I ussually run my boiler without a cap on the chimney and put a cap on for the summer but sicnce I am going to burn this summer I said I can leave it off. So I got home last night to light a fire and as I was cleaning my secondary chamber I pulled out some ashes along with a bird which came flying at my face...needless to say it scared me pretty good. So not only will a bird build a nest in your chimney looks like they will live in your stove too.