We have a Rheem heat and air unit RRGG07E37JKR, thirteen years old, which no longer puts out heat. The air still works when switched to "cool" at the thermostat. The fan works when switched from "Auto" to "fan" in either heat or cool mode. Any advice will be appreciated. Saun and Phil

When you turn it to heat does anything start or happen?

Nothing happens other than the click heard when rotating the thermostat dial (the magnet making and breakinig the switch inside the glass tube).
Again, the a/c works fine, just nothing heat-wise. Saun and Phil