How Does a Chimney Work?

Draft: hot gas rising through a sealed tube

Exhaust from a fire weighs less than the cooler air around the house, so it floats upward the way a hot-air balloon does. The flue turns that lift into a steady current: a closed vertical passage holding a column of warm gas, with heavier outdoor air pressing in at its base. Technicians call the result draft. It is a small pressure difference, yet it is enough to push smoke up and draw fresh combustion air into the fire behind it.

Two things set its strength. One is height, since a taller column of warm gas has more lift than a short one. The other is the temperature gap between the flue gas and the outdoors: the hotter the gas compared with the air outside, the harder it rises. That is why a chimney pulls hard on a cold night with a well-established fire, and feels sluggish on a mild, damp afternoon or in the first minute after a match is struck.

The stack effect: the house behaves like a chimney too

Buoyancy works on the building as well. In heating season the air indoors is warmer than outside, so the whole house acts like a short, wide chimney. Warm air presses out through gaps near the upper floors and attic, and cold air leaks in low, around basement windows, sill plates and door thresholds. Somewhere in between sits a level, called the neutral pressure plane, where the house is neither pushing air out nor pulling it in.

Where a fireplace sits relative to that level matters. Below it, rooms run at a slight negative pressure compared with outdoors, so the house competes with the chimney for air. In a tall brownstone or a three-story rowhouse with a garden-level or parlor-floor fireplace, the lower floors pull steadily, and a cold, unused flue can become an air intake that brings outside air, and whatever smell lives in the flue, down into the room. Bath fans, a dryer and a range hood add to that pull, because every bit of air they blow out has to be replaced from somewhere.

In summer the effect reverses and weakens. Air-conditioned indoor air is cooler and heavier than outdoor air, so it tends to sink and escape low while outdoor air enters higher up. A chimney in that season can run backward, with cool air spilling out of the firebox. Most smoke and odor complaints from a fireplace that is not lit come down to this contest between the house and the flue.

The parts, from firebox to cap

Each piece of a masonry fireplace chimney has a job in keeping the current moving in one direction.

• Firebox: the chamber lined with firebrick and refractory mortar, angled to throw heat forward and guide smoke back and up
• Throat and damper: the narrow slot above the fire, and the metal plate that opens it for a fire and closes it when the fireplace is cold
• Smoke shelf: the ledge behind the damper that turns back cold air dropping down the flue
• Smoke chamber: the sloped, funnel-shaped space that gathers smoke from the wide throat into the narrower flue
• Flue liner: clay tile, cast refractory or stainless steel that keeps heat, smoke and moisture inside the passage
• Crown and cap: the sloped slab that sheds rain off the masonry, and the cover that keeps out rain, animals and sparks

Why flue size has to match what burns below

Draft depends on the gas staying warm all the way up, and the size of the flue controls that. An oversized flue spreads a modest volume of smoke across a wide cross section, so it moves slowly, gives up its heat to the masonry and loses lift. An undersized flue cannot carry what a large fireplace opening produces, and the surplus rolls out the front.

For an open fireplace, flue area is sized against the area of the fireplace opening, which is why a wide, tall opening served by an ordinary flue smokes no matter how dry the wood is. For a stove, insert, boiler or water heater, the appliance maker's installation instructions set the flue size, and a stainless liner is often run down an old masonry flue to bring it down to that size.

Boilers and water heaters run on the same physics

In many older homes across New York, New Jersey and the Philadelphia area, the chimney's working job is venting an oil or gas boiler and a water heater rather than a fireplace. Exhaust enters through a connector pipe and a thimble in the chimney wall, then rises on its own heat. Gas appliances usually carry a draft hood and oil burners a barometric damper; both let room air mix with the exhaust so the draft holds steady when wind gusts or the weather swings.

Gas and oil exhaust carries a lot of water vapor. If it cools too much on the way up, that vapor condenses on the flue walls together with combustion acids that eat at mortar joints and clay tile. One trap shows up often: the old boiler is replaced with a high-efficiency model vented out a side wall, and the water heater is left alone in a flue sized for both. Its small, cool exhaust stalls in the oversized passage, condenses and can spill at the draft hood. Relining the flue to suit the remaining appliance solves it.

Forces that push back against the draft

Outside conditions can overpower a chimney. Wind sweeping over a taller neighboring building, a higher roof ridge or a big tree creates eddies that bear down on the outlet, and a stack too short for its roofline sits right in that turbulence. Inside, a tight house with strong exhaust fans can simply out-pull a cold flue.

Startup is the weak moment for any chimney. A flue full of cold, heavy air resists the first smoke until enough heat goes up to flip the column. Opening the damper early, warming the flue with a little burning paper held near the throat, and cracking a window on the fireplace floor all help the draft establish itself before the fire grows.

Keeping the path clear and sealed

Draft only does its job through an open, tight passage. Creosote narrows the flue and becomes fuel for a chimney fire. A fallen tile or a nest can block it outright. Cracks let heat and exhaust leak sideways into the masonry and the framing around it. NFPA 211 expects chimneys, fireplaces and vents to get an inspection every year, with cleaning whenever deposits have built up, so changes like these are found while they are still small.

Some symptoms tell you the system is not drawing as designed: smoke in the room, soot on the face of the brick above the opening, an exhaust smell near the boiler, rust or white crust on the connector pipe, and water in the firebox. Each one points to a part of the path that has stopped doing its job.