I have a heat pump/furnace combination. The system is set so that above 40 degrees, the heat pump is the heat source. Below 40 degrees, the furnace is the heat source. When the heat pump is in use, the system works perfectly. When the furnace is in use, the thermostat turns on the furnace at the appropriate time, based on the temperature settings, but will not turn off the furnace regardless of how hot it gets. Once the furnace is started, the only way of turning off the furnace is to turn off the power to the furnace. I don't know how to fix the problem.