Katonah Fire units were dispatched early Saturday evening to a carbon monoxide incident with symptoms at a house on Harris Road. Upon arrival, Car 2212 reported that one occupant of the house was complaining of a headache and had departed the premises. Upon initial investigation of the house, readings of 30 parts per million CO were found. Firefighters turned on their SCBA and continued to check the house. Further investigation revealed that the hot water heater was the most likely source of the CO. The homeowners were advised to call a service to repair the hot water heater. KFD personnel secure the heater and shut off the gas supply to it. Once CO levels were down to zero, KFD went back in service. Units were on scene for about an hour.