2009 Chevy Impala will only blow cold air

There is a very good possibility that you need to change your thermostat because it is stuck open. Generally how a thermostat works is that it does not allow antifreeze into the engine block until the engine reaches a certain temperature (usually 185-215 depending on vehicle). Once that temperature is reached the thermostat opens up allowing antifreeze to flow into the engine block maintaining a steady temperature. If your thermostat is always open, antifreeze is always running into the engine block and cooling it even before it reaches its normal temperature. This keeps the engine running cooler for much longer, which results in you not being able to get heat.