Lennox Blower Motor Issue

Discussion in 'HVAC Heating & Cooling' started by AlGreen, May 21, 2021.

  1. AlGreen

    AlGreen Member

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    New Jersey
    For the past several days the AC for my upstairs has intermittently not responded to thermostat calls for cooling. I have two zones, with the upstairs one consisting of a Carrier Infinity 24ANB6 central AC, an old Lennox Elite G24M2 furnace, and Honeywell VisionPro 8000 thermostat. Those times when the AC isn't cooling, the Carrier is showing code 83 for low-pressure lockout, and the Lennox blower fan is making a humming sound but not spinning.

    With the thermostat set to Fan: Auto and System: Cool (temp adjusted below threshold for cooling) I checked the AC voltage across the ACB Cool and Neutral terminals, and measured 12oV. Then (with power off) I checked the 5uF capacitor for the blower motor, and it measured spot on (ESR was also low-0.06 ohms). The blower wheel spins freely when I move it by hand. At this point, I feel fairly confident that the motor needs to be replaced, however, I'd like someone who has more experience with HVAC stuff to chime in and confirm.
     
  2. fitter30

    fitter30 Well-Known Member

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    Try changing the run capacitor. If that doesn't help probable the motor. Indoor fan motor if picking aftermarket motor go by amp draw not hp. If your motor is 6.1 amps and aftermarket 1/2 hp is 6.0 go with a.3/4 hp.
     
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  4. AlGreen

    AlGreen Member

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    Thanks. The cap tested fine, but I can try replacing it anyway.

    For the motor, I'd like to go with an aftermarket to save some money. My concern is that while I can match up the electrical specs, I don't know if there's any way to get the physical dimensions correct without removing the old motor and measuring everything. Here's a pic I took of the label on the old motor.

    Attic Blower Motor.jpg
     
  5. fitter30

    fitter30 Well-Known Member

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    1075 rpm ,frame size 48 = 5.5" diameter, 1 /2" shaft, rotation note the arrow on label and is it shaft end or back end. Might have to get a belly band mount to adapted to blower housing
     
  6. WorthFlorida

    WorthFlorida The wife is still training me.

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    The best way to test a cap is with it disconnected. Then use an ohm meter. If you get a consistent low ohm reading then it is bad. Most caps for motors and compressors swell when they good bad, or the small hole on top is leaking oil.
     
  7. AlGreen

    AlGreen Member

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    The cap is good. But I'll replace it and the motor at the same time.

    One thing I'm wondering about: It appears that the old motor has the red wire for the low speed tap connected to the ACB Cool terminal on the control board, rather than the black wire for the high speed tap being connected to it. That black high speed tap wire is connected to the ACB Low terminal. So essentially they're reversed. Would someone have done that on purpose during installation?
     
  8. WorthFlorida

    WorthFlorida The wife is still training me.

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    Don’t go by color alone. For cooling the fan is usually set for the high speed, a low speed for heating or fan “on” setting. To connect the motor it should be a keyed connection so reversal should not be possible. After it is installed turn on the AC and the fan should run at high speed. Turn off the AC and switch to heat mode and turn up the thermostat. The fan will run slower. If it checks out. Then you’re ok.
     
  9. AlGreen

    AlGreen Member

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    I know the motor side of the wiring is a keyed plug that can only be connected one way, but it's the control board side of the harness where the wires are individually plugged into the various terminals that's connected incorrectly. We've lived in this house for about six years, and I always thought that the fan didn't blow as hard in AC mode as it does from our downstairs Lennox furnace. I think it's because someone (for some reason) connected the low speed motor tap to the ACB Cool terminal on the board and the high speed motor tap to continuous Fan. Even though I'm replacing the motor anyway and will connect the new one as you described (high speed tap to ACB Cool, med-high to ACB Heat, low to Continuous), I though it was weird to see the old motor connected wrong.
     
  10. AlGreen

    AlGreen Member

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    Is is possible that when the AC was installed (Carrier Infinity 24ANB6), the tech purposely connected the low-speed motor tap to the ACB Cool terminal on the control board to get the right CFM for the system?
     
  11. WorthFlorida

    WorthFlorida The wife is still training me.

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    I had a two story home for 26 years with two AC units. No matter what some HVAC designers say, the second floor has to be a larger unit in BTU's than the floor area calls for. The second floor unit handles a lot of heat rising from the first floor and your current one may not be able to cool enough. New AC units get more efficiency by pushing a larger volume of air for more contact time with the coils to extract heat and moisture. It also slows down the air flow some that makes it quieter. The wires might have been reverse for this reason where as at high speed the air wasn't cooling enough for comfort perhaps during the hottest days. At high speed the noise can be loud if air handler is near the bedrooms.
    My previous home had two 2 ton units and the second floor was always much warmer and the AC would run most of the time. When I replaced them I upgraded the second floor to a 2.5 ton unit and it was much better. However, I had to keep the second floor thermostat 2 or 3 degrees warmer than the first floor so both units ran about the same. If the thermostats were set the same, the second floor would cool the first floor, second floor would be warm and our master bedroom on the first floor would be warm since the air wasn't flowing in that part of the house.
     
  12. fitter30

    fitter30 Well-Known Member

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    Blower speed usually is on hi for ac medium for heat. To check cooling speed need a.pocket thermometer take reading at filter rack and 18" from evaporator 18*- 24* difference. Suction line ( insulated line)at condenser should be cold after 30 minutes run time.
     
  13. AlGreen

    AlGreen Member

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    Thanks guys.

    WorthFlorida: The second floor furnace is directly above the master bedroom, so keeping the noise down may have been part of the reason why the installers decided to use the low-speed tap for the AC. We have two Carrier Infinity units for house, with the second floor one being a 2 ton, and the first floor 2.5.

    fitter30: Appreciate the tip for how to check cooling speed. I'll give that a try after I install the new motor.
     
  14. AlGreen

    AlGreen Member

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    I replaced the motor and cap earlier today. I'm going to leave the speed for AC as it was. Thank again for your help.
     
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