Replace the filter on a 3-ton air handler by turning the system to Off at the thermostat, locating the filter compartment, removing the old filter and replacing it with a new one. Depending on the amount of use and air quality, filters normally require replacement every three to six months.
Turning off the system before beginning the work prevents dust from collecting on the heat exchanger coils in the air handler. Most thermostats have a switch that allows the user to choose a heat, cool or off setting. If the system has a fan setting, the owner should make sure it is set to automatic or off before removing the old filter.
The filter is usually in the air handler or a cold air return duct, depending on the age of the system. If it is in the air handler, the access door usually has a label to alert the resident to its access. If it is in the cold air return duct, access requires opening the duct cover.
When removing the old filter, note the arrow that indicates the direction of flow. Disposable filters have cardboard frames. If the filter has a metal or plastic frame, clean it according to the manufacturer's directions and reinstall it. Manufacturers print the size of disposable filters on the cardboard frame. Use this size when purchasing a replacement.