No heat on 1997 Ford Taurus

When heater is on, only cold air blows out.

2 answers
heater blows cool air -replaced heater valve-replaced thremorsat[-replaced hot and cold swwitch
Is the engine running at normal operating temperature? If it is running cool, then the thermostat may be stuck open. If that is OK, then the system that controls the airflow past the heater needs to be inspected.