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Chimney Doctor's Homeowner's Guide

Solving Drafting Problems For Fireplaces

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Written By: William B. Hussel, President, Chimney Doctors
November 28, 2001

Chimney Draft

The main function of a chimney is to create draft for combustion and to transport the flue gases and smoke out of the building. A good draft is vital for a good combustion. We consider a normally good draft to be between 10-20 Pa (1-2 mm VC). The chimney creates the draft, not the appliance.

A tall chimney gives more draft. If the draft is insufficient it can be a solution to build a taller chimney. The chimney diameter should never be less than the diameter on the appliance flue outlet. A circular chimney liner normally gives a better draft than a square. Use of flue pipe elbows reduces the draft.

Introduction

  • Solving drafting problems is usually a trial and error procedure. This report lists several procedures that can be used to improve and/or solve fireplace drafting problems. Most people initially try raising height of the chimney; however, this procedure usually does not work as described below. The most common problem is where the chimney flue is too small (as compared to the size of the fireplace opening). In many cases, 8” x 12” flue tiles were used by the mason constructing the fireplace instead of 13” x 13” flue tiles which are now the minimum size required by building codes.
  • In many of the other drafting problems, a series of trial and error steps are needed to determined which items help improve the drafting situation.
  • Generally the smoke does not move quickly up the flue in a straight line, but moves up in a turbulent/rolling manor very slowly. Some of the smoke (as it rolls towards the flue) makes it’s way past the opening of the fireplace and escapes into the room. Fireplaces that have this problem have smoke stains above the fireplace opening from the smoke escaping at the top edge of the fireplace opening.
  • Many drafting problems can be solved with very reasonable expense. Ultimately, if all the procedures listed below are tried and nothing solves the drafting problem, a fireplace fan placed at the top of the chimney is the “last resort”. These fans are generally installed for a cost of $2,000 to $3,000 and it is recommended that they have a rheostat switch placed near the fireplace so that the homeowner can regulate the volume of air moving up the chimney. Only in extreme cases does this fan need to be used.

Types of Problems

PROBLEM #1: CHIMNEY FLUE TOO SMALL

  • As mentioned in the introduction, this problem is the most common cause of smoky fireplaces. Refer to attachment 1, which is a chart showing the ratio of the fireplace opening to the flue size based on the height of the chimney. This chart can be used to specifically determine the exact minimum flue opening size for given fireplace opening with a given chimney height. The chart (attachment I) is a relatively new (more technical) way to calculate flue size.
  • Generally the “old” ratio was 10 to 1. This “10 to 1” ratio has been used over the last 50 years or so and is a simple/easy way to remember the proper ratio. This rule means that for every 10 square inches of fireplace opening, the flue size needs to be 1 square inch. The fireplace opening is the opening measured where air can enter the fireplace opening. For fireplaces with glass doors, the opening should be measured inside the metal frame, not the entire outside dimensions of the metal frame. If the glass doors have an air intake area, the measurements should include the air intake area. If a fireplace measures 36” wide x 30” high the total area opening is 1,080 square inches. Using the “old rule”, the flue size needs to be at least 108 square inches (1080/10).
  • In measuring nominal flue sizes, the dimension listed is the outside of the flue. For example, a 13”x 13” flue size means that the flue tile is 13” x 13” outside dimension. The wall thickness of flue tiles is approximately ¾ of an inch; therefore, the inside dimension of a 13” x 13” flue tile is actually 11 ½” x 11 ½”. When using the “old rule”, you can see that a flue tile with 11 ½” x 11” ½” inside dimension would be more than adequate to vent a fireplace opening with a 36” wide by 30” high opening.
  • If the flue opening is too small, the fireplace opening must be reduced, or the flue size must be increased, so that the proper ratio (“old rule” is 10 to 1) is maintained. An easy device to decrease the fireplace opening is to install a “smoke guard”. This device is sold by most fireplace stores and chimney companies. It is a 4” wide rectangular device, which is usually placed at the top of the fireplace opening. It is very important that a high temperature sealant be used to seal around the smoke guard so that no smoke can escape along the top or sides of the smoke guard. Two smoke guards can also be installed along the sides of the fireplace opening so that the total opening of the fireplace is not only reduced in the height dimension but it is also decreased in the width dimension. Fireplaces that have extremely wide openings (for example: 48” wide) normally need smoke guards installed on three sides in order to bring the flue to fireplace ratio within a reasonable number. If the homeowner objects to having individual smoke guard pieces installed in the fireplace opening, a one piece custom smoke guard can be manufactured from ¼’ steel plate and mounted in the fireplace permanently. Temporary smoke guards however can be used to test if the reduction in fireplace opening will solve the problem. Again, it is important that the smoke guards be completely sealed between the metal and the fireplace masonry to eliminate any smoke escaping in between the metal and masonry.
  • Another experiment that works well is to use a piece of sheet metal to temporarily position in the fireplace opening to determine how large the smoke guard needs to be in order to solve the drafting problem. In some extreme cases, the smoke guard needs to be as much as 8” wide along the top to bring the “ratio” into a reasonable number. The temporary piece of sheet metal should be held in the opening with some mechanical device, not duct tape. The metal heats up from the heat of the fire and transfers the heat to the duct tape which will quickly fail.
  • The reason that the ratio of the fireplace to flue size needs to be approximately 10 to 1 is that the velocity of the air moving into the fireplace opening needs to be fast enough to prevent the smoke from escaping from the fireplace opening. If a ratio of 10 to 1 is used, the air moves very quickly into the fireplace opening and quickly up the flue, and therefore eliminates any possibility of escaping smoke. The fast moving air also eliminates the rolling/turbulent smoke responsible for smoke escaping.
  • Another good solution in making the fireplace opening smaller is to install glass doors. Use a glass door model that has a reasonably wide frame on all 4 sides. Once the glass doors are installed and properly sealed to the masonry, the fireplace opening is reduced considerably. Another experiment that can be used to determine if glass doors will solve the problem is to cut the opening of the intended glass doors from a large piece of sheet metal. Cut the opening of the intended glass doors and place the large piece of sheet metal in front of the fireplace. Fiberglass insulation can be used to help prevent smoke from escaping around the back of the sheet metal. If the test piece solves the problem, glass doors can be purchased to permanently solve the problem.
  • Another typical problem that falls into this category (flue size too small) is with two sided (see through) fireplaces. These designs in effect have double the fireplace opening size as a conventional fireplace. It is therefore necessary that the flue size be at least two times larger than a conventional fireplace with one opening. Most masons do not understand the importance of using an extremely large flue tile for a two sided fireplace. A good solution to solve a chronic smoking problem with a two sided fireplace is to close off one of the two sides by either using a piece of refractory glass, or by using brick/stone to match the existing fireplace opening. No matter which method is used in closing off one of the openings, the opening must be completely sealed to eliminate any smoke escaping from the sealed side.

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PROBLEM #2: INTERNAL FANS/AIR-TIGHT HOUSE

  • When fireplaces have a large roaring fire, a tremendous amount of air is pulled up the chimney. This air must be replaced inside the building with outside air. The term “make-up” air is normally used since it describes the volume of air that must be made up inside the building to replace the air passing up the chimney. If this “make-up” air is not available in adequate quantities, a smoking problem can result. If the building has internal fans that exhaust air from the house, the problem is made worse. Examples are: Jenn Air™ exhaust fans, conventional kitchen exhaust fans, attic ventilation fans, radon exhaust systems and bathroom ventilation fans. This problem is very common with fireplaces located in restaurants because of the high volume exhaust fans required for the cooking areas drawing air out of the building, which competes with the fireplace. Usually this problem in restaurants is so significant that an exhaust fan for the fireplace is usually needed for these extreme conditions. Furnaces and hot water heaters also use air for combustion and use up internal air in competition with the fireplace. Newer furnaces and hot water heaters that are “high efficiency” models usually bring fresh air in from the outside of the house, and therefore do not compete with the fireplace.
  • Newer homes that utilize Tyvec™ house wrap and good air-tight windows also do not allow adequate make-up air to enter the house as the fireplace is being used.
  • Changes in the air-tightness of a house may cause a drafting problem with the fireplace. Painting the house or installing new air-tight windows are examples.
  • In order to determine if inadequate make-up air is the problem, open a window or door approximately 3” - 4” to allow air to enter the house as required by the fireplace. It is important that when the door or window is opened, air should be blowing into the house. Therefore, choose a window or door opening on the windward side of the house. If a door or window is opened on the opposite side of the house and air is drawn out of the house, the opened window or door actually makes the problem worse. If the problem is solved by opening a door or window, a fresh air intake needs to be installed in the back wall of the fireplace to allow adequate air for the fireplace on a regular basis. This fresh air intake device is actually a “valve” which can be opened or closed to the fireplace to allow adequate make-up air to enter directly into the firebox. This would obviously eliminate any cold air/drafts within the house in cold weather, which would occur when door and windows are open.
  • Another good fresh air intake design is to use a 6” galvanized pipe to ventilate the ash dump pit to outside air. The ash dump door is then opened to the fireplace and allows adequate air to directly enter the fireplace opening. It is a good idea to have a steel plate mounted 2” - 3” above the ash dump area so that ashes are not blown around by the incoming air. Never vent a fresh air intake to the garage, since combustible fumes may be pulled into the fireplace.
  • If the temperature outside is relatively warm or the air is damp and “heavy”, the fireplace will not draw as well as with colder temperatures.
  • Another creative idea is to hang (use duct tape) a piece of toilet tissue approximately 1-2 feet from the top opening of the fireplace. As different experiments are used (i.e. opening or closing windows, increasing chimney heights, etc.), the movement of the toilet tissue will indicate the strength of chimney draft. This technique can be used essentially with all trial and error procedures and the draft strength can be determined as these different procedures are tried. It’s usually best to have actual fire in the fireplace when performing this test. Be careful not to make the tissue too long which would cause the tissue to ignite.

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PROBLEM #3: CHIMNEY HEIGHT

As previously mentioned in the introduction, most homeowners will attempt to solve a fireplace drafting problem by raising the chimney height. It is a well known scientific fact that tall chimneys draw better than shorter chimneys. This fact is correct since hot air rises and the more hot air that is contained within the chimney the stronger the draft. If this technique is going to work, it usually works with shorter chimneys. The additional height added to the chimney is proportional to the increase in draft. For example, if a chimney height is increased by 2 feet and the chimney is 10 feet high, the effective improvement is 20%. On the other hand, the same 2 feet added to a chimney that is 30 feet high is only 7%. To experiment with increasing chimney height, simply use a temporary piece of metal extension inserted inside the flue of the chimney. If extremely cold temperatures are present, the temporary metal extension should be insulated. Here again, use the toilet paper technique to determine the strength of draft with the extension compared to not having the extension. If the temporary extension seems to solve the problem, the chimney can be permanently increased in height at a later date.

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PROBLEM #4: COLD CHIMNEYS

  • As previously mentioned, hot air rises. The hotter the air, the stronger the draft. If a chimney is located with a large portion of the flue area surrounded by cold air, the draft of the chimney is reduced because of the cold chimney. Even after a fire is burning for long periods of time, the outside chimney provides a permanent heat sink which continually draws the heat away from the rising warm air inside the chimney.
  • Solutions to this problem include insulating the flue tiles using Vermiculite insulation when the chimney is originally constructed. Another solution is to remove the flue tiles and install an insulated stainless steel chimney system which has poured insulation around a stainless steel tube. This design is extremely effective because all heat sinks are eliminated. The stainless steel metal rises in temperatures very quickly and keeps a high temperature because of the surrounding insulation medial. Properly installed insulated stainless steel lining systems usually always increase the draft of the fireplace. Depending on the internal dimensions of the chimney chase, it is also possible to increase the size of the flue when installing a new stainless steel insulated chimney liner. At least 1” of insulation needs to surround the stainless steel liner tube.

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PROBLEM #5: INCREASE THE LEVEL OF BURNING LOGS

Another easy, inexpensive procedure to try is to raise the log grate by using bricks, or installing short pieces of hollow pipe onto the legs of the grate. By decreasing the distance between the burning logs and the damper, the smoke has to now travel a shorter distance and is less likely to wander outside of the fireplace opening. Another procedure that can be used while using this procedure is to also block off the bottom of the fireplace opening by installing bricks across the opening. For example, if the smoking problem is resolved by increasing the height of the grate by 8 inches and by also installing 8 inches of bricks across the opening of the fireplace opening, the problem can be permanently solved by simply permanently installing a raised floor of the fireplace by 8 inches. This technique not only decreases the distance that the smoke has to travel, but also in effect decreases the size of the opening which increases the air velocity into the fireplace as described in section I.

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PROBLEM #6: INADEQUATE DAMPER OPENING

  • As describe in a previous section, the flue size opening must be within approximately 10% of the size of the fireplace opening. It is also important to have an adequate damper opening size to allow smoke to easily pass from the fireplace area into the flue area. If the existing damper doesn’t fully open, or if the design originally was too small, the damper opening must be increased to solve the smoking problem. Generally, the damper opening must be 90% of the flue size.
  • One solution is to completely remove the constricted area and to install a top sealing damper mounted to the top of the flue tile.

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PROBLEM #7: IMPROPER DEPTH OF FIREBOX

Attachment II shows the ideal dimensions of fireplace dimensions. In our experience, we have found many smoke drafting problems that are a result of the fireplace either being too shallow (more common) or fireboxes being too deep (less common). If the fireplace is too deep as referenced in the attached chart, simply construct a new back wall of the firebox by dry stacking refractory brick to test the effect of reducing the firebox depth.

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PROBLEM #8: FIREPLACE GRATE PLACEMENT

It is usually a good idea to place the grate and burning logs as far back in the firebox as possible if a poor drafting fireplace exists. This process increases the distance that the smoke must travel before it can escape from the fireplace opening.

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PROBLEM #9: SEASONED FIREWOOD

Some fireplace drafting problems can be cured by simply decreasing the amount of smoke going up the chimney. This can be accomplished by simply using extremely dry/fully seasoned firewood. This dryer wood produces less smoke than wet wood. If any moisture/sap boils out of the end of the logs, the wood is too wet.

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PROBLEM #10: OPENING GLASS DOORS

  • It is usually a good idea to slowly open glass doors to avoid creating a vacuum. This technique is also true with sensitive fireplaces when doors to the house or to the room are quickly opened to create a temporary vacuum in the room.
  • It is also a good idea to keep the glass doors tightly against one another when they are fully opened. If the doors are slightly closed, a “miniature chimney” effect can occur between the two (2) pieces of glass of the glass doors. This “miniature chimney” will allow smoke to be drawn from the fireplace to escape into the room.

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PROBLEM #11: DOWN DRAFTS

  • If nearby trees, buildings or roof surfaces are higher than the top of the fireplace chimney, down drafts can force air/smoke down the chimney and cause smoking problems inside. Usually this problem is the cause of internal smoking if smoke can be seen in a turbulent downward movement from the top of the chimney. You can also see smoke or have smoke smell at the ground level at the base of the chimney if this is the problem. On properly functioning chimneys, the smoke rises away from the chimney and is not blown downward from down drafts. Some trees grow very quickly and can therefore create a new problem as the new growth interferes with the draft.
  • Usually the most economical solution is to either install rain caps which divert the wind from being blown down the internal flue tiles, or by installing a “Vac-U-Stack”. The Vac-U-Stack is a round device with stationary vanes which produces a venturi effect thereby creating a vacuum on the inside of the chimney as the wind is blown against the vanes. If the wind is blown downward or horizontally across the Vac-U-Stack, a vacuum is created on the inside of the flue tiles which increases the chimney draft.

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PROBLEM #12 FIREPLACE EXHAUST FANS

As mentioned in the introduction, this is usually the “last resort” if all other mentioned procedures simply will not solve the problem. This device is a specially designed fan system mounted in an aluminum housing which mounts on the top flue tile. A licensed electrician is needed to run power through a rheostat switch (usually located near the fireplace) to the fan on top of the chimney. The rheostats switch can be used to control the speed (and noise) of the fan. The noise is similar to an exhaust fan for the kitchen.

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PROBLEM #13: FLUE OBSTRUCTIONS

Check to see if any of the following obstructions are blocking the flue: birds nests, squirrel nests, soot/creosote and deteriorated flue tiles or masonry.

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PROBLEM #14: SMOKE TRANSFERRING FROM ONE FLUE TO ANOTHER FLUE

  • This problem is evident when smoke travels up one flue but also transfers over and is drawn to a second adjacent flue. Unwanted smoke usually appears in the basement area as a fireplace is being used upstairs, or the smoke exits a second unused fireplace during the time the primary fireplace is being used.
  • This problem is brought about by the smoke either transferring at the top of the flue tiles or internally. To determine which problem is evident, the secondary flue tile needs to be completely sealed at the top using duct tape and plastic. If the primary fireplace is used and the problem doesn’t reappear with the use of several fires, the problem is with smoke transferring at the top. This problem is permanently solved by either increasing the height of the flue tile of the primary fireplace, or by installing airtight top sealing dampers.
  • If closing off the secondary flue at the top does not solve the smoke transfer problem, the problem is an internal smoke transfer problem. This problem can only be resolved by relining one of the flues with a continuous seamless stainless steel insulated lining system. Once this system is installed, it is not possible for the smoke to transfer from one flue tile system to the other.

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IF ADDITIONAL ANSWERS ARE NEEDED TO SOLVE YOUR FIREPLACE OR CHIMNEY PROBLEMS, CONTACT CHIMNEY DOCTORS DIRECTLY AT (414)-463-8984.

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