Lots of people don't realize it, but when you use a heating appliance that is vented to the outside, every cubic foot of air that goes up and out the stack has to be replaced in the room. With the regular old fireplaces and pot belly stoves we know and love, that replacement air is going to be many, many cubic feet per minute and is going to be in the form of cold air seeping in through cracks, under doors, around windows... any way it can get in. Otherwise there would be a vacuum formed in the house, your ears would pop, the canary would die and eventually your house would implode.
Well, okay, I got a little carried away... actually, if your house is that tightly sealed, your wood burning device would burn slowly and it would be difficult to get a good draft going up the chimney. A lousy draft = a smoky wood burner or fireplace with a lot of the smoke ending up indoors.
My old house is nowhere close to being tightly sealed. When I had a good fire going in the wood burner, I could put the back of my hand up to the crack in the front door jamb and feel the cold air being pulled in. A lot of the work of the heater was going toward heating that cold air! I finally decided to do something about it. But what? The air intake on the front of my stove was a round design with a built-in adjustable damper and it swung open with the door when it was opened to add wood.