Our house was built in 1955. Our Heatilator is original to the house. We are the second owners. Previous owners didn't use the fireplace except at Christmas. We have used it quite often. I noticed plugs on the lower side vents. These are on both lower vents. I've googled and the search came up empty. I can't find any pictures or explanations for the plugs on the side vents. What are these for? There isn't any sort of fan or blower in our fireplace. It doesn't appear that a fan or blower was removed at any point either. Our chimney sweep was at a loss as well.