Lennox G40UH - Bad Limit Switch?

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Old 10-31-15, 04:58 PM
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Lennox G40UH - Bad Limit Switch?

Hey Guys,
First time poster here. Turned on my heating of the first time this weekend, only to realize cool air coming out of the vents, no hot air at all!

On a call for heat, the blower motor turns on and just circulates the air

Removed the cover to the furnace and noticed that the inducer motor wasn't running. The Green light stays on and the Red flashes slowly. According to the manual (http://www.lennox.com/pdfs/installat..._G40UH_IOM.pdf) it means 'Primary or secondary limit switch open'

I disconnected the wires from the first limit switch and tested it for continuity, My multimeter beeped meaning that switch is fine.

The 2nd limit switch is somewhere near the blower and I cant see it, not sure if that one is bad (the blower turns on though on call for heat). The blower was replaced earlier this year and maybe the technician forgot to plug some wires in?

I thought the control board was bad, so I replaced it. The problem still persists.

Not sure what to do further. Please can y'all help.
 
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Old 10-31-15, 07:14 PM
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Welcome to the forums.

The two limits near the burner are the flame rollout switches. They should be connected together and are not your problem.

The other limits should be shown on your wiring diagram on the blower door. The schematic shows 1-2 limits. They are connected between pin 1 and 7 on the control box. You should be able to determine the wiring color and follow the wires to the limits.
 
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Old 10-31-15, 09:17 PM
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Thanks PJ, There is 1 limit switch right next to the blower motor which seems closed. There are two flame rollout switches too (which have a red button in the middle).

I haven't been able to validate if the second limit switch which is behind the blower motor is working or not

Also how do I check and see if the blower motor is bad? (though the lights indicate that the limit switches are bad)
 
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Old 11-01-15, 12:02 PM
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The high limit switch(es) connect to pin 1 and 7 on the main board connector plug. It's not the rollout switches.... that would be a different code. Pins 1 and 7 need to show a short to be normal.

The wiring of the limit switches is a loop. You don't necessarily need to check the sensor....follow the wiring.
 
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Old 11-01-15, 04:38 PM
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A better way to test for such conditions is to connect one test lead to the furnace sheet metal and use the other to test for 24 VAC.

If you test the beginning of the circuit for the limit switch(s), you should find 24 VAC.

Then test the connection from the other end of that circuit. If you have 24 VAC there, the limit switches are closed. If you have zero voltage, one or both are open.
 
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Old 11-01-15, 08:44 PM
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i am not seeing 120v to the inducer motor from the circuit board terminals. I put in a new circuit board and same issue. I'll jumper R and W in order to bypass the thermostat and see if the inducer motor works. I installed a new Emerson Sensi wifi thermostat a few months back, maybe that is causing issues.
 
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Old 11-01-15, 09:14 PM
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If a safety line is open...... like the board is telling you..... the inducer will not start.
This is not a stat problem.
 
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Old 11-02-15, 02:29 PM
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It sounds like your primary limit is OK but that you have a secondary limit that is open. But you need to check that and make sure.

Many furnaces DONT have a secondary limit, so you need to verify that you have one. Secondary limits are usually manual reset. So if they open you have to push the reset button on them to close the switch again.

Secondary limits are often there to verify that the fan that circulates air around the house is working properly. If that fan doesn't start and work properly, the furnace can overheat in a way that wont be detected by the primary limit installed near the heat exchangers.

So if the secondary is open, you need to verify that the fan motor is starting and running reliably and effectively, and that air is passing around the heat exchanger freely (no plugged filter or shut off warm air vents.)

An open secondary limit could be a fluke. If everything seems OK you could reset it and give it another try.
 
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Old 11-02-15, 04:14 PM
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Wink

woohoo....we have heat! Thanks Seatlle and PJ for your help.

I traced the wires from pin 1 and 7 and realized that the plug to the secondary limit switch behind the blower motor was disconnected. The blower motor was replaced 5 months back, so maybe the technician 'forgot' to plug in the secondary limit. Eitherways, this issue is now resolved!
 
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Old 11-02-15, 04:21 PM
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Good job.... great news.
 
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Old 11-04-15, 03:30 PM
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<<The blower motor was replaced 5 months back, so maybe the technician 'forgot' to plug in the secondary limit. >>



Unfortunately, I've done stuff like that once in a while.


You OUGHT to catch most mistakes of that kind by cycling the furnace through the heating cycle a time or two before leaving, but if you aren't thorough, you can leave mistakes like that in the system you have "repaired."
 
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Old 12-30-15, 02:21 PM
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I'm having a similar issue so I'll attach it to this thread...

My unit keeps shutting down and blinking the "Primary or secondary limit switch open" error, except that I measure no resistance across the limit switches. When I turn the unit on and it calls for heat, I hear a switch clicking on/off somewhere near the control board and the voltage swings at pin 1 and 7 as the switch cycles. As for the 24V supply from the transformer, I see 24V at pin 3, but no voltage at pin 6. Has my transformer gone bad, causing the problem with the limit switch circuit?
 
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Old 12-31-15, 01:40 PM
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Follow up to my previous post...

I believe the control board itself needs replaced, because I'm getting 24V through the thermostat when heat is called for, but the 24V is lost somewhere between coming in to connection W and going to pin 1. According to the basic schematic, none of the limit switches are between W and 1, so something on the board is causing the problem.
 
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Old 12-31-15, 01:51 PM
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Is your furnace make and model number the same as in the original post?
 
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