Blue shield fall service. Replaced filters. Blower motor run capacitor checked good, motor and wheel are clean, hot surface ignitor checked good, cleaned and checked flame sensor, pressure switch checked good, inducer motor run load amps are within mfg specs, all limits and safeties checked good, manifold pressure was 3.5 in wc, within mfg specs, air temperature rise was 44 deg, within mfg specs. Checked tstat operation, system operating normally. Carbon monoxide- 0 ppm. Found flue not connected to furnace, adjusted flue piping and reconnected flue to furnace.