Everything breaks down eventually. In order for mechanical devices to work, there’s going to be some level of friction that’s not only wearing away at the pieces but also causing temperature spikes that weaken the molecules in the parts. For air conditioning units, the stresses put on them by being outdoors can also cause them to wear prematurely. For heating units, cold Maryland winters can cause them to turn on multiple times an hour, putting stress on both their electronic and physical components.