Ideally, you’d never need to replace an air conditioner in the middle of summer. An older air conditioner which needs to be put out to pasture should ideally be replaced in the fall or early spring, when there’s no demand for it and you can proceed carefully with your decisions.
Unfortunately, not every old air conditioner is polite enough to wait until September to break down, and if you’re facing a serious repair call in the middle of a heat wave, it may be more beneficial to simply replace the failing system instead of spending money unnecessarily to prop the older one up.
Every plumbing system encounters clogs now and again. It’s an unfortunate reality of owning a home. While common, however, clogs should usually be dealt with by a professional plumber, who has the training and equipment to do the job far more effectively than any store-bought snake or cleanser.
Ducts get damaged just like any other portion of your home. In some cases, that can be direct damage, as when rats or squirrels get into the crawlspace and gnaw on the material, or someone inadvertently creates a problem while working on some other part of the house. Other times, it can come from simple wear and tear, such as when two fitted parts lose a bolt and create an opening. 
