Heat turns off but fan keeps running

Greg Welter

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.

Thanks.

bill_71

it actually sounds like the "heat pump" portion of your heating system isn't working quite up to par. the two bulbs you see on your thermostat are for staging of the heating system..the first stage is your heat pump, the second stage is your electric heat strips...or supplimental heat, or auxiliary heat.
from what you are describing the heat may be working fine when the thermostat is 2-3 degrees below room temperature. this will keep both stages of heat on and fan running. once the second stage of your thermostat is satisfied (1-2 deg below rm temp) then your second stage heat shuts off and your fan and heat pump stays running in order to reach the thermostats setpoint. if the heat pump wasn't putting out any heat then you have nothing but a fan running and that will blow around room temperature air.
now you stated that the outdoor unit shuts off but the fan stays running inside which sounds a bit backwards from my theory. there is a fan "on/auto" switch on the thermostat that should be in the auto position unless you want the fan to run 24/7. an a/c and heat pump guy should be called to check the performance of you heat pump but a quick and simple test would be to put your thermostat on "emergency heat" and see how evertything responds. this function will bypass the outdoor unit and run only your electric heat and essentially turn your thermostat into a single stage "on/ off" device. * note* if this heat pump was installed in the mid 80's then i would highly recommend keeping it on emergency heat anyways once the temperature drops below 30-35 degrees unless you like wasting electricity.