How Does a Chimney Work? Draft, Parts and Why It Smokes

Air expands and gets lighter as it heats. Inside a flue, the column of hot combustion gas weighs less than a column of outside air of the same height, so the outside air pushes in at the bottom and the hot gas rises out of the top. The taller and warmer the column, the stronger the pull.

That is also why draft is weakest at the start of a fire. A cold flue holds cold, heavy air that resists moving until it warms. Anything that cools the gas on its way up, such as an oversized flue, a flue on an outside wall or wet fuel, slows the draft and leaves more residue behind on the walls.

Masonry chimneys and factory-built metal chimneys follow the same logic even though they are built differently. The table covers the pieces of a typical masonry fireplace chimney, the kind found in many Tacoma houses.

Several of those parts are written into the 2021 Washington State Residential Code, which took effect statewide on March 15, 2024. A masonry chimney has to be lined, with clay flue tile, a listed lining system, a listed factory-built chimney or another approved material able to resist corrosion and cracking at temperatures up to 1,800 degrees Fahrenheit. Interior chimneys need a 2-inch air gap to combustible framing, and exterior ones 1 inch.

On the roof, the stack's top must clear its own roof opening by 3 feet or more, and stand 2 feet taller than any roof or wall closer than 10 feet, a rule that exists partly for draft: wind swirling around a roof ridge can press down on a short chimney. A chimney wider than 30 inches that does not sit on the ridge needs a cricket, a small peaked diverter behind it that sends water around the stack instead of letting it pool.

Tacoma adds its own requirements when an appliance goes in: stoves and fireplaces must be listed by an approved testing agency and meet state or EPA emission standards, and the city asks installers to confirm an existing masonry chimney is lined first.

Draft depends on the difference between the flue and the outdoors, so weather matters. On cold, still winter evenings, cool air settles near the ground and smoke has trouble dispersing; Washington's Department of Ecology identified exactly those evenings as the time when wood smoke used to push the Tacoma-Pierce County area over the federal fine-particle limit.

Wind has the opposite effect in a different way. WeatherSpark reports that Tacoma's wind blows mostly from the south from about September to June, with a windier stretch from mid-October to late April. Gusts across a short or poorly capped chimney can push smoke back down, which is why a fireplace may draw well one night and puff into the room the next.

Census estimates put the median Tacoma home at 1967, and almost two in three of its housing units predate 1980. Many of those chimneys were built for open fireplaces and have since had an insert, a gas appliance or a new furnace connected to them. Each change alters what the flue carries, and NFPA 211 and CSIA treat swapping in a dissimilar appliance as a reason for a Level II inspection before the new one is used.

If you are not sure what your chimney has inside it, an inspection answers that question. Ours is $99 plus tax, and that amount comes off the bill for any repair you then approve.