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Kenton Guidry
Kenton Guidry, HVAC Technician
Category: HVAC
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Experience:  12 years install/service experience.
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We have a Lennox G23 series furnace. The pilot is lighting,

Resolved Question:

We have a Lennox G23 series furnace. The pilot is lighting, but blower will not kick on. The diagnostic code indicates "PRIMARY/SECONDARY LIMIT SWITCH OPEN". What is the primary/secondary limit switch?
Submitted: 7 years ago.
Category: HVAC
Expert:  Kenton Guidry replied 7 years ago.
Do you have an electrical meter to determine what needs to be done?
Customer: replied 7 years ago.
By electric meter do you mean little green lights that flash differently to reflect different problems? If so then yes. Otherwise I don't know what you mean by an electric meter
Expert:  Kenton Guidry replied 7 years ago.
No I mean a meter to measure voltage.
Customer: replied 7 years ago.
We do not have anything on the furnace to measure voltage
Expert:  Kenton Guidry replied 7 years ago.
You will be unable to determine your problem.

I can tell you what the problem could be but I am not sure you want to pay for me to guess.
Customer: replied 7 years ago.
how much will your guess cost me? If it is $15.00 or less go ahead and guess.
Expert:  Kenton Guidry replied 7 years ago.
The limit switch will cause your system to trip based off of the temperature inside of your furnace.

If your blower motor does not come on then it will cause this to trip.

If you can get a hold of an electrical meter you can measure the voltage going to the fan motor to see if it is getting power.

If the motor is getting power and does not run then your motor will need to be replaced.

If you do not have voltage you will need a meter to diagnose why the control circuit is not supplying power.

If your fan motor is coming on, and you are getting high limit switch trip, you will need to make sure the air filter is is good shape.

A reduction in airflow will cause the heat to stay in the furnace and get so hot the trip happens.
Customer: replied 7 years ago.
The blower works because when we turned on the AC, the blower worked. Also we just put in a new furnace filter.
Expert:  Kenton Guidry replied 7 years ago.
The blower motor runs at two different speeds.

The motor high speed during the cooling, and low or medium when heating.

The motor gets the power from a relay when the cooling is energized.

Older furnaced use a temperature switch to energize the low speed for heating.

New furnaces with control boards send power via relays within the control board.

Are you certain your fan motor is coming on during the heat cycle?
Customer: replied 7 years ago.

Are the fan motor and blower one in the same. We are not quite sure if we are understanding what you are saying. We don't think anything is coming on after the furnace ignites. It tries to prove itself for about 2-3 minutes and then just shuts down.

Expert:  Kenton Guidry replied 7 years ago.
Yes, the fan motor and blower are interchangable terms.

Would you look at the following link and tell me if this is the same furnace you have?

Customer: replied 7 years ago.
yes this is our furnace, the G23 series
Expert:  Kenton Guidry replied 7 years ago.
Please turn your furnace off.

Wait one minute.

Then turn it back on.

After that watch the furnace start up.

You will need to remove one of the panels to observe the following.

Gas furnaces have a start sequence they must go through.

The first step is that the furnace will get the signal from the thermostat to turn on.

Then the induced draft fan will come on. Not all systems have this fan.

After that, if you have an induced draft fan, the pressure switch will close because of the induced draft fan running.

Once that happens your ignition will attempt to ignite the flame.

If you have a spark ignition the ignitor will spark. If you have a hot surface ignitor, the ignitor will glow very bright from being so hot.

This is when the gas valve should open.

The gas will not ignite.

Then the flame sensor will sense the flame.

Once the flame is sensed, the heat exchanger will warm up.

After the heat exchanger warms up the indoor blower motor will start.

Please let me know how your system runs compared to the above description.
Customer: replied 7 years ago.
The furnace goes through all the steps you listed until it gets to the indoor blower motor starting. At that point the furnace shuts off and the diagnostic lights indicate PRIMARY/SECONDARY LIMIT SWITCH OPEN. Prior to this while the furnace is cycling, the status/diagnostic codes indicat NORMAL OPERATION - SIGNALED WHEN HEATING DEMAND INITIATED AT THERMOSTAT
Expert:  Kenton Guidry replied 7 years ago.
This is where you will need a meter to determine if your fan motor is out or the control board is faulty.

The reason the furnace is shutting down is because the fan motor is not coming on.
Customer: replied 7 years ago.

Is there a meter specifically designed for this and where can it be purchased?

Expert:  Kenton Guidry replied 7 years ago.
You would be better off getting a technician to do this.

The work will have to be done by a licensed contractor or the part you replace will not be warrantied.

Especially if we find out it is the control board.

If you decide you want to continue you can purchase an electrical multimeter at lowes or home depot.
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