How to replenish heat
Either your heater core is not getting hot or the blend doors are not working. First check coolant level, if you're low it can prevent heater core from getting flow from radiator. You can also squeeze the heater hoses with engine running to see if they feel hot/pressurized. If heater core is getting hot but air is not flowing across it from blower motor, you may have a stuck blend door. Usually that is caused by a bad actuator.
The coolant/engine system is OK....temp gauge up and good heat out of rear heater... problem must be as you suggest - blend doors in plastic duct collector inside of dash... could be valve to front heater core not fully opening but it seems more likely to be "flappers" =JAH
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