2004 Ford F-250 Heater blows cold air

Are you low on coolant? If not you might have a air bubble in the coolins system, let the truck heat up to normal temp. You should have at least 1 bleeder valve. (By the thermostat, water pump, or on one of the lines from the heater core) open one or all the bleeder you find. They should spit a steady stream. So bleed untill it does, might have to close them shut off the truck let it cool and repeat. Hope it helps