The short answer
Brown tar staining and condensation in a chimney are usually caused by flue gases cooling too much, so moisture and unburnt residues condense on the flue walls instead of leaving as vapour. The main culprits are burning damp or unseasoned wood (which carries a lot of water and produces smoky, low-temperature combustion), slow, slumbering fires that never get hot, an oversized or cold flue (for example a stove fitted into a large unlined chimney, or a flue on a cold external wall), and poor draught. The deposits are acidic and flammable, so they can stain through the chimney breast and raise the risk of a chimney fire. The fixes are dry, seasoned fuel, hotter burning, and a correctly sized, insulated liner.
Tar and condensation are a sign the flue is running too cool for the fuel being burnt. Here is the mechanism and the cure.
Tar and condensation
- Core causeflue gases cooling too much
- Worst fueldamp / unseasoned wood
- Flue problemoversized, cold or external
- Deposits areacidic and flammable
- Main fixesdry fuel, hot burn, sized liner
Why tar and condensation form
When fuel burns, the flue gases carry water vapour and combustion by-products up the chimney. If the flue stays hot enough, these leave as vapour at the top. If the gases cool too much on the way up, the moisture and unburnt residues condense onto the flue walls as a sticky brown-black tar (creosote). Over time this build-up can soak through the masonry and show as brown staining on the chimney breast inside, often with a strong smell. Crucially, these deposits are acidic (which attacks mortar and liners) and flammable (which is what feeds chimney fires), so they are more than a cosmetic problem.
The common causes
The biggest single cause is burning damp or unseasoned wood. Wet wood carries a lot of water, so much of the fire's heat is wasted boiling it off, the fire burns cool and smoky, and the flue runs cold — ideal conditions for tar. Slumbering a stove (running it shut right down for long, low burns) has the same effect. An oversized flue — such as a small stove venting into a large old unlined chimney — lets gases slow and cool, and a flue running up a cold external wall loses heat fast. Poor draught compounds all of these. In short, anything that keeps the flue cool encourages tar and condensation.
| Cause | Why it leads to tar/condensation |
|---|---|
| Damp / unseasoned wood | Water cools the flue, smoky burn |
| Slumbering the fire | Low temperatures, incomplete burn |
| Oversized flue | Gases slow and cool |
| Cold external flue | Rapid heat loss up the flue |
| Poor draught | Sluggish, cool flue gases |
Indicative; usually a combination is involved. Sources: HETAS; NACS guidance.
How to prevent and fix it
The first and lowest-cost step is fuel and how you burn: use dry, well-seasoned wood (ideally with a low moisture content, which you can check with a cheap moisture meter), or seasoned/kiln-dried logs and approved smokeless fuels, and run the fire hot enough rather than constantly slumbered. Structurally, fitting a correctly sized, insulated flue liner keeps the gases warm and matches the flue to the appliance, which is one of the main reasons stoves are relined when fitted into old chimneys. Regular sweeping removes deposits before they build up dangerously. Where tar staining has already come through the chimney breast, the underlying cause must be fixed first; surface treatments alone will not hold back active tar.
Frequently asked questions
Why is my chimney breast turning brown?
Brown staining on a chimney breast is usually tar (creosote) soaking through the masonry, caused by flue gases condensing in a cool flue. Burning damp wood and an oversized or cold flue are common reasons; the cause must be fixed, not just covered.
Does burning wet wood cause tar?
Yes. Wet or unseasoned wood carries a lot of water, so the fire burns cool and smoky and the flue runs cold. That lets moisture and unburnt residues condense as tar, which is both acidic and flammable.
Can a liner stop tar build-up?
A correctly sized, insulated liner keeps the flue gases warmer and matches the flue to the appliance, which reduces condensation and tar. It works best alongside dry fuel and burning the fire hot enough rather than slumbering it.
Sources & further reading
Figures on this page are typical UK ranges drawn from published sources and depend on your specific chimney. They are guidance, not a quotation.