Chimney Smoke Test: What It Shows and When You Need One

What a smoke test is

In a smoke test, the flue being tested is closed at the top, usually with a cover or an inflatable plug, and a source of dense test smoke is set at the bottom, typically a smoke pellet or cartridge made for the purpose. The smoke fills the flue under slight pressure. Then the technician and an assistant watch everywhere it might come out: the chimney breast in each room, the attic around the chimney, closets, the basement, the outside brick, and any other flue in the same stack.

A tight flue holds the smoke and lets it out only where it is supposed to go when the top is uncovered. A cracked or broken flue lets it leak through joints and gaps, which is exactly where exhaust would leak during normal use.

Smoke test, draft test and inspection

These three are often confused, and each answers a different question.

A chimney can pass one and fail another. A flue with a cracked tile halfway up may still draft strongly, so the draft test looks fine while exhaust seeps through the crack into a wall cavity. A perfectly tight flue can still draft poorly because it is too short or too large. That is why the right check depends on the symptom.

When a smoke test is worth doing

Not every chimney needs one. It earns its place when there is a reason to suspect the flue is not tight, or when leaks would be hard to see any other way.

• After a chimney fire, to check whether heat cracked the flue
• When smoke or exhaust smell shows up in rooms away from the fireplace
• In rowhouses, twins and brownstones with flues shared or next to a neighbor's
• Before deciding whether an old clay-tile flue can be reused for a new appliance
• When a boiler or water heater flue runs through finished spaces with no visible access
• Buying a house with an unlined or poorly documented chimney

Attached houses and shared stacks

In New York brownstones, Philadelphia rowhouses and New Jersey twins, chimneys often sit in party walls with several flues side by side, some belonging to the house next door. Old mortar between those flues, called the wythe or withe, can deteriorate until smoke and exhaust pass from one flue into another. Neighbors then smell each other's fireplaces, or a boiler's exhaust drifts into the flue serving an unused fireplace next door.

A smoke test is one of the clearer ways to show that. It also means coordinating: neighbors should be told before testing so smoke appearing in their home does not alarm them, and their own appliances on adjacent flues may need to be off.

How the test is run

The fireplace and any appliance on the flue are turned off and cold. Smoke and CO alarms near the test area may be noted in advance, since test smoke can set off smoke alarms. The top of the flue is sealed, the smoke source is lit at the base, and the opening at the bottom is closed so smoke is pushed up and outward. One person watches inside the house and attic while another watches outside.

After a few minutes, the top cover is removed and the flue is allowed to clear. Any spots where smoke appeared are marked for closer inspection. The smoke used is made for testing, but it is dense and unpleasant, so windows are opened afterward.

Reading the results

Smoke escaping somewhere tells you a path exists; it does not always tell you exactly where the defect is. Smoke can travel along a gap behind plaster and appear a floor away from the crack. So a smoke test is followed by tracing: inspecting the flue from inside with lights, looking at the area behind where smoke appeared, and checking the adjacent flues.

A small leak at a connector or cleanout door is often a simple fix. Smoke appearing in the attic, inside walls or in a neighbor's flue usually points to a flue that needs repair or relining before it is used again.

What usually follows a failed test

The common outcomes, from simplest to most involved, are sealing a loose connector or cleanout door, repairing a section of mortar or tile that can be reached, installing a new liner sized for the appliance so the old flue no longer carries exhaust, and in some party-wall chimneys, relining several flues so each is sealed from the others. Where a flue is no longer needed, it can be capped and sealed instead.

A liner is often the cleanest answer for an old clay flue that leaks in several places, since repairing tiles deep inside a chimney is difficult.

Whatever the repair, the flue is usually tested again afterward, so the report can show it now holds smoke where it previously leaked.

Can a homeowner do a smoke test?

Smoke pellets are sold to the public, but a real test needs the top of the chimney sealed, which is roof work, and people watching every space the smoke might reach, including neighboring homes. Lighting a smoke source in an open fireplace without sealing anything mostly shows that the chimney draws. If you suspect a leak, especially exhaust from a boiler or water heater, keep CO alarms working and have the flue checked rather than testing it yourself.

Where it fits in an inspection

We use smoke testing as a tool within an inspection when the situation calls for it, not as a routine add-on. If a test is recommended, the reason, the scope and the price are stated upfront in writing, and the results go into the written report with the locations where smoke appeared and what we recommend.

Timing matters too. A smoke test is most informative on a cool, still day, when the flue is not being pushed around by wind or warmed by an appliance that just shut off. After a chimney fire, it is done once the flue has been cleaned, because heavy deposits can temporarily plug cracks that would otherwise show.