Furnace comes on when it wants to, even when thermostat is removed or off. Just air, not heat. Heater man said it was the circuit board needing to be replaced. True or not? Cost of circuit board is $368. Could it be something simpler? Heater is about 15 years old... or more? When unit comes on, just fan.. Thermostat has no control over the heater. What else might it be?