strange smell of burnt oil at times

hi we have just got an old escort estate and runs fine but strange smell of burnt oil at times more so when heater on but when heater on full clears any ideas !!! runs o,k and pulls well have checked for head gasket trouble ie no water in oil or oil in water only had a couple of weeks so dont realy no the car well when heater is on full no smell yet when on low ei no 1 it smells of burnt oil inside the car !!!!! any ideas