Why Does a Stove Pipe Drip and Leak Tar? Causes and Fixes

Short answer: the brown-black, sticky liquid dripping from a stove pipe is smoke, water vapour and creosote (often called tar) condensing on the cold inner surface of the pipe. A university extension bulletin states that creosote condenses when pipe and flue surfaces fall below about 121 °C (250 °F), and below 66 °C (150 °F) forms a thick, sticky, tar-like deposit. The commonest causes are damp wood, a long cold pipe, air turned down too far, too many elbows and a wrongly fitted pipe joint.

Creosote is not just dirt: the U.S. FEMA notes that all its forms are highly combustible and that, if it builds up enough and the flue gets hot enough, it can cause a chimney fire.

Pipes, elbows and bases: Stove accessories →

Why does a stove pipe drip? Causes and fixes

How do you fit a stove pipe correctly?

• Direction: In one pipe manufacturer's instructions the bottom of the pipe goes inside the stove collar. Condensate running down the inside then flows inwards at the joint, not out of it.
• Screws: The same instructions call for three sheet-metal screws equally spaced around each joint, especially with elbows. The joint spacing of 90° elbows may in some cases permit creosote leakage.
• Tape: The TKİ guide says pipe-elbow joints and the flue entry can be sealed with non-combustible aluminium tape, and that the pipe end must not be pushed too far into the flue opening.
• Slope: The horizontal run should rise towards the flue; pellet stove installation manuals ask for at least a 3% slope (pellet stove flue). For a wood or coal stove, check your manual for slope and length.

Ignova's 13 cm matt anthracite pipe comes in 25, 33, 50 and 70 cm lengths with an elbow (manufacturer): 13 cm stove pipe and elbow.

How do you clean tar out of a stove pipe?

The university bulletin says to take the pipe sections outdoors and brush them inside with a wire brush of the same diameter; that chemical cleaners are not meant for chimneys with heavy deposits; and that burning out a chimney on purpose is a risky way to keep it clean. Wear gloves, a mask over mouth and nose, and goggles. If you notice chimney fire signs (a roar, shaking pipes), get everyone out and call 112: chimney cleaning and stove maintenance.

Why does dry wood make a difference?

According to FEMA, with unseasoned wood much of the energy first goes into driving off the water in the cells; the smoke stays cooler and more creosote builds up. The EPA wants wood below 20% moisture, checked with a moisture meter: how to tell damp wood. Firewood options are on the oak firewood and firewood pages; you can also measure the moisture of the wood you receive yourself.

What does it depend on?

• Fuel moisture: The single most effective variable.
• How you burn: A low, long fire cools the flue gas.
• Pipe and flue: Length, elbows, insulation, outside walls and diameter; weak draught increases deposits.
• Maintenance frequency: With heavy use, have the flue checked mid-season too; a new stove's paint smell on first firing is a separate matter: cast-iron stove first firing.
• Stove choice: If you are thinking of a new stove, see the output you need in the stove sizing calculator.

Safety note

No stove is "smokeless"; pellets, wood and coal all produce smoke and carbon monoxide when they burn. A flue of the right diameter that draws properly and sealed pipe joints are essential. Use a carbon monoxide detector at home; with headache, dizziness or nausea, ventilate, get outside and call 112: stove gas symptoms and carbon monoxide detectors.

Frequently asked questions

Is water dripping from a stove pipe normal?

No, constant dripping shows condensation inside the pipe. Look at the moisture of the wood, how far you turn the air down and the length of the pipe.

How do you clean tar from a stove pipe?

Take the pipe down and brush it inside, outdoors, with a wire brush of the pipe's diameter. Do not try to burn off heavy deposits.

How often should a stove pipe be cleaned?

The university bulletin recommends checking pipes at least once every 2–3 months of use; have the chimney cleaned by a professional every year (USFA).

How do you seal stove pipe joints?

Fit the pipe the right way round (bottom end inside the stove collar), screw the joints and seal them with non-combustible aluminium tape (TKİ).

Does a pellet stove pipe drip too?

Yes, condensation also forms in a pellet stove's cold, uninsulated, long horizontal pipe: what a pellet stove flue should be like.

Sources: creosote condensation temperatures, causes and cleaning, Michigan State University / University of Idaho extension bulletin E-1388, "The Creosote Problem: Chimney Fires and Chimney Cleaning" (archive); FEMA, "Chimney Fire Precautions"; wood moisture, U.S. EPA Burn Wise, "Best Wood-Burning Practices"; pipe joints, DuraVent, DuraBlack installation instructions; tape, elbows and flue entry, Turkish Coal Enterprises (TKİ), “Guide to Using Coal in Stoves”; pellet stove slope, CADEL installation manual; chimney inspection, U.S. Fire Administration, "Heating Fire Safety"; pipe sizes, manufacturer page Ignova 13 cm stove pipe and elbow.