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I have a Miller furnace, there is a small opening in front where the oil line comes in from outside and connects to the burner, a rectangular shape opening that also lets cold air in. Not sure if I can plug this hole with say a towel or if its some sort of cold air return that needs to be left alone.

Any advice appreciated.

Thank-you.

That opening might possibly be a fresh air source for the burner. The usually have some type of fresh air coming in because they don't want it drawing air from inside the home. Pictures would help a little more.