Kia Rondo . The engine occasionally overheats . The heater fan blows hot

Depends, does it "overheat" when driving above 35mph or below that speed, overheating only at slow speeds oten means there's an issue with the fan. If it is at any speed it is probably the thermostat or water pump. Remember that the heater is not routed on the same circuit as the radiator, the heater takes its coolant straight from the engine to ensure the front windscreen defrost works as soon as possible in winter time. So I would not rule out the thermostat as a possible cause because the heater blows hot. You can however use the heater as an additional radiator in case of possible overheating by turning it to full hot and full blower.

that was the part I was missing The thermostat is on the intake, not on the output like the old style engines Problem fixed Thnank you