heater not heating on 1996 GMC Suburban 1500

Replaced thermostat and heater only gets luke warm.

Asked by for the 1996 GMC Suburban 1500
Maybe the thermostat is staying open too much. Is the engine warming up fully-if not then recheck the termo. and make sure it opens at the right temp. Also, you may have another coolant system issue that is related to this.
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I had a very similar problem. The heat was barely warm and the higher you turn on fan, the cooler it got. For me, I had to replace the heater core and that solved my problem. Just one solution to think about.