Some people can probably do it themselves in the same way some people can service their own cars... Others are just not that technically equipped... The heat pump, has a strainer and a circulation pump, which are open to the municipal water and a possibly 50 year old plumbing system. Each time there is a water cut, all kinds of junk gets into pipelines, which will eventually find its way to those parts. Fridges and Aircons are internally closed to outside problems, you have control over the water that goes into your engine. Pipe insulation deteriorate, which plays a BIG part in efficiency.