A method is described for identifying faults relating to an HVAC system, such a clogged filter. Sensor data is used to estimate HVAC system efficiency. Trends in system efficiency are then used to identify faults such as clogged filters. The sensor(s) can include one or more of the following types: optical sensor, temperature sensor, pressure sensor, acoustic transducer, humidity sensor, resistive sensor, capacitive sensor, and infrared sensor. The efficiency estimation can also be based on conditions external to the building, such as data from exterior sensors and/or data gathered from third parties such as government or private weather stations. The efficiency estimation can also be based on performance metrics such as the time used to reach a set point temperature. The fault identification includes filtering out non-fault related events.An instrumented air filter for use with an HVAC system is also described. The air filter, which can be disposable, can also include one or more of the following: a wireless communication device adapted to transmit data from the sensor to a receiver; a power harvester (e.g. using piezoelectric elements) positioned and adapted to generate power from air passing through the filter; and a power storage device adapted to store power generated by the power harvester.A method is also described for estimating HVAC system capacity based on user-inputted characteristics, such as the physical size, of an air filter used in the HVAC system.