no heat

I have a 97 GMC Savanna, recently replaced the heat control valve and thermostat. The engine will not get to temperature. Before I replaced the thermostat, the gauge registered around 100, then afterwards it goes to somewhere between the 100 and hovering around the big line at I guess about...

i have a 2003 gmc savana that will only blow cold air. i dont think it is the thermostat as it does not over heat and i can feel the heater core gets warm from anitfreeze. i dont know where to go from there. any help would be appreciated. thank you chad