Lennox - pressure switch


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Old 12-19-06, 06:01 AM
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Lennox - pressure switch

Good morning,

We have a Lennox furnace (80MGF3-75A-11) that tripped the circuit last night. I reset the breaker on the electrical panel and the furnace came back on for about two minutes. All of a sudden we heard a loud pop come from the furnace room and the furnace shut off again. Once again the circuit was tripped on the electrical panel. I turned the switch for the furnace power off and reset the circuit breaker. Just an FYI, the furnace room is on our second floor and the electrical panel is in the garage.

When I turned the switch to the furnace on I saw the two green LED lights on the board in the furnace blinking faster than usual. I looked on the troubleshooting sticker on the back of the furnace panel and it said this indicates a "heating demand". I could hear a click from the furnace which I assume may have been the igniter trying to start the burners. After about 3-4 minutes of the two LED lights flashing fast I heard 2 more clicks and then the LED lights changed to #1 staying off and #2 flashing. The sticker said this indicates that the pressure switch has opened.

I am not sure what troubleshooting steps to take. Any help is most appreciated. Thanks for your time.
 
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Old 12-19-06, 06:14 AM
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Did you see what the loud pop was about?

Pressure switch is a round thing that has a rubber hose on it. I am guessing the one you got is going to the small inducer fan. Remove the house from the switch and blow into it. then put back on, and remove from the fan housing.. blow/suck with care and see if you can tell if the switch is opening and closing.. If yes. then check the flue pipe to make sure it's clear.

If no luck, I would not mess with it anymore to may be something major wrong if you had to reset the breaker twice, and had a loud pop noise. Call a service tech.
 
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Old 12-19-06, 06:35 AM
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Unfortunately, I didn't get a chance to see what the pop was from. I inspected everything and there was no visible damage to anything on the furnace. I assumed after I saw the LED lights had indicated the pressure switch was open that it may have been what I caused the loud sound. There is a tube/hose that leads from a round silver metal device to what I believe is the motor. I will try to remove that and blow in it like you suggested when I get home this evening and give you an update.

Ideally, I would have just called someone to take a look at it. But, considering the time of year and we just had a baby, money is a bit tight right now. I truly appreciate the advice. Thank you
 
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Old 12-19-06, 07:01 AM
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Not knowing how far away is the furnace is from the garage and what door/wall is between you and the furnace.. I don't think you could of hear the pressure switch pop that far.
 
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Old 12-19-06, 08:04 AM
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Sorry, I should have explained it more in depth. After I reset the breaker in the garage I went upstairs to see if the furnace was working and it was. I then went about 10 feet away from the furnace room, which I left the door open to, and sat in our loft. I was relatively close to the furnace room when I heard the loud pop and since the door was open to the furnace room it was pretty obvious that was where the noise came from. I assumed it was from the furnace because after the pop the furnace shut off. I went down to the garage and reset the breaker and went back upstairs to the furnace. That was when I noticed the two LED lights flashing faster than they usually do. Then after a few minutes I heard the "clicks" again the LED lights switched to the #1 being off and #2 flashing. According to the troubleshooting sticker, that indicates the pressure switch is open. Hope that was a bit more descriptive.
 
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Old 12-19-06, 08:28 AM
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Did you smell anything after the pop? Smoke, burning smell, plastic, etc.?
 
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Old 12-19-06, 08:35 AM
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No, there was no smell or any smoke. Just the pop. If I had to explain the sound of the pop, I would compare it to squeezing bubble wrap.
 
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Old 12-19-06, 04:41 PM
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All right, I tried blowing/sucking on the hose and I could hear something in it that sounded like it was opening and closing. I also tried to look in the duct and could see nothing obstructing it but it hand about halfway up the attic and there's no way for me to get on the roof because we live in a townhouse. Is it possible that I have a bad pressure switch and could just go buy another one? I am just curious if there's anything else I could try before having to call someone.
 
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Old 12-19-06, 04:52 PM
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if you feel comfortable doing this, remove the flue pipe from the top of the furnace where it connects to the inducer housing and just set it to the side, then turn the knob off on the gas valve, with a flashlight look down in the inducer motor to see if anything is obstructing the small wheel inside, spin it with a screw driver to make sure its not locked up. also if you havent already done this make sure that the fitting the small tube slips over on the inducer housing is not clogged, take a small pin or piece of wire and push through just to be sure then reconnect the hose, you said you heard a click when sucking on the hose from the psi switch end which is telling you that the bladder is more than likely ok and the switch will make granted there is a vaccuum from the inducer. turn the furnace back on and see if the sequence of operation gets farther than before, if you get a new code like a no flame sensed code since you turned the gas off then youll know you have a drafting problem and that it lies in your flue or chimney and not the furnace. sorry for the paragraph lol
 
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Old 12-19-06, 07:15 PM
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I tried disconnecting the duct from the motor and there's nothing binding or obstructed. Everything spins freely. After I reconnected all the ducts I turned the gas back on and tried turning on the furnace. The same thing happened with the LEDs. Both lights blinked fast then after a few minutes I heard a click and then only the #2 led was flashing. When I had the duct disconnected I could feel a light draft coming down from the roof at the open end of the duct. Does that mean anything?
 
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Old 12-19-06, 07:30 PM
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the draft you feel is no big deal.

just to cover the bases:

Did you try to run the furnace with the flue pipe disconnected to cancell out an obstruction? what happened?
Does the inducer motor spin freely and when you turn the furnace on it runs and sounds ok
You checked to be sure that the port/fitting on the inducer housing is 100% clear, and when you suck on the tube to the psi switch you can hear a clicking as if a switch was making when you suck in and opening when you let off.
Nothing else happens after that except for the code flashing psi switch open correct.
Does the pressure switch have just 2 wires hooked to it?
Do you have a jumper wire with alligator clips on each end? or similar
Do you have a multimeter?
 
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Old 12-19-06, 07:44 PM
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Did you try to run the furnace with the flue pipe disconnected to cancell out an obstruction? what happened? YES, I got the same LED indications as previously.


Does the inducer motor spin freely and when you turn the furnace on it runs and sounds ok? It spins freely by hand but nothing happened when I turned the furnace on.


You checked to be sure that the port/fitting on the inducer housing is 100% clear, and when you suck on the tube to the psi switch you can hear a clicking as if a switch was making when you suck in and opening when you let off. YES


Nothing else happens after that except for the code flashing psi switch open correct. CORRECT

Does the pressure switch have just 2 wires hooked to it? YES
Do you have a jumper wire with alligator clips on each end? or similar NO
Do you have a multimeter? NO

I can probably get a multimeter tomorrow from a friend. We're just going to use space heaters tonight. I truly appreciate all of your help this evening. I'll check back tomorrow.
 
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Old 12-19-06, 07:58 PM
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Ok, the small inducer fan motor is not running at all?

That has to run before anything can happen. if you get a meter, check to see if you got power going to the motor.
 
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Old 12-19-06, 07:59 PM
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so the vent motor doesnt run when you turn the furnace on, just stays still, and all the wiring to it is connected well?
 
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Old 12-20-06, 05:17 AM
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That's correct. I'll double check it tonight and try again, but last night the fan didn't do anything when I turned the furnace on with the duct removed from the top of it. I was able to spin it by hand but it didn't do anything after that. Is there any difference between spinning the wheel on the fan from the inside after the duct is removed or just spinning the wheel that is connected to the front of the fan? It seems to do the exact same thing.

I am going to try to borrow a meter tonight. The only one I have is one of the cheap ones that you touch the red and black wires to it and a ground and it lights up saying if it is 50/120/200V. Will that do the trick or do I need the multimeter?
 
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Old 12-20-06, 05:29 AM
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You could use that meter. but get a good one if you can.
 
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Old 12-20-06, 06:24 AM
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All right, I have a friend that probably has one. I'll see if I can borrow it tonight.
 
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Old 12-20-06, 03:17 PM
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multimeter test:
unplug the inducer motor, there should be a plastic plug about 6 inches or so from the motor itself that you can disconnect. once done take the meter and hook the leads to the two wires coming from the circuit board. turn the furnace on and make sure the thermostat is calling for heat. if you get 120 volts or something similar within 5 volts or so then your motor is bad open winding.
if you dont get any voltage and the wiring is connected tight to the board then you have a bad circuit board.
 
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Old 12-20-06, 04:29 PM
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Thanks for the walk-thru. I don't see a cord coming out of the motor. All I see are two wires on the top of it. I unplugged those and used the cheap wire tester that I have and nothing happened. I put the red prong from the tester in one of the motor wires and the black prong on the switch box for a ground and nothing lit up. I did that for both disconnected wires coming off of the motor while they were still connected to the board. Hoping that I did nothing wrong, I take it my board is most likely bad? Do some motors not have visible power cords or am I just a dork? I looked everywhere around it and there's only those two wires coming from the top that I tested.
 
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Old 12-20-06, 04:37 PM
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You got the right wire.there is two wire.
 
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Old 12-20-06, 06:24 PM
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Originally Posted by Jay11J
You got the right wire.there is two wire.

I don't know what you mean by that. I tested both wires that I disconnected from the motor. Neither of them had any voltage indicated with the meter.
 
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Old 12-20-06, 07:32 PM
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You got the right wire when you were trying to do a reading.. Some people call wire a "cord", where a cord is where you have two wires tied into it.

Trace the wire down to the board to make sure a wire didn't short out. That maybe a pop is caused by a wire shorting out. if all looks good, then go down to where the wire is conntected to the board. and take a reading for power. If you read nothing, then it's the board.. if you got power, then trace the wire again to make sure all is ok.
 
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Old 12-20-06, 08:21 PM
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those two wires are your power wires for the motor, one hot and one neutral. if you unplugged the connector that isolates the motor from the power source and checked the power from the board side of the connector and had nothing with the furnace on and calling for heat than your board is bad, pretty cut and dry
 
 

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