AC technology is essentially the same as that for heat pumps, which I do see more frequently nowadays, with a concomitant increase in recommended repairs. But what I want to focus on in this blog post is what, if anything, a homeowner can do to repair these appliances.
Repair issues of air conditioning vary with the type of installation. The most common type is a centralized system that distributes cool air throughout the house using the heating ducts and registers. The localized window units operate similarly on a much smaller scale. Evaporative coolers are really different and not pertinent to this blog. For basic principles of operation, see my article on air conditioning repair conditioning work
The four main components of conditioners are the blower (typically the furnace blower), the evaporator coil (inside and directly downstream from the blower), the condenser coil (outside), and the means of distribution. (There is also the piping carrying Freon between the two coils in both directions in a closed loop.) When AC problems occur, they are usually condenser issues, as it is the component that tends to work hardest. Next in line is the evaporator coil. Most servicing should be done by an HVAC professional, but there are some simple tests the homeowner can periodically conduct (along the lines of a home inspection) to assess performance.
The simplest tests check adequacy of air flow. Hold a piece of yarn in front of each register to see if it flutters. If it doesn’t, there may be a leak in the duct leading to that register. Now dangle a tissue next to the air return to see if it gets sucked against the grille. Failure of this test implies there isn’t enough draft air going through the system, and it suggests that there may be a leak in the blower compartment.
With an accurate thermometer, check the amount of cooling the air conditioner is producing. Take a reading right where the air returns to the blower and take your other reading at a room register (this is where it is coldest). The differential should be between 14° and 22°F. Something less implies that one or both coils aren’t working properly and need to be serviced. Access to the condenser coil is difficult in that the whole unit is self-contained, but investigate the evaporator coil inside the furnace housing. See if it has become encrusted with frost or ice.
Outside, see if the condenser is making unusual noises. Look for mechanical damage and water damage. Test the temperature of the liquid (copper and pencil-sized) and suction (hot-dog-sized and insulated) lines that connect the two air conditioner coils. The liquid line should be about 20° warmer than outside air, while the suction line should be the temperature of ice water.
Also, observe the on and off behavior of the condenser. If it seems to be cycling too frequently (known as short-cycling), it is not matched well to its load. Repairs are in order if any of your tests fail. See this heating and air conditioning article for more general information on both of these systems.