The heater in your Ford Focus works off of the radiator. Coolant is pumped through a heater core and radiates heat. This heat is then blown into the passenger compartment by a fan. While your heater should provide years of trouble-free use, you will need to troubleshoot it if you are having problems getting the heat to work properly in your Focus.
Turn the vehicle on and allow it to warm up to normal operating temperature.
Turn the heater fan on "high".
Check for any leaking antifreeze or coolant in the passenger side foot well. The leaking coolant indicates that your heater core has failed and will be the reason why you are not getting hot air to blow into the cabin.
Watch for puffs of white smoke coming out of the heater vents. White smoke is actually steam and is an indication that there is a leak somewhere in the system. The main reason for heater failure is a leak in the system. If there is a leak, you'll need to have the system checked and repaired by a professional mechanic.