Hmmm. My next guess would be an out of adjustment water valve and or blend door. If the door isn't adjusted right when your on full heat it may not be full closed. kind of like the front door to your house. So the heat is being cooled by the cold outside air making it just warm. Or the water valve could have broken internally.
To rule out the water valve you need to undo the cable that goes to it. try to turn it by hand. It should only turn so far. Kind of like a ball style water valve in your house. If it spinns around and around its bad.
Then read the attachment and try adjusting the cables. Disconnect the cables under the passenger dash and then turn the knob to full heat then fallow the adjustments. I like to work my way down the line doing the blend door first then go under the hood and set the water valve.