Cummins Big Cam 400

Like they said check to make sure the hot water valves are on. You can start at the firewall and trace heater hoses back to them. If they are on the possible causes are low coolant or no circulation. Also check that between radiator and a/c condenser isnt stopped up.
The causes of no circulation are thermostat, water pump, or restriction in the capillary tubes inside radiator and heater core. Start with checking to see if upper and lower hoses are same temp. How long does it normally take to run hot?

 

Old post I know, but coolant hose going in to thermostat housing has a cone shaped metal screen in it , real easy to check, pull it out and see if it is clogged. betting it is.