Blocked Heater Core

I have a 2010 GMC 3500 Savanah van with 6.0 motor. No heat in van. Engine operating temp is normal. Heater hose into core is hot. Hose coming out is barely warm. I am of course hoping for another explanation other then blocked heater core. The truck only has 45k miles. I am ready to backflush , but wondering if there can be any other explanation. A/C works normal Thanks for any suggestions

I do not know but it is possible there is a control that allows flow to the heater core of warm coolant that may have failed.