Outside Air Source
This air vent may alleviate smoking problems.


Fireplace problems?

If you are having a problem either keeping your fire lit, installing an outside air source may be beneficial. Keeping a fire lit takes different factors including air and fuel for burning. The products of combustion include water vapor and carbon monoxide. Being sure that the fireplace will vent properly is very important to the safety of the home as a whole. One way to improve air flow is to install an outside air source. An outside air source is a vent that provides fresh air to feed the fire.

Building codes in most parts of the country have require these during new construction. Thus, most homes already have them. They look like a small rectangle iron grate, and most often located on the back wall of the firebox. These normally slide to the right and left to open or close. There is a variety of styles around the country, some being round in shape. They to supply air from the outside so as not to rob  air from the living space of the home. My experience is that many will not open. They’re rusted shut or the homeowner doesn’t know how to open them. We have sealed our homes up tight. Think about our air tight windows, doors, weather stripping, and attic insulation. That is why building codes require outside air for all appliances. This includes fireplaces, hot water heaters, and furnace equipment


A fire that burns well will actually increase draft. In order for a chimney to draft properly, there must be a good air flow from the firebox to the top of the chimney. The fire in the firebox warms the flue system so that the products of combustion will flow up and out of the chimney. If the flue system is too cold the outside air coming down the chimney may be heavier than the products of combustion coming up the chimney which can cause a smoking problem. If installing an outside air vent does not solve your problem, a less intrusive repair would be to install a SmokeGuard which makes the size of the firebox smaller so that the products of combustion may be vented more efficiently.


Fireplace Smoking Problems
Installing a SmokeGuard is simple.


Ample Air

An outside air source can make starting a fire in your chimney easy for beginners. With ample outside air and a good set up of kindling and good, seasoned wood, anyone can get a nice fire burning. One thing I will caution you about when it comes to outside air sources is to watch them when they get painting over. Whether the vent is an outside air source or simply a vent on the front of a wood stove feeding the fire in the stove with air from the room, if you get the area painted be sure that the air vents are not painted or sealed shut. This can cause problems, especially if it is a vent on the front of a wood burning stove that requires the air in order for the wood to fully combust.

Consulting a professional

Having an outside air source is generally not a costly process, but consulting a professional over where to systematically place the vent is a good idea. The professional The professional should  discuss with you both indoor and  outdoor variables such as other competing appliances in the home, large hills, trees, or other obstructions that possibly may be a factor for a sluggish draft.

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Clay Lamb
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20 thoughts on “Outside Air Sources

  1. I just read through this article and was wondering, we have and woodburning stove insert and when we turn on kitchen or bathroom fans they draw the smoke back down the chimney into the house. Would opening this vent help with our problem? We realize our home is pretty air tight and we usually open a window before using the fans but sometimes it’s not enough.

  2. We have big problems with a lot of cold air coming inside the house thru the outside air intake door and the chimney trap door.
    Would it help to put a big piece of non-combustable insulation inside the bottom vent-type doors (under the firebox) where the outside air intake door is ?

  3. I have a masonary fireplace. I have to crack a window to get it to draw well. I raised the fire grate up and installed a smoke deflector and it helped. I want to install a 4 inch pipe in the fire box for air. Will this work ? If so where should I put it in the fire box?

  4. Greg, A big congrats goes out to you on all that you have done. It is obviously a serious negative pressure air in balance in your home. As far as the outside air source I’m somewhat surprised that you don’t already have one of these in your fireplace. May I suggest going to a local brick supplier in your area, or even go online to investigate these vents. Different Mfg. have different requirements for the positioning of their outside air vents. Some are 3′ round, while another manufacturer may produce a vent about the size of one of the firebricks found on the back wall of your fireplace. As far as saying this will solve your problem, would depend on the severity of the negative air pressure issue. I have seen negative air problem resolved by the homeowner simply changing out their expensive furnace air filter to a much cheaper filter, (replace more ofter) that allowed the proper air balancing to be pulled from the room within the home “evenly”. Due to the much thicker “filter mesh media found in the expensive filter, this would not allow air movement from within the cold air room vents and started to down the chimney, located in the basement.

  5. I have a masonry corner fireplace with ash dump in the center of the fire box floor. I would like to raise the floor of the firebox and convert the ash dump as the source of outside air. The dump entry is 7.75×4 inches or 31 sq inches. Will this be large enough? Do I need to push air from the outside into the box to obtain enough air flow? The firebox dimensions are wxhxd 35x23x22
    I plan on installing ceramic glass doors that are “air tight”.

  6. Thanks for all the stats but here’s the problem. I think you obviously agree you have an air balancing problem within the house otherwise you wouldn’t be seeking an outside air source. Most outdoor air sources are too small to start with, but much bigger you’re going to be sitting in the living room with a winter coat. This will definitely be a trial and error. P.S. I’m sorry, I’m laughing here as picture you and your family sitting by the warm fireplace with big winter coats around the fire! Burn safe and warm even if you have to wear your winter coat. Clay

  7. My wood fireplace had been converted to gas DoD it still need this vent open? Also when the fireplace pilot light is out in the winter would cold winter air come in and create moisture inside the walls?

  8. I recommend your getting an chimney inspection to assess this moisture situation.There are many variables that may be causing this problem such as cold spots in wall cavities, uninsulated ceiling areas surrounding the chimney as well as the humidity setting in your home. Additionally may I suggest your getting the manufacturer’s info off of the Mfg. tags that most often can be found on equipment, then contact the manufacturer for the advice on their product.
    You can use the sweeps finder on http://www.nscg.org
    Clay Lamb

  9. Hello i have a wood burning fireplace. The fireplace is in a closed room, tight windowsn and no additional air source excpet when i open a window. What do u suggest to as air supply for me not to have to deal with a smoking chinmey?

  10. Let me start this off. The whole air balancing is more of an art than science because they are so many competing sources for fresh air. If it were mine, I would start with the least expensive thing first. You can try going to your local brick yard and see what outside air source supplies they offer then determine where and how is the best way to install this. Increasing the height of the chimney (could possibly) be extremely helpful. Keep trying because I bet you’ll win!

    Burn Safe,
    Clay Lamb

  11. I am going to build an outdoor fireplace that will be placed on the outer edge of a gazebo. It will have a 42″ wide x 30″ tall opening into the fire box area. My chimney flue will be 12″ x 12″ inside x 10′ high above the damper. Do I need to design a fresh air vent opening into the firebox?

  12. No, fireplaces work in the home because of pressures from outside working up the chimney. Fresh air won’t be required here because you’ll have outside air all around, unless a rigid home inspector has reason for it. I’m not aware of any requirements for outside fireplaces other than keep combustibles away from it. Enjoy great evenings using that fireplace!

  13. Hi Clay. I recently removed the hearth from my brick wood burning fireplace. To my surprise there was a 4-inch corrugated plastic tube coming up out of the floor with the other end facing the fireplace wall. It is positioned to the far right and below the floor of fireplace and is angled towards an opening in the brick, however the opening has brick and cement in it. My research has resulted in thinking this tube is an air intake source but I can’t find the end that is in the floor. There is no outside vent on the house and it was within a brick hearth. Can I eliminate it?

  14. First of all there shouldn’t be any plastic in your fireplace at all. Sounds like a construction default and needs to be removed out of there. I’ve seen it where they put concrete around it to seal it out, but that shouldn’t be there. It should a continuous noncombustible conduit from the fireplace to the outside.

  15. Hi. I have a stainless vent on the outside of my chimney about 4 feet off the ground. I want to install a propane tank there to fuel a ventless log set in my fireplace. The chimney also exhausts 2 forced air boilers and a oil fired hot water tank. The propane company will not place the tank within 10 feet of that vent, but that’s exactly where I need it. Any clue what this vent may do? Thanks, Paul G

  16. I am going to defer to what your local LP provider is saying. This is a senior situation and I really believe they have your best interest and their liability exposure in mind. Clay.

  17. I used to have an open front wood burning fireplace that was installed in the 80’s, but just had an ironstrike wood stove insert put in. When it gets below zero outside I have moisture build up all over the unit if it’s not in use. What should I do? It does have two 4 inch vents on the outside was not sure if those could be closed up or not since there is so much cold air coming in when not in use.

  18. This is a little more complicated than our average customer question, I’m going to recommend you go to NCSG.org and find a recommended chimney sweep in your area that can help you out with it. Best, Clay.

  19. I have a masonry fireplace with an air vent that goes from the firebox to a brick-sized hole on the exterior of the fireplace; they never finished the vent with an exterior cover of any kind and bugs can get into the house via this air vent. What can I use to cover the opening on the exterior? I have a cutting wheel and can cut a grove into the bricks if necessary to fit a flat cover.

  20. You should be able to find a grill cover at your local brickyard that you may have to cut in, take a picture with you and take measurements.

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