Hello: This may not be a heater problem at all. If cold air is coming in the vent all the time, than air must be leaking (or being blown) out of the house somewhere else. Are upstairs windows always open? Is an exhaust fan running? If you can isolate the heater from the rest of the house, do so and see if that changes things 😎 Another possibility is that it’s wind driven. If so, the vent termination may need to be moved to above roof line. Lastly, perhaps American knows of a damper that can be safely put in the line to prevent back draft.