Heater will not heat

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  #1  
Old 01-10-09, 01:31 AM
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Heater will not heat

I have a Lennox G40UH (X) it doesn't let the combustiuon air inducer turn on and the code it has says the primary/secondary limit switch is stuck open I located both these how ever I do not know how to test them my house has 2 of these units and the other operates fine thank you for all your advice.
 
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Old 01-10-09, 08:30 AM
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First you have to locate the limit switches. They should look something like this:


The primary switch is probably located in the "vest panel" the vertical partition inside your furnace to which the burner assembly is attached. It will probably not be a manual reset switch and should reset itself. (It will not have the a reset button, red in the photo.)
The purpose of this switch is to open and protect you and your family if for some reason the heat exchanger should overheat.

The secondary limit switch will be located somewhere in the blower compartment, perhaps on the blower scroll. It will probably be a manual reset switch. You press on the button to reset the switch. You should hear a click when it resets.
The purpose if this switch is to open and protect you and your family if the circulating air blower should malfunction.


There may be other limits of the manual reset type that are located on the burner assembly or draft hood. Their purpose is to protect you if for some reason flue products or flames spill back into the furnace.

You can test to see if the switches are "stuck open" by removing the wires and checking them with an ohmmeter. You should have continuity across the terminals.

In some older furnaces (and some not so old mobile home furnaces) there is a fan switch that looks just like a limit switch. This is not a limit switch and actually works in reverse of a limit switch. It closes to turn the blower so at room temperature it should be open. Complicating things, this switch is often mounted right next to the primary limit switch.

If a auto-reset limit switch becomes stuck open it is probably a malfunction of the switch itself although I have seen incidences where a switch was damaged due to extremely high heat. If it happens again something is very wrong. Fix it!

If a manual-reset switch continues to open something is very, very, wrong.... Fix it before it kills you!

By the way (catching breath)
Originally Posted by chp10_8 View Post
I have a Lennox G40UH (X) it doesn't let the combustiuon air inducer turn on and the code it has says the primary/secondary limit switch is stuck....
Are you sure of the failure code? Usually when there is an open limit switch, the control causes the fans to run continuously. If you are reading the code correctly it looks like your problem is probably a dead blower motor... or perhaps a defective control board which is not turning on the blower properly. Either could cause a manual-reset switch to trip.

Just curious: The "UH" in the model number means that this furnace can be installed in either the upflow or horizontal positions. Is your furnace standing up or on it's side? This could have a baring on limit problems.
 

Last edited by xpogi; 01-10-09 at 09:03 AM.
  #3  
Old 01-10-09, 08:36 AM
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You need to identify the circuit that contains these limit switches. It will start at the ignition control module and go through one or more switches before going back to the ignition module.

This should be easy to identify from the cicuit diagram, if you can read it or puzzewl them out. You can probably do the latter if you are willing to spend some time thinking about the diagram and comparing that with the furnace.

You need an multimeter that reads AC voltages. Connect one probe to the furnace chassis and with the thermostat calling for heat you should have 24VAC traveling all the way through that circuit. It's probably going to stop at the switch that is open when it should be closed.

If the switch is resettable, describe the switch and where it's located --- you need to find out why it opened.

If it's not resettable, it may be stuck open and need replacement. You can tap on it gently and see if that jogs it closed and starts the furnace operating.
 
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Old 01-11-09, 02:48 AM
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ok I just switched the circuit boards and the heater still doesn't operate the same when the limit switches are switched now im stuck please assist.
 
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Old 01-11-09, 10:10 AM
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Originally Posted by chp10_8 View Post
ok I just switched the circuit boards and the heater still doesn't operate the same when the limit switches are switched now im stuck please assist.
We all must be missing something here. You need to take the burner compartment door off and then give your furnace a call for heat and note just exactly what happens and in what sequence. Does the inducer blower come on and then shut off? Does ignitor glow red at all? Does the fault code start immediately after you give the furnace a call for heat or is there a delay? Write it down and check it a couple times so that you are sure it does the same thing every time. And lastly, are you very sure of the fault code? This is beginning to sound like a pressure switch related problem to me but that should be a different flash code.
 

Last edited by xpogi; 01-11-09 at 10:38 AM.
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Old 01-11-09, 02:03 PM
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Originally Posted by SeattlePioneer View Post
You need an multimeter that reads AC voltages. Connect one probe to the furnace chassis and with the thermostat calling for heat you should have 24VAC traveling all the way through that circuit. It's probably going to stop at the switch that is open when it should be closed.

If the switch is resettable, describe the switch and where it's located --- you need to find out why it opened.

If it's not resettable, it may be stuck open and need replacement. You can tap on it gently and see if that jogs it closed and starts the furnace operating.
chp,

You never answered the above quote. You may be presuming that because blink code suggests limits, that it is confined to just the limits. I take the blink codes with somewhat of a grain of salt. It may be any disruption on that 24 volt pathway(that the limit is also on). Check it out.
 
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