I'm putting in a 2000 watt 240volt wall heater that calls for 14gage wire with a 15 amp circuit breaker. There is an existing 12gage wire with a 20amp 240volt circuit breaker not being used. Can I just use that?
Using 12 AWG wire for a 15A circuit is just fine. However, if the device calls specifically for a 15A breaker then you must use a 15A breaker. You can't upsize the circuit just to save a few $. That means replacing the 20A breaker with a 15A breaker. Note also that the breaker needs to be a double-breaker or a handle-tied pair of breakers, not just 2 single 15A breakers side-by-side.