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Heat turns off but fan keeps running
I'm taking care of a cabin that was built in mid-80s with a heat pump system. Typically, we run the system with the thermostat set about 68 deg, concurrent with a wood fireplace burning. We notice a problem in the winter where the temperature will get to the point that the heat pump turns off, but the fan continues to run, blowing cold air throughout the house. We notice that if we adjust the wall thermostat we can get the fan to turn off. It appears that there are two set different set points, one for the fan and one for the heat. When I took the plate off the wall thermostat I noticed that there are two mercury switch bulbs, one with a red wire attached and one with blue. I'm not sure about this but it appears that the two bulbs are mounted on a frame that may be able to adjust the relative angle between the bulbs.
Is this normal? Can you help me with a better understanding of the system? (BTW, the exterior of the wall thermostat is marked Ruud, but inside it is marked Honeywell. The plant in the basement appears to be a mix of Ruud and Carrier equipment.