Chimney Downdraft: Causes and Fixes

You light a fire and instead of rising, the smoke rolls out into the room. Or the house smells like an ashtray on windy days even when the fireplace is cold. That is chimney downdraft: air moving down the flue instead of up.

The fix depends on the cause, and there are three main ones.

The Three Causes of Downdraft

Wind Downdraft

Wind blowing across the top of a chimney should help draft by pulling smoke out. But wind hitting a higher roof, a tree or a nearby building can swirl down onto the chimney top and push smoke back in.

Signs

• Puffs of smoke on gusty days.
• Worse when wind comes from one direction.
• The chimney sits lower than nearby roof ridges, trees or buildings.

Fixes

• Wind directional cap: a rotating or vaned cap that turns so its opening faces away from the wind.
• Draft inducing cap: a fixed design that uses wind from any direction to pull smoke out.
• Taller chimney: extending the flue so it clears nearby obstacles. Model codes set a minimum of 3 feet above the roof and 2 feet above anything within 10 feet, often called the 3-2-10 rule. Chimneys that meet the minimum can still suffer in windy spots.
• Trim trees that overhang the chimney.

Pressure Downdraft

Modern homes are tight. Exhaust fans, clothes dryers, range hoods and furnaces all pull air out of the house. If not enough air can get back in, the house goes into negative pressure, and the chimney becomes the easiest path in.

Signs

• Smoke spills when the range hood or bathroom fan runs.
• Cold air or a smoky smell comes from the fireplace when it is off.
• Opening a nearby window fixes it.
• The fireplace is in a basement.

Fixes

• Crack a window near the fireplace when lighting a fire.
• Turn off exhaust fans while starting the fire.
• Makeup air: an outside air kit for the stove or fireplace, or a makeup air system for big range hoods. Ask an HVAC contractor.
• Seal the chimney when not in use with a closed damper or a top mount damper cap.

A cap alone will not fix pressure downdraft.

Cold or Short Flue Downdraft

Draft depends on warm air rising. A cold chimney, especially one on an outside wall, holds a column of cold air that has to be pushed out before smoke can rise. A flue that is too short or too big for the fireplace may never draft well.

Signs

• Smoke at the start of every fire that clears once the fire is going.
• Exterior chimney in a cold climate.
• A large old flue serving a small stove insert.

Fixes

• Prime the flue: hold a rolled up burning newspaper near the damper for a minute before lighting.
• Start fires hot with plenty of kindling.
• Liner: a correctly sized insulated liner warms faster and drafts better.
• Extend the chimney if it is too short.

Can a Chimney Cap Cause Downdraft?

A correctly sized cap rarely causes downdraft. But a cap can make things worse when it is:

• Too small, so the outlet is choked.
• Set too low over the flue.
• Clogged with creosote or leaves.
• A rotating cap stuck facing the wrong way.

If downdraft started right after a cap went on, check the cap first.

Do Wind Caps Really Work?

Wind directional and draft inducing caps do help with true wind downdraft. They do nothing for pressure problems or cold flues. That is why diagnosing the cause first saves money. If smoke spills only on windy days, a wind cap is a good bet.

Cold Air Coming Down the Chimney

Cold air sinking down a chimney when the fireplace is off is a form of downdraft too. It wastes heat and brings in smells.

• Close the throat damper when the fireplace is cold.
• If the damper is warped or missing, a top mount damper cap seals the flue at the top with a gasket.
• Glass doors on the fireplace reduce drafts into the room.

Smells from the Chimney

When air comes down a chimney, it carries the smell of creosote with it. The smell is worse in summer humidity and after rain. Sweep the chimney, cap it to keep rain out, and fix the pressure problem.

Safety

Downdraft on a gas, oil or wood appliance can pull carbon monoxide into the home. Make sure you have working carbon monoxide alarms on every floor. If an alarm sounds, get out and call 911 or the gas company. Do not keep using an appliance that spills smoke.

When to Call a Professional

Call a certified chimney sweep if:

• You cannot tell which cause applies.
• Downdraft continues after simple fixes.
• You suspect the flue is the wrong size or damaged.
• The appliance is gas or oil and spills combustion gases.

A sweep can measure draft, inspect the flue and recommend a cap, liner or height change.

A Simple Downdraft Test

Before buying anything, run a quick test to narrow the cause.

1. Cold check: with the fireplace off, hold a lit incense stick or match smoke at the open damper. If smoke is pulled into the room, air is coming down the chimney.
2. Window test: open a window near the fireplace and repeat. If the smoke now rises, house pressure is a factor.
3. Fan test: close the window, turn on the range hood, bathroom fans and dryer, and repeat. If the problem gets much worse, pressure is the main cause.
4. Wind check: note wind speed and direction on days with problems. If it only happens in wind from one side, wind is the main cause.
5. Warm up test: light a small hot fire with lots of kindling. If smoke spills at first and then clears, the flue is cold.

Write down what you find. It will help a sweep or HVAC contractor diagnose the problem.

Stack Effect in Simple Terms

Warm air in a house rises and leaks out through the upper floors and attic. That pulls air in at the lower levels. A fireplace on the lowest level, such as a basement, may become an air inlet rather than an outlet, so air flows down the chimney.

The taller and leakier the house, the stronger this effect. It is one reason basement fireplaces often struggle with draft.

Two Flues on One Chimney

When two flues sit side by side, smoke from one can be pulled down the other. This often happens when a furnace or second fireplace runs while the first fireplace is cold.

Fixes include:

• Raising one flue a few inches above the other so smoke does not drift across.
• A multi flue cap with a divider.
• Closing the damper on the unused flue.

Wind Caps Compared

Rotating caps have moving parts that need occasional cleaning and lubrication. Fixed caps have no moving parts. Powered inducers need wiring and are usually installed by professionals.

Why Downdraft Gets Worse in Certain Seasons

• Spring and fall: outside temperatures close to inside temperatures mean weak natural draft.
• Summer: a cold fireplace and hot outside air can reverse the flow, bringing creosote smells into the house.
• Winter: strong draft when the fire is hot, but a cold flue at start up.

Downdraft and Gas Appliances

Gas furnaces and water heaters that share a chimney with a fireplace can be affected by downdraft. If combustion gases spill back, carbon monoxide can build up. Signs include soot around the appliance draft hood, condensation on windows, and headaches or nausea in the home. Have a qualified technician run a draft and spillage test if you suspect a problem.

Costs of Common Fixes

• Opening a window: free.
• Wind cap: usually more than a standard cap, plus installation.
• Outside air kit: depends on the appliance and access.
• Chimney extension or liner: a larger project; get quotes from a sweep.

Start with the cheapest tests and fixes, and move up only when the cause is clear.

The Bottom Line

Chimney downdraft comes from wind, house pressure or a cold flue. Watch when it happens to find the cause. Wind caps help with wind, makeup air helps with pressure, and liners, height and hot starts help with cold flues. Never ignore smoke or carbon monoxide in the house.