thermostat stuck closed

with coolant full and the car is cold ( squeeze the radiator hose with your hand). You should be able to squeeze it and it will be cold. Crank the car and run to normal temperature and again squeeze the hose. If the hose is now firm and warm the thermostat is not stuck closed . if the heater blows cold but the heater core hoses are warm it is circulating but the heater core blend door may not be working.

over heating ? How soon and is the coolant full? any smoke coming out the exhaust or milky oil?

Check other threads or detailed diagnostic procedures I wrote for coolant issues. Specifically check for percolation, as symptoms point to the HGs. Bad t-stats in Subarus are unheard of...same with wp's.