Keep the Central Air Conditioner Running Through the Warm Months: Filter, Thermostat, and the Service Log

A short set of checks you can run yourself, a service visit you schedule, and a log that tells you what the system is actually doing.

A central air conditioner spends most of its hours on a few simple conditions. Keep those conditions steady and the system gives you clearer signals, so the upkeep here is about giving it an easy season and writing down what changes. Start before the first hot stretch arrives.

The filter

Look at the filter every few weeks while the system runs hard. Hold it up to a light. If you cannot see light through the pleats, replace it. Write the size printed on the frame on the paper you keep with the unit, and buy that same size. A loaded filter lets less air move across the coil, which makes the system run longer to deliver the same comfort.

The thermostat and the registers

Set the fan to auto unless you want air moving all the time. Replace the thermostat batteries when the display dims. Keep supply and return registers open and clear of furniture and rugs. Closing many registers at once raises the pressure in the ductwork and can freeze the indoor coil. Large swings in the setting make the system work harder to catch up, so a steadier setting is easier on it.

The outdoor unit

Walk outside once a season and look at the outdoor unit from each side. Clear leaves, grass clippings, and loose debris from around the cabinet. Leave the fins alone and do not aim a hard stream of water at them. The technician can clean the coil with the cabinet open during the service visit.

The service visit

Schedule a service visit once a year, before the heavy season. Ask the technician what the visit will measure. A useful list includes the electrical connections, the contactor, the capacitor, the coil condition, the airflow across the indoor coil, and the condensate drain. Ask for the readings written down. A note that says all is fine tells you much less than the numbers behind it.

The log

Keep a notebook with the unit's paperwork. Write down each filter change, each service visit, each repair, and any change you notice in how long the system runs. That record is what you will use to judge the unit when the question of replacement comes up. The central air conditioner page on this site gives the typical service life and the window for planning.