My goodman furnace heats up, blows hot for about 5 minutes, then goes cool air and won't warm up again for at least 20 minutes... meanwhile the fan never turns off... this unit has run all day and only raised the room temp 4 degrees... its a small room !

Open all registers.

Make sure the air filter is clean and the return no obstructed.

I don`t understand (its a small room) maybe you could give us more info,, including model numbers and what you are heating with the furnace

Is the fan set to on or auto on the thermostat. But do as hvac1000 says, sounds like the furnace may be going out in high limit, which would cause blower to run constantly, with little heat getting through the register.