed wire to the Fan terminal on the control board.

This is a great feature but it can cause a recall when replacing an older board that used the heating speed if you forget to connect a speed tap to the Fan terminal on the replacement board.

 

Last edited by Houston204; 04-15-14 at 05:44 PM. Reason: diagram added
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Old 04-16-14, 07:10 AM
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Come to find out, a bolt on the motor mount was loose (sheared off threads). Replaced the bolt and the sound has been eliminated.

Another question though, I have a 2 ton system on a 2,000 sqft home with one return (14x30); fan motor is 3/4 hp. The blower is 18-24" behind the intake with no ducting; merely a cavity with a platform that the furnace sits upon with air being drawn. There is noticable air rushing noise through the return. I am considering opening up the louvers on the grill grate, in addition to looking into options for filters to reduce the air noise. Relocating the return with a longer run is not an option right now. I've seen opinions on installing duct lining and DIY baffles, as well as modifying grill louvers and the use of less restrictive filters.

Any recommendation on the type of filter to help reduce resistance within the intake? Other ideas?

Thank you.
 
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Old 04-16-14, 05:39 PM
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A 3/4 hp motor on a two ton system sounds like an awfully big motor. Are you sure it's only a two ton?

I prefer the 1" thick glass fiber filters. People will say they don't filter well but last summer I opened up my 25 year old system to look at the evap coil & found it to be clean. The coil has not been cleaned since I installed it & all I've ever used is the filters I mentioned above.

I don't fully understand your return system. Is it essentially a box on the end of the air handler going to the return grill?
 
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