Model TDE080 gas furnace, circa 1996. The problem is that it senses it needs to heat and the fan comes on, but no heat. If I trip the circuit breaker and reset it the system will work for one cycle, but the next time it needs to heat there will be no heat. One other thing I noted - if I turn off the unit from the inside wall unit, the fan will continuously run until, as I mentioned, I reset the system. What needs to be replaced?