Why Is My Chimney Leaking Black Liquid?

What the liquid is

Every wood fire leaves deposits on the inside of the flue. Creosote is what smoke leaves behind when it cools before it gets out of the chimney, and it builds up in recognisable stages. First comes a dry, flaky soot that brushes off easily. Next comes a crunchy, tar-like layer. The last stage is a hard, shiny glaze that looks like dripped black paint and clings to the tile.

Water changes all of them. It softens and partly dissolves the deposits and runs down the flue as a dark, oily liquid with a strong smoky or sour smell. Because it carries acids from the smoke, it is hard on everything it touches: it rusts the damper, eats into mortar joints, and leaves stains on brick and plaster that are very hard to remove.

Old fuel counts too. Many rowhouse and brownstone flues in the region carried coal smoke for generations, and soot left in those flues can still bleed out as a dark liquid when water reaches it, even where the fireplace has sat unused for years.

Rain, condensation or a smoky fire

Rain is the first suspect. An uncapped flue, a cracked crown or a gap where the flue tile meets the crown lets water in, and it picks up deposits on the way down. Wind-driven rain in a nor'easter can soak the top of a chimney even through a cap that is too small.

Condensation is the second. A small, smouldering fire in a big, cold flue makes smoke that cools quickly and condenses on the walls as liquid creosote. A gas or oil boiler venting into an unlined or oversized masonry flue does the same with its water vapor, and that condensate picks up old soot as it runs down. The timing usually tells them apart.

• Black drips after rain: water entering at the cap, crown or flue top
• Black drips while a fire is burning: liquid creosote from a cool, smoky burn
• Brown or black streaks near the boiler: condensation in the heating flue
• Water running down the inside of a metal liner: a failed top seal or a missing cap

Reading where the stain appears

Where the liquid surfaces narrows down the cause. Drips in the firebox or on the hearth come straight down the flue. Dark streaks running down the outside brick from the crown suggest water and deposits spilling over the top, or seeping out through the brick of an unlined flue. Brown stains on the plaster of the chimney breast in an upstairs room mean the liquid is soaking through the flue walls, which happens most in unlined or badly cracked flues.

In the basement, black streaks below the cleanout door, or a tarry puddle behind it, belong to whatever flue that door serves. In a stack with several flues, knowing which flue serves which appliance is the first step.

When water runs down a metal liner

Wet soot in a stove, or a clump of black sludge in the tee at the bottom of a liner, means water and deposits are meeting inside the liner. The cap and the top plate where the liner is sealed to the crown are the first places to look; a liner installed without a proper top fitting lets rain run straight in.

Water inside a liner can also be condensation, if the liner is too big for the appliance or was meant to be insulated and was not. Either way, it is worth tracing before the next cold stretch, when that moisture can freeze at the top.

Why it is more than a stain

Black liquid means there is enough creosote in the flue for water to carry it. Creosote is fuel. A thick layer, especially the glazed kind that forms from slow, cool fires, can ignite inside the chimney, and a chimney fire can crack tiles and reach the wood framing around the stack. The acid in the liquid also wears out tile and mortar, which opens gaps for heat and exhaust to escape into the house.

Hold off on using the fireplace or stove until the flue has been swept and checked. If the stain is coming from a boiler flue, the boiler cannot usually be left off for long, so book the check promptly and make sure CO alarms near it and on the sleeping floors are working.

Burning habits that make liquid creosote

When the drips appear during fires, the fix is partly in how the fire is run. Cool, smoky fires make the most creosote, and these are the habits behind them.

• Burning wood that is not properly seasoned; wet wood hisses, steams or bubbles at the cut ends
• Closing the damper or the stove's air control down to stretch a fire overnight
• Small, slow fires in a large, cold flue built for a much bigger fireplace
• Burning cardboard, wrapping paper or trash, which smoke heavily and leave residue
• An exterior chimney that stays cold until a fire has been burning for some time

What the technician checks

Tracing black liquid means following the water and the deposits at the same time. The visit usually covers these points, from the bottom of the chimney to the top, and ends with an explanation of which flue is leaking and why.

• Which flue serves which fireplace, stove or boiler, and where each one ends at the top
• The type and depth of deposits along the flue, and whether any of it is glazed
• Cracked or missing tile, open joints and any path for liquid into the surrounding brick
• The cap, the crown and the seal around the flue tile or liner top
• For a heating flue, the flue size against the boiler and the condition of the connector pipe

Cleaning up and fixing the source

The deposits are swept out first. Ordinary soot and crunchy creosote come off with brushes; hard, glazed creosote needs a separate removal treatment. Then the water is stopped at its source: a cap and crown repair for rain, a liner sized to the boiler for condensation, and drier wood and hotter fires for creosote that condenses during burning. Damaged tile may need repair or a liner.

Stained brick can often be cleaned once the leak has stopped, though old stains may not come out completely. Pressure washing and strong acid cleaners can damage the soft brick and lime mortar of an older house, so the cleaning method should suit old masonry. Stained plaster has to dry out fully and be sealed with a stain-blocking primer before it is painted, or the dark marks bleed back through.