Is the air flow strong even though it's just warm? If you feel the plenum, is it warm or hot? You'll want to feel it right where the core sits. Hows your coolant look? Is there a lot of calcium on the core?
You might consider a home flushing kit. Or add an additive to flush the system out. If you jsut want to flush the core then disconnect the lines that go through the firewall and use a hose to see if there is flow. Hold the hose up to the outlet so it will help remove the scale.
A 2.5L heater should heat up pretty quick.