Units from Headquarters Station were alerted for a CO detector activation in an apartment building. Engine 1-1 and Tower 1 arrived with a partially evacuated building and the caller stating they had several CO detectors activated within the building. Crews had high reading of CO steps inside the door. Crews remained on scene for 2 hours investigating throughout the old 3 story building. The source of CO was found to be a blocked flue from a furnace. Chief 1 had command and the scene was turned over to the building owner for repairs.