Chimney Safety by the Numbers
Every figure on this page is attributed to its publisher, and the one drawing has a full text version, so it reads the same on a phone, a screen reader or a printout.
By Mark Jason, owner of NWI Chimney Sweep · figures checked against their published sources
The parts of a chimney, top to bottom
A masonry chimney has seven working parts: cap, crown, flue liner, flashing, smoke chamber, damper and firebox. The drawing shows where each one sits.
Text version of the graphic. Everything the drawing shows is repeated here and in the sections that follow, so nothing depends on seeing the image.
| No. | Part | Where it is | What it does | How it usually fails |
|---|---|---|---|---|
| 1 | Cap | On top of the flue | Keeps rain, birds and animals out; mesh arrests sparks | Missing, rusted or blown off |
| 2 | Crown | Slab covering the top of the brickwork | Sheds water away from the flue and past the brick face | Cracks; no slope or overhang |
| 3 | Flue liner | Inside the stack, firebox to top | Contains heat and exhaust and carries them out | Cracked tile, open joints, creosote |
| 4 | Flashing | Where the chimney passes through the roof | Seals the joint between shingles and brick | Open counter-flashing joint; tar patches |
| 5 | Smoke chamber | Above the damper, below the flue | Funnels smoke from the wide firebox into the narrow flue | Rough stepped brick that collects deposit |
| 6 | Damper | Throat of the fireplace | Opens for a fire, closes to keep heated air in | Warped or rusted so it will not seal |
| 7 | Firebox | Where the fire burns | Holds the fire against firebrick | Eroded mortar joints, cracked firebrick |
The bar at the foot of the graphic shows the three creosote stages and the method for each: stage 1 dusty soot comes off with a brush, stage 2 hard flakes need rotary sweeping, and stage 3 shiny glaze needs chemical treatment plus rotary tools. The full table is further down this page.
Home heating fires in the United States
Home heating equipment caused an average of 48,530 U.S. house fires a year from 2014 to 2018, according to the National Fire Protection Association’s Home Heating Fires report (published 2021). These are national annual averages, not local figures.
The leading cause is a dirty flue
In the same NFPA report, failure to clean was the leading factor contributing to home heating fires, mainly creosote in the chimneys of solid-fuel appliances. A newer single-year estimate from the U.S. Fire Administration counts 32,200 home heating fires, 190 deaths and 625 injuries in 2021, and ranks heating as the second leading cause of home fires that year. The two agencies count differently, so the figures should not be read as a trend.
The three stages of creosote
Creosote builds in three stages, from dusty soot to hard flakes to a shiny glaze, and each stage needs a different cleaning method. The stage, not the calendar, decides how a flue has to be cleaned.
| Stage | Looks like | Forms when | How it comes off |
|---|---|---|---|
| 1 | Dull, dusty or flaky soot | Dry wood, hot fire, good draft | A standard brush on rods |
| 2 | Hard, crunchy black flakes | Restricted air or a flue that stays cool | Stiff brush or rotary power sweeping |
| 3 | Shiny, tar-like glaze | Wet wood, smouldering fires, oversized or cold exterior flue | Chemical treatment plus rotary tools; brushing alone will not move it |
Burn-season routine in six moves
A safe burning season comes down to six habits, each one backed by a named authority: NFPA 211, EPA Burn Wise, the U.S. Fire Administration or the CPSC.
Schedule the yearly inspection
NFPA 211 calls for chimneys, fireplaces and vents to be inspected annually for soundness, deposits and clearances.
Meter the firewood
EPA Burn Wise: wood burns best below 20 percent moisture after at least six months of seasoning.
Clear three feet
The U.S. Fire Administration advises keeping anything that can burn at least 3 feet from a fireplace, wood stove or heater.
Open the damper fully
A partly closed plate cools the flue gas and pushes smoke back toward the room.
Can the ashes
EPA Burn Wise: remove ashes into a covered metal container and store it outdoors on a non-flammable surface.
Test the alarms
The Consumer Product Safety Commission advises a CO alarm on every level and outside sleeping areas.
Carbon monoxide and wood smoke
More than 200 people in the United States die each year from accidental carbon monoxide poisoning linked to consumer products, according to the Consumer Product Safety Commission. Wood smoke figures below are from the Environmental Protection Agency’s Burn Wise program.
Blocked flue
A nest, fallen tile or collapsed liner sends exhaust back indoors. Gas appliances give little visible warning.
Oversized flue
A small appliance on a big cold flue drafts weakly, condenses moisture and can spill at start-up.
House under suction
Kitchen and bath fans or a dryer can pull a tight house negative and reverse a fireplace’s draft.
Dryer exhaust: the code limits
The International Residential Code, Section M1502, limits a clothes dryer exhaust duct to 35 feet by default, less a deduction for every elbow. Local amendments and the dryer manufacturer’s instructions can change the allowed length, so treat this as the default.
| Item | IRC M1502 default |
|---|---|
| Maximum duct length, dryer to outside hood | 35 feet |
| Deduct for each 90-degree elbow | 5 feet |
| Deduct for each 45-degree elbow | 2.5 feet |
| Duct | Smooth metal, 4-inch nominal diameter |
| Flexible transition hose behind the dryer | One piece, 8 feet at most, not run inside walls |
| Outside termination | Backdraft damper, no screen |
Doing the math
Worked example: a run with two 90-degree elbows and one 45 has 35 – 5 – 5 – 2.5 = 22.5 feet of straight duct to spend. Many laundry rooms moved to the middle of a house exceed that without anyone having done the arithmetic.
Where to go from here
Each panel of this chimney safety infographic maps to one service we provide. The yearly visit is a chimney inspection to NFPA 211, deposits come out with professional chimney sweeping, a failed or oversized flue is corrected by relining with a stainless steel liner, and the laundry numbers are checked during dryer vent cleaning. For the do-it-yourself side, follow the homeowner chimney maintenance guide.
About the numbers
How many house fires are caused by chimneys each year?
NFPA’s Home Heating Fires report for 2014 to 2018 estimated an average of 48,530 home heating equipment fires a year in the United States and found that fireplaces or chimneys were involved in roughly three in ten of them. Failure to clean was the leading contributing factor.
What month has the most heating fires?
According to the same NFPA report, nearly half of home heating equipment fires, 48 percent, occurred in December, January and February, with January the leading month. That is why a chimney inspection belongs in early fall, before the heating season starts.
What does NFPA 211 say about how often a chimney should be inspected?
NFPA 211, the Standard for Chimneys, Fireplaces, Vents, and Solid Fuel-Burning Appliances, calls for an inspection at least once a year to confirm the system is sound, free of deposits and has correct clearances. NWI Chimney Sweep works to NFPA 211, which is a published rulebook and not something a company can be awarded.
How thick does creosote have to be before it is dangerous?
There is no single official thickness, but a rule of thumb widely used in the chimney trade is to sweep a masonry fireplace flue once sooty build-up reaches about 1/8 inch, and sooner if any glazed creosote is present, because that much creosote is enough to fuel a chimney fire that can damage the liner or spread to the house.
Why does the moisture content of firewood matter so much?
Energy that should heat the flue is spent boiling water out of the log. The smoke leaves cooler and wetter, and it condenses on the liner as creosote. EPA Burn Wise sets the target at less than 20 percent moisture.
How long can a dryer vent be under the building code?
The International Residential Code, Section M1502, sets a default maximum of 35 feet from the dryer to the outside hood, reduced by 5 feet for every 90-degree elbow and 2.5 feet for every 45-degree elbow, unless the dryer manufacturer’s instructions permit more.
Why are screens not allowed on a dryer vent hood?
Lint mats against a screen within weeks and chokes the airflow, which overheats the dryer and lengthens every cycle. The code calls for a backdraft damper instead, which keeps pests and cold air out while letting lint pass. If your dryer vent hood has a screen or cage fitted, it should be removed.
Are these statistics specific to Northwest Indiana?
No. They are national figures from NFPA, the U.S. Fire Administration, the CPSC and the EPA. We do not publish chimney fire statistics for Lake County or Porter County because we have no verified source for them, and we will not estimate.
One number to act on: the yearly inspection.
If it has been more than twelve months, call and we will get the flue looked at before you light it.
Call (219) 203-9797