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.
November 10, 2013
What a great web site. Thanks.
One question. I have a direct vented unit and have a fair understanding of how it works but I don’t know how the thermocouple controls that part of the main gas valve feeding just the pilot. Can you help. Again, thanks.
November 12, 2013
Good question! The thermocouple creates a small amount of voltage when heated (20-30 millivolts). This voltage opens an electromagnet in the control valve to feed gas to the pilot. If the thermocouple cools off (ie the pilot goes out), the electromagnet cannot stay open and gas will be shut off to the pilot.
Fireplace manufacturers use a thermocouple for the pilot light because it cools off much faster than the thermopile.
October 18, 2017
Need some help fireplace will stey lit all day long, with the door open (direct vent) as soon as i shut the door the flame turns all blue and goes out! Vent is facing wind direction but has a fancy vent on the outside to prevent wind from coming in the vent is not obstructed what else can i do??
November 18, 2017
Check the venting for blockage (including the cap). Make sure the venting is connected and not damaged or disconnected. Also, make sure the fireplace was installed within its tested venting limits. Venting limits can be found in your installation manual. It is definitely an issue with the venting.