A sensor network system for determining a chimney maintenance schedule comprises a sensor unit (16) arranged for placement in or proximate to a chimney (6). The sensor unit comprises at least one sensor arranged to measure a parameter of the chimney (6) and use the measured parameter to generate chimney health data associated with the chimney (6). The sensor unit includes a transmission module arranged to transmit (20) the chimney health data to a remote analysis unit (18). The remote analysis unit (18) is arranged to receive chimney profile data associated with the chimney (6) and to estimate a chimney health level associated with the chimney (6) from the respective chimney health data and chimney profile data. The remote analysis unit (18) determines the chimney maintenance schedule from the estimated chimney health level.