This problem just started a couple days ago.
Heater blows out cold air this started 2 days ago.Is it the thermostat. on 1999 Ford Windstar
If it don't run hot,it's not the thermostat.Check the water level in the radiator and if that don't work then the heater core is stopped up.
A leak any where in the coolant system could cause an air pocket in the heatern core.
The fan is broken