I once owned a yugo back in the 80s, and the only thing good about that car was that it was equipped with a rear window defroster, so when it broke down in cold weather, which was frequently, my hands stayed toasty warm while pushing it. As for the hot air problem, it is probally the water valve that leads to the heater core. From my recollection, there is a wire cable coming from the temperature knob from the dash which is routed to the engine compartment to the water valve. When the temp knob is set to heat, it moves the cable which in turn opens the valve, thus letting hot water from the engine block to the heater core. The cable could be broken or just disconected. Or, the valve itself may need to be replaced, which is a fairly simple job, The difficult task is finding a new replacement part. I would suggest not installing a used or junkyard part as they are most likely rusted and non functional. In conclulsion, this is not a serious problem, but only a minor inconvenience during the warmer months.