A brand new Ac carries a 5-year warranty on the compressor and a 1-year warranty on other parts. Second hand Ac will not let you have this advantage. Also, if the Ac at your parent's house require service every four months, then there is some major problem with it. Air conditioners require servicing usually once in a year. All that one needs to do is to keep cleaning the air filter regularly which could be done by any one.