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Tell me please model number of your Lennox unit, the size of the house, how long the system has been installed, and how hot it is outside.
We can go from there.
You have provided the model number of the thermostat only. I need the model number of the Lennox unit sitting outside the house..... and need to know the square footage in the house, and if its a new system or not. I need all of that data in order to assess what the possible causes are.