Let me start by saying this could either be a thermostat or a heater problem. We have a Maple Chase that's the equivalent of a Robert Shaw 9620. After reading different posts here, I found the correct owners manual online. Our problem is that the heater runs and runs but it's blowing very cold air out of the vents. This started yesterday, when the green AUX light lit up. I wouldn't have paid attention to it but it just wouldn't get warm. Even set to 69-70 it only got to 64. Last night was even worse. The heater ran almost all night but it was 50 downstairs this morning. Even my husband complained and he loves the cold. The batteries are fine, I'll change the filter tonight (to be safe), the settings change at right time/temp. We always set it to HEAT, but should we be setting it to EMER HEAT? The manual says to use EMER if the primary stage of heat isn't working but I don't even know what the primary stage is. The manual also says to check the differential setting but I don't want to mess around with it if the problem is with the heater and not the thermostat. I would really appreciate any advice. Thanks very much!

Your heat pump condenser section is not working. That is the primary stage. It is the outside unit with the big fan on top. The second stage is the electric heat inside the blower unit inside. Without the outside unit making heat, the only heat you have is the electric backup. You need a HVAC service person to fix the heat pump condenser.

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Auxiliary heat light is lit and won't go off. Why?

Tunk2009:
"auxillary heat light is lit and won't go off. why?"

The aux heat light means that the unit is running on electric heat only. The thermostat reads the change in temperature in the room, and finds that it is not rising at an acceptable rate. So it automatically puts the electric heat on.

Most likely, your heat pump condenser section is not working. That is the primary stage. It is the outside unit with the big fan on top. The second stage is the electric heat inside the blower unit inside. Without the outside unit making heat, the only heat you have is the electric backup. When the electric heat is on, the AUX light is constantly on.

You need a HVAC service person to fix the heat pump condenser.