Heat does not work; no evidence that heater core has failed; fan works on 2001 Ford Escort

When the fan is first turned on, there is warm air but it quickly turns cold

Asked by for the 2001 Ford Escort
Heater core can be flushed with fair results, worth a shot though.
Check answer 1 first.
1 more answer
possible coolant level or thermostst could be stuck open