Russellm

Hi guys I need some help! Got a 2004 impreza sport got an issue with my heating.. So start car from cold heating comes along set dial to 25 degrees and get warm inside.. After 20 mins or so of driving it start to blow cold air even though the dial hasn't changed position. Turn dial up to 27 and hot air returns but after a few minutes it goes cold again When on 30 its hot all the time