Picture this: it’s a cold winter’s evening, so you decide to build a fire in your fireplace and snuggle up under a blanket, only to find that your whole house is filling up with smoke. Not a very appealing picture, and certainly not what you expect from your fireplace.
A fireplace that fills your house with smoke is not only a nuisance, but can be a safety hazard as well. Below are many common reasons you could be getting smoke in your house, how to identify the cause for your issue and how to fix it.
The most likely reason that your house is filling with smoke when you light a fire is that the damper is closed. The damper should be fully opened when the fireplace is in use, and closed when it is not.
All indoor fireplaces have a damper that must be opened before you light a fire in your fireplace. A closed damper blocks off the chimney to keep rain and debris out of your house and to keep conditioned air (i.e., heated or cooled) in. When the damper is closed while a fire is burning, the smoke is blocked from exiting through the chimney and has nowhere to go but back into your house.
Your damper will be located in one of two locations – in the throat of the fireplace or at the top of the chimney. The operation of the damper depends on what type you have.
The majority of dampers are cast iron throat dampers. This type of damper is a metal door located right above the firebox at the base of the smoke chamber. You can easily check whether it is open or closed by looking up into the chimney with a flash light.
Cast iron throat dampers are opened and closed using a handle that is attached to the damper door. To open the damper simply lift the handle, push it all the way forward, and pull back to latch into place.
If you forgot to open the damper before lighting a fire, use caution when opening it, as the hot metal damper can cause severe burns. We recommend using a fireplace tool to safely open the damper while a fire is going in the fireplace.
The other type of damper you could have is a top seal damper. These dampers are located at the top of your flue and completely close of the chimney. They are opened and closed using a stainless steel cable or chain that runs down the chimney to a handle located inside the fireplace.
If your damper is open and you’re still getting smoke in your house, it’s possible that you have a chimney blockage. These can take a couple different forms.
If you have a wood burning fireplace, one of the most common and most dangerous chimney blockages is creosote. When wood is burned slowly, it produces tar and other organic vapors that combine with expelled moisture to form creosote. The creosote vapors condense during cooling while combustion by-products go up the chimney. This condensed creosote can then accumulate on the flue lining.
We recommend having your fireplace and chimney professionally inspected once a year (prior to the heating season) to determine if creosote build-up has occurred. If a significant layer of creosote has accumulated (1/8” or more) it should be removed to reduce the risk of chimney fire.
Critters, such as squirrels, birds, and bats, have been known to make their homes in chimneys. Their nests and debris can build up over time, causing blockage issues. Even spider webs in flues that haven’t been used for a long time can redirect air flow.
If all you have is animal nests and debris, contact a professional sweep, who can safely and effectively remove the blockage. If you have live animals causing a blockage, do not light a fire until you have had an exterminator remove them.
Many fireplaces are topped off with either a chimney cap or a chimney pot. These accessories are both decorative and practical, as they help keep rain and debris out of your chimney.
A chimney pot or chimney cap that is too small can cause smoke to reenter your house. The base of the chimney pot or cap often fits over the top of the flue, but should never be smaller than the flue. The top of the chimney pot shouldn’t be much narrower than the flue itself – “choking down” the diameter can cause smoke draw issues.
In general, tops and caps should have a combined opening that is four times the cross-sectional area of the flue itself. If your chimney pot or cap is too small, consider replacing it with a large unit to alleviate some of your smoke issues.
A well-functioning house will have close to neutral indoor air pressure and will work to keep the pressure natural. When a fire is burning in a fireplace, the fire is pulling a tremendous amount of air up the chimney. In order for the neutral pressure of the house to be maintained, the air that is escaping out of the chimney must be replaced with new “makeup” air.
Often times, when smoke is blowing back into your house, it means there is a downdraft and the room is too negatively pressurized. In other words, the easiest way for the makeup air to replace the air being lost or exhausted somewhere else in the house is down the chimney.
An easy way to test if pressure differential is causing the smoke draw issues is by holding a burning stick of incense in the throat of the fireplace. If the smoke goes up the chimney, pressure differential is not your culprit and you can move on.
However, if the smoke from your incense blows down and back into the room, that means your chimney has a downdraft. To solve this problem, you’ll need to find a way to neutralize the downdraft. You can do this by opening a window or door low in the house, turning off a fan or your furnace or closing upstairs windows. Try these things one at a time (while still using your incense to test) and give the air enough time (2-3 minutes) to reverse itself. See what it takes to control the indoor air pressure so that there is no downdraft in the chimney and the smoke begins to draw upward.
Please note, every home is different so what works for one may not work for all. A good rule of thumb is to think about reducing the air escaping high in the house and increasing the air coming into the house on the lower floors. Note, it may make a difference which window you open or close, especially in a breeze – when you open a window, check to ensure the outside air is actually flowing in.
Once you’ve gotten your pressure under control, you’ll want to test to make sure you have adequate airflow for the fire.
Modern houses tend to be very tight (think spray foam insulation, house wrap, weather-sealed windows, etc.) and much more energy efficient than homes built prior to the 1970s. Because these homes are so tight, it is difficult for ventilation and combustion air to easily enter the home, meaning that there is not enough makeup air in the house for the fireplace to draw properly.
To test whether airflow is causing your smoke issues, you’ll need to simulate the air loss caused by a fire – this can be done by turning on a kitchen fan to create a downdraft. Once you’ve got the fan turned on, use your lit incense again to see where the smoke is going.
Then add ventilation air (opening a downstairs window that is blowing air into house, etc. as described above) until the cold chimney drafts up, even with the kitchen fan still on. Going through this process will help you determine how much air is needed for the fireplace.
While the quantity of combustion air required varies based on the type of fireplace, type of chimney and size of the fire that is burning, a general rule of thumb is 1 cubic foot per minute of make-up air per 1 square inch of flue area
Obviously, opening a window every time you want to burn a fire isn’t an ideal long term solution for every homeowner. For a permanent fix, you can install one of the following:
Because your fireplace’s flue runs up through the roofline and outside the home, external weather conditions can sometimes cause smoke to fill your house. Two of the most common weather culprits are extreme cold and cross drafting.
In very cold temperatures, an unused flue can fill up with high-density cold air that itself blocks the flow of smoke up the chimney. This is especially true if your chimney is located on the outside of the house. In these cases, take steps to "warm" your flue by building a small fire of newspaper at the back of the firebox. Continue to feed more newspaper until the smoke from the fire disappears up the flue.
*Note: A masonry chimney will take considerably more time to "warm" than a metal chimney.
Wind can cause smoke issues a couple of different ways. The first way is known as Dynamic Wind Loading. This is caused when wind blows on one side of the house, causing a high-pressure zone on that side of the house and creating a corresponding low-pressure zone on the other side of the house. This causes a pressure differential, which can be solved by opening a window on the windward side of the house. For a more permanent fix, you’ll want to tighten up the leeward side of the house (negative pressure side) and install an outside air kit
Wind can also cause turbulence at the top of a chimney, not allowing the smoke to escape, or can even blow the smoke back down the chimney into the fireplace. This is especially common if there are tall roofs or trees nearby. You can fix this by replacing your rain cap with a draft inducing cap.
The stack effect describes the movement of air into and out of the building, chimneys, stack, etc. In most homes, there is typically a neutral pressure level about halfway between the ground floor and the roof, meaning everything above that level is positively pressurized (air will leak out) and everything below is negatively pressurized (air will leak in).
In older houses and oftentimes tall houses, the negative pressure rarely exceeds 8 Pascals, which is about the difference in pressure in 10ft. of altitude. Furnaces and fireplaces usually perform well pulling against a negative pressure of up to 8 Pascals.
Modern houses, however, tend to be more tightly built using spray foam insulation, caulked windows and sealed doorways. They also tend to be full of powerful kitchen and bathroom exhaust fans, so the neutral pressure level may be higher in the house (if it exists at all). This means the negative pressure in the lower areas of the house can far exceed 8 Pascals, causing a downdraft in a chimney.
You can help reduce stack effect in your home by stopping air leaks in the uppermost parts of your house, including attic access hatches, ceiling light fixtures, and poorly fitting windows. However, you can’t really change the kitchen and bathroom exhaust fans that blow air out of the house. In this case, the stack effect can only be corrected by adding as much air to the house as is being blown out of it, i.e. opening a window or adding an outside air kit.
If you’ve followed all the troubleshooting tips above and nothing has stopped smoke from blowing back into your house, it is possible that your fireplace was constructed incorrectly.
When a house is taller than the chimney, the house has a “taller effective stack” and can act as a better chimney than the chimney itself, pulling air in through the flue and pushing it out through cracks/leaks higher in the house. Chimney height can also create a downdraft situation if the chimney does not extend high enough into air with a low-enough density to create an ambient updraft. In general, a taller chimney will typically draw better.
If your fireplace smokes because your chimney is too short, the problem is likely worse when the wind blows. Not only is a too short chimney unlikely to vent properly, it may also be a serious fire hazard, risking setting the roof on fire.
While not universally true, we recommend following the 2-10-3 rule – a chimney must be 2ft. taller than any structure within 10 ft. and stand 3 ft. taller than the point at which it penetrates the roofline. That said, if you are constructing a chimney, we always recommend consulting a professional as well as checking local building code.
A throat or smoke chamber that is too tight for the size firebox will create smoke draw issues. The throat front and back should fit well at the top of the firebox and not restrict the opening air space. If you have a throat damper, ensure it does not restrict the throat opening either. Check to see if firebrick or other material has been added to the back of the throat, which would reduce the throat opening area and should be removed. Feel around for smooth, streamlined surfaces with no obstructions that would restrict or redirect air flow.
For a fireplace to draw well, you need to be sure the flue size is adequate for the amount of smoke that can be created in the firebox. A good rule of thumb is that the fireplace opening be no more than 10-12x the area of the flue. A smaller flue simply can't process the amount of smoke, causing some to spill back into your home. For metal chimney systems, we recommend at least a 14” flue for 36” and larger boxes.
Flue size should be matched to firebox size in accordance with Section R1003.15.1 or R1003.15.2 (IRC 2015).
However, there are some exceptions to this rule
And remember, bigger is not always better. Your flue should be the appropriate size for your fireplace. If a flue is too large, the amount of heat produced by a small fire might not be enough to drive the draft upward.
A house that has multiple flues sharing one chimney chase may transfer smoke to one another when one is burning and the other is not, causing smoke to reenter the home.
When you light a fire in one fireplace, you are creating a negative pressure inside the house. If the house is fairly airtight then the easiest way for the fire to get air is through the open flue of the other fireplace –in other words the smoke goes up the chimney and gets sucked back down the other flue along with the necessary fresh air.
To fix this, you can extend the height of the flue that is causing the smoking to make one flue higher than the other. This allows the smoke to escape without getting pulled back down with the fresh air. A minimum of an 8-12” height differential is recommended. If adding height to the chimney is not an option, you can also consider adding an outside air kit (also called a fresh air kit) to the fireplace that is causing the smoking.
Smoke spillage can be caused by a variety of different factors, some of which are easier to fix than others. The most common causes of smoke entering the home are:
However, this list is not exhaustive of all possible scenarios, nor can it be – other potential influences include:
If you have a fireplace that is filling your house with smoke and you can’t determine the cause, or you don’t feel comfortable doing it yourself, seek the help of a licensed professional who can help pinpoint the issue and offer viable, long-term solutions.
