How Direct Vent Fireplaces Work
Millivolt systems can be either direct vent or natural vented fireplaces. Direct vent fireplaces will have a sealed pane of glass on the front of the fireplace. It will also use 100% outside air to fuel the fire and is completely sealed off from your home. The two big advantages to direct vent fireplaces are high efficiency and the ability to install them in almost any room.
Natural Vented Fireplaces rely on natural draft and stack effect. The front of the fireplace will either have doors that open up, or no doors at all. Open hearth units will heat up the vent or flue, causing air to rise in the chimney. Air replaced to fuel the fire will come from inside your home. Naturally vented fireplaces may give you more ambiance but they actually may cause your furnace to run more. As air is pulled from your room and pushed up the chimney, your home will need to make up that air. Many times, especially in older homes, it will leak outside air though old windows, electrical outlets, or wherever it is easiest to get it. If you have a newer home that is more air tight, you may develop back draft problems and the fire may vent back into your home instead of up the chimney.