First off, a properly operating gas furnace or water heater should not leak carbon monoxide at all. You should purchase a carbon monoxide detector and install it in your utility room. Mine is mounted right next to the furnace and water heater.
Second, it appears they meant to use that as a fresh air intake and someone has used it as a cable TV chase. Your furnace and water heater don't need that much draft air for combustion. Even if your house is super sealed a much smaller opening would suffice. My house uses natural gas for both as yours does and there are no vents for combustion air. Houses in northern climates have never had vents until recently and things have worked just fine since the house usually leaks enough to provide the fresh air. Only recently they have started building supersealed homes that might need a fresh air intake for combustion air.
Get the CO detector and then cover the open vent and see if you have any problems. If everything continues to work without any CO detector alarms then you are good and should save some money on your heating bill. Even if you have issues you should install a proper fresh air heat exchanger and stop wasting heating dollars.