At 22:37 hours, Station 20 was dispatched for a carbon monoxide detector activation at 3794 School Road. Chief 20 arrived on scene and established the School Road command. The Chief confirmed that there were two carbon monoxide detectors in the home and they were both still in alert. The crew from Rescue 20 formed two teams of two and sent Team A into the structure with two Galaxy meters. The meters showed 40 parts per million as soon as the team made entry. The highest readings were found in the basement with 90 parts per million. The source of the CO was an instantaneous hot water heater. Team B controlled the utilities and then helped establish ventilation. An exhaust fan was placed in the stairwell from the basement and a PPV fan was placed in the front door. The home was ventilated in about an hour and turned back over to the homeowners. Crews were out approximately two hours.