I really like the quiet gravity feed furnace; it was one reason I bought this 1954 house. But I'm starting to get concerned about it being unreliable. We had our first colder day here in Wyoming and I turned on the thermostat and the familiar sound of the gas manifold ignition didn't occur. I remember that last year I had to gently tap the valve before it would light off. That isn't working now. Could it be a bad valve? How hard are they to find?