Why is my 93 ford ranger 5 speed 3.0 heater now blowing out cold air?

I have a 93 ford ranger. Last winter the heater worked descent. Blowing above warm air. Not as warm as my newer model car but it worked. The beginning of this winter it wasn't quite as warm as last year. Within 2-3 weeks its now blowing out cold air (defrost panel floor all vents). Last night at roughly 25-30 degrees I drove it approx. 10 miles occasionally turning heater/defrost on to try to defrost windshield some and upon returning home it sounded like boiling water under the hood.

The heater valve can be broken the blend door under the dash can be stuck Check the floor boards for moisture if the core goes bad its usually leaking in the cab passenger side floor boards a broken heater by pass line but check the heater valve first it could even be a bad vacuum line