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I'm getting a primary/secondary limit switch code from my…

Hi I'm getting a...

Hi I'm getting a primary/secondary limit switch code from my furnace and a bad smell. The furnace makes a loud humming sound.

Contractor's Assistant: Did you check the furnace's filters?

Yes, the furnace filter was filthy. I've since removed it and started the furnace without a filter but same problem

Contractor's Assistant: Do you plan on doing the work yourself?

Yes, I would like to try.

Contractor's Assistant: Anything else we should know to help you best?

I've verified the intake and cleaned and drained some fluid from the tubing. Otherwise, it is fine.

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Answered in 30 minutes by:
3/11/2018
gotoman for ac
gotoman for ac, HVAC Technician
Category: HVAC
Satisfied Customers: 5,315
Experience: 51 years of experience install design and repair of all makes and models of AC
Verified

Russ here. Can you give me model number of furnace.

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Customer reply replied 1 month ago
Lennox Elite Series
90UGFA3/4-100-6

Make sure main blower runs ok. Turn fan to on to check motor. There should be a manual reset limit switch on the fire box. The box just before the exhaust vent. Will be located on the side or bottom of the box. Russ

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Customer reply replied 1 month ago
I've turned the furnace off. There was a strong smell coming through the vents.

Just try the fan in the on position to see if it works. The fan may be the problem. Russ

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If the fan does not work the furnace will heat up the limit will trip. The limit is a high temp safety. Russ

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Customer reply replied 1 month ago
Turned the furnace on. Flame lit, some clicking noises but I don't think the fan came on. Finally, the limit switch opened and shut down the flame. There is a humming sound coming from the furnace. is this normal?
Shouldn't the fan still be running?
Customer reply replied 1 month ago
Removed the panel and it seems the humming sound is coming from the fan. Now when the furnace tries too start up again the limit switch comes on almost immediately.
Regan Land
Regan Land, HVAC Technician
Category: HVAC
Satisfied Customers: 767
Experience: Over 20 years of experience, NATE Certified, Licensed HVAC Contractor, Certified IAQ Specialist
Verified

Hello, it looks like Russ had to hop off. I'll be happy to assist from here. Try to go to the thermostat and turn heat off, then turn the fan from 'auto' to 'on' - do you hear the loud humming when you do this?

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Customer reply replied 1 month ago
Hello,
Turned the heat off and turned the fan from auto to on. Yes, I do hear the loud humming sound.
Customer reply replied 1 month ago
The fan is not spinning and no air coming from vents

Okay, thank you. Now try turning the fan back to auto and rech in and try to spin the blower wheel/fan motor by hand. Does it spin freely or does it feel stiff/stuck?

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Customer reply replied 1 month ago
the fan does spin freely. It doesn't feel stiff or stuck.

Okay - so that means one of two things is happening. Either the motor capacitor is bad or the motor itself is bad. It's also possible that the control board isn't sending proper voltage, but it should be okay since it's doing it in heat speed and fan only speed. The capacitor is much cheaper to replace, so I typically recommend starting there (especially since the motor spins freely). However, it may not be until tomorrow before you can get one from a supply store.

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Customer reply replied 1 month ago
Can I test the capacitor with a multi-meter? What reading should I get?

Absolutely! Does your meter happen to test microfarads (uF)? If not, I'll explain how to test in the ohms setting.

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Customer reply replied 1 month ago
No, I don't think it measures microfarads. Can you walk me through the ohms?

Yes - Set the meter to test at least 1000 (1K) Ohms. Turn all power off to the furnace, then use a screwdriver to short between the two terminals to discharge the capacitor. Then remove both wires from capacitor and place each lead from meter to each side of the capacitor. Watch the reading on the meter closely - it should jump up quickly then return to OL. It will do tis each time to touch both leads to the capacitor. If it doesn't jump up then the cap is bad.

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Customer reply replied 1 month ago
Okay, I'll try it and let you now.

Sounds good.

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Customer reply replied 1 month ago
How do I short the capacitor with the screw driver? I don't think I discharged it. When I try to get a reading with both leads on the capacitor, there is no change at all. The reading remains at 1 when set to 2000. Tried setting it to 200 and still nothing at all.

You will use the screwdriver to short across the terminals that the wires are connected to. Even without it shorted, if the meter isn't changing then it is almost most certainly bad. If you think you'll be needing heat tonight then you might be able to contact a local HVAC company and meet the on-call technician to get one. Otherwise you can pick one up at a local supply house tomorrow. Make sure you get one with the same rating. Hopefully the motor is good and just replacing the capacitor will get you back up and running.

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Customer reply replied 1 month ago
Can I discharge it with the screwdriver after it has been removed from the wiring? I just want to be sure I have done it correctly.

Yes, you can discharge it at any time. Discharging it just makes sure it's not holding a charge that could zap you when removing the wires. Testing and readings would still be the same either way.

Regan Land
Regan Land, HVAC Technician
Category: HVAC
Satisfied Customers: 767
Experience: Over 20 years of experience, NATE Certified, Licensed HVAC Contractor, Certified IAQ Specialist
Verified
Regan Land and 87 other HVAC Specialists are ready to help you
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Customer reply replied 1 month ago
Thank you!

You're very welcome! Thank you for the positive rating! If you have any additional questions you can still ask them here. This question will stay open for 30 days.

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Regan Land
Regan Land
Regan Land, HVAC Technician
Category: HVAC
Satisfied Customers: 767
767 Satisfied Customers
Experience: Over 20 years of experience, NATE Certified, Licensed HVAC Contractor, Certified IAQ Specialist

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