Abstract: A filter monitoring apparatus 10 is provided for determining when an air filter 12 in an air conditioning and heating system 14 is clogged. The monitoring apparatus 10 includes an air pressure sensing switch 28 including an actuating arm 34 which monitors air flow through the filter 12 and closes a switching member 30 when the air flowing through the filter falls below a predetermined level. When the switching member 30 closes an activation signal is fed from a power source 24 to activate a horn 26. Once the horn 26 is activated a predetermined sound is transmitted which indicates the air filter 12 is clogged or dirty and should be changed.