Abstract: An apparatus including an air filter for an air conditioning device, a warning device, a computer processor, a computer memory, a count setting device, and a reset device. The computer processor is programmed to set a value of a timer count variable to an operator input start value in the computer memory in response to the count setting device. The computer processor is programmed to count down the value of the timer count variable and when the value of the timer count variable has counted down to an alarm level, to activate the warning device. The reset device is actuated by an operator to cause the computer processor to deactivate the warning device and to reset the value of the timer count variable to the operator input start value, and to cause the value of the timer count variable to be counted down.