By ROBERT MILLS, Sun Staff -Seven people were hospitalized yesterday when an old heating system began pumping potentially deadly carbon monoxide through a Kensington Drive home, prompting an early winter warning from firefighters.

In light of the incident, in which residents of the home luckily noticed something was wrong, Chelmsford Fire Capt. Richard Miller said all homes should have a carbon-monoxide detector and regular maintenance performed on heating systems.

The victims of yesterday’s incident were not identified, but all seven are adults who were taken to Lowell General Hospital, according to a report.

Three people were treated for carbon-monoxide sickness and released from Lowell General, a spokesman said. It was not immediately clear if the other four had been diverted to another facility or simply went to Lowell General to be with their friends.

Miller said firefighters were called to 17 Kensington Drive for a reported odor of gas after someone began feeling ill and believed an odor of gas was in the air.

That prompted a call to firefighters, but Miller said there actually wasn’t any gas in the air, which was confirmed by firefighters and Keyspan gas crews, who could find no gas leak.

Instead, they discovered that lack of maintenance on the home’s original 50-year-old furnace had caused the carbon monoxide to be emitted.

Carbon monoxide, which is colorless and odorless, is deadly in high enough doses.

“Once the levels go over a certain amount, you’re not going to wake up from it,” Miller said.

Miller said furnaces that old, especially when not maintained yearly, can burn less efficiently, creating a yellow flame that fails to burn all gases like carbon monoxide, instead of the blue flame that should burn inside gas stoves.

Firefighters got the people out of the home, gave them oxygen and told them not to use the furnace again, Miller said. It was unclear where they were staying last night.

With winter approaching and residents throughout Greater Lowell bundled up inside, Miller said it is important to have a carbon-monoxide detector.

“They should go hand in hand with fire detectors in today’s day and age,” Miller said. “Everybody should have one.”

The odorless gas kills about 500 Americans every year, according to the Web site of the U.S. Centers for Disease Control. It can be emitted by any fuel-burning heater or water heater, the Web site says.