It happened just before 11:00 Thursday night at the Spring Oak Assisted Living facility on the 300 block of South White Horse Pike.
Officials say crews arrived and located the source of the CO.
They disovered a malfunctioning heating unit on the roof that caused the gas to back up into the building.
Residents were not evacuated from the home but were moved to the cafeteria area, then moved back to their rooms early Friday morning.
No injuries were reported.