2004 Ford F250 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