Why Is My Chimney Smoking So Much?

What the smoke color tells you

Step outside and look at the top of the chimney while the fire burns. A fire burning well in a warm flue gives off very little you can see, maybe a thin, wispy haze that disappears within a few feet of the cap. Lots of smoke means some of the wood is going up the chimney unburned, as gases and fine particles that should have been flame.

Color helps narrow it down. Keep in mind that a fire you just lit, or one you just loaded, smokes more than one that has been burning a while, and that cold, damp air makes any plume look thicker.

• Thin, wispy, nearly clear: a clean fire in a warm chimney
• Thick white or gray that billows: wet or unseasoned wood
• Dark gray or black: a smoldering fire starved of air, or plastics and trash
• White plume from a boiler chimney on a cold day: mostly water vapor
• Black puffs from an oil boiler chimney: the burner needs service

Causes, most common first

In the homes we visit, the reasons behind a heavily smoking chimney fall into a fairly steady order. Fuel comes first, then how the fire is built and fed, then the chimney itself.

• Wet or green wood: hissing logs, thick white smoke; burn split wood dried under cover
• Fire starved of air: lazy flames, dark smoke; damper fully open, smaller loads
• Oversized logs or deep ash bed: smolders instead of flaming; split smaller, clear excess ash
• Trash, cardboard, treated wood: dark, smelly smoke; burn only clean, dry firewood
• Cold or oversized flue: smoky starts, slow draft; warm the flue; liner if oversized

Wet wood is the usual culprit

Freshly cut wood is full of water. Before it can burn, the fire has to boil that water off, which cools the fire and produces a lot of thick, white smoke. Seasoned wood has been split and left to dry, under cover, until most of that water is gone, usually over at least a warm season.

Dry wood is lighter than wet wood of the same size, has cracks running across its ends, and makes a hollow knock when two pieces are struck together. Wet wood hisses and bubbles at the ends as it heats. A simple moisture meter, pushed into a freshly split face, takes the guessing out of it. Wood that was dry when bought but stored uncovered on the ground can take on water again.

A fire that is choking

A fire needs air to burn clean. Closing the damper partway to make a fire last longer, packing the firebox with big logs, or burning on a deep bed of ash all slow the burn into a smolder. On a wood stove, turning the air control right down after loading does the same. Smoldering fires make the most smoke and the most creosote.

Burn smaller split pieces, leave room for air between them, keep the damper fully open and clear out excess ash, leaving a thin bed. On a stove, let the fire burn hot for a while after each reload before turning the air down. Do not burn trash, cardboard, painted or treated wood, or wrapping paper. They smoke heavily and leave deposits that are hard to remove.

When the chimney is part of the problem

Sometimes the fuel is fine and the flue is not. A chimney on an outside wall stays cold, so the smoke cools before it rises and the fire struggles to draw. A flue that is much larger than a stove or insert needs lets smoke slow and linger. A partial blockage from creosote, a nest or fallen tile narrows the path. Each of these makes even good wood smoke more.

A chimney that is too short for the roof, or overshadowed by a taller building next door, can also be pushed around by wind, which leaves smoke hanging around the top and sometimes pushes it back down.

Why a smoky chimney needs sweeping

Smoke that cools on its way up the flue condenses as creosote. A season of smoky fires can coat the flue with a sticky or glazed layer that is much harder to clean than dry soot and burns very hot if it catches fire. Heavy buildup also narrows the flue, which slows the draft and makes the next fire smokier still.

If you have been burning wet wood or seeing a lot of smoke, have the chimney swept and checked before you keep using it. NFPA 211 calls for cleaning when deposits build up, and smoky fires build them up fast.

What a technician checks

When a chimney smokes heavily despite dry wood and good habits, the inspection looks at the draft path from the firebox to the top. That means the damper and smoke chamber, the size of the flue compared with the fireplace opening or the stove outlet, deposits on the flue walls, blockages, the cap and its screen, and the height of the chimney relative to the roof and nearby buildings.

On stoves and inserts, the liner and connector are checked for size and for heavy creosote near the appliance, where cool smoke collects first. The visit should end with a clear cause, not just a cleaning, so the next fire burns better.

Boiler and furnace chimneys

Many homes have a gas or oil boiler venting into the chimney. On cold mornings, a gas boiler's exhaust makes a white cloud that looks like smoke but is mostly water vapor condensing in the cold air; it vanishes quickly and leaves no smell or soot. That is normal. Dark smoke or soot from an oil boiler chimney is not. It means the burner is not burning cleanly and the flue may be collecting soot, so both the burner and the chimney need service.

Smoke in the street and the neighbors

On close-packed blocks, dense smoke drifts into nearby windows and yards, and some towns have rules about nuisance smoke and what may be burned. Dry wood and a hot, well-aired fire cut visible smoke down a great deal. If neighbors complain about smoke even when you burn well, the chimney height and draft are worth checking.

A quick look from the street a few minutes after each reload is the easiest way to judge your own burning.