The signs are damp brown or black patches on the chimney in the attic or a room, drips at the cleaning door, an acrid smell and crumbling masonry above the roof. We find the cause with cleaning and a camera, and the solution is most often a liner of the right diameter, insulation and condensate drainage. Describe in the form where you see damp and which appliance is connected to the chimney.
Why condensation forms in a chimney
- A new appliance in an old flue: a condensing gas boiler, a pellet boiler or an efficient wood boiler produces much cooler flue gases than the masonry stove the chimney was built for.
- A flue channel that is too wide: the flue gases move slowly in it and have time to cool before they reach the top.
- A cold part of the chimney: the flue runs through an unheated attic, along an outside wall or high above the roof, and the walls there are cold.
- Wet firewood: wet wood produces a lot of water vapour, and the flame is cool and smoky.
- Burning on a low flame with the damper closed: the stove smoulders for hours, and the flue gases in the chimney are so cool that they condense.
- A long horizontal flue pipe between the appliance and the chimney, in which the flue gases cool down before they even reach the chimney.
Condensation most often appears right after a heating change. With the old stove the chimney was dry for years, but in the first winter after the boiler is installed brown stains appear in the attic. This means the flue does not suit the new appliance, and the solution is usually chimney liner installation.
Condensation or rainwater
A damp chimney can also be caused by rain. The solution is different, so first you need to know which it is:
| Sign | Condensation | Rainwater |
|---|---|---|
| When it appears | during the heating season, especially in cold weather | after rain or snowmelt, also in summer |
| Colour and smell | brown or black, sticky, with a smoky or acrid smell | usually clear or with lime deposits, no smoky smell |
| Where it shows | along the whole height of the chimney, at the cleaning door, at the flue pipe connection | in the upper part, in the attic under the roof |
| What we do | liner, insulation, condensate collector | cap, crown slab, chimney head repair |
If water comes from above, chimney cap installation helps, as does restoring a cracked head: chimney repair. Often both causes are present at once, and a camera shows this clearly.
Why condensation is dangerous
Condensate is acidic. It soaks into the mortar joints and bricks, freezes and expands in winter, so the masonry cracks and crumbles, most of all above the roof. Through the cracks flue gases can get into the attic or a room. In a solid fuel flue condensate mixes with soot and forms tar (creosote), which sticks to the walls and can catch fire. On removing tar: chimney tar removal.
If the flue is damaged, it must not be used. If tar has already burned in the chimney, call 112 first and then read chimney soot fire: what to do.
Cabinet Regulation No. 238 “Fire Safety Regulations” of 19.04.2016, sub-point 89.1: it is prohibited to use a damaged heating appliance, device or flue; sub-point 89.2: it is prohibited to overfire a heating appliance; sub-point 89.11: it is prohibited to clean a flue by burning it out. LBN 231-15, point 50: flues are made of non-combustible, heat-resistant and corrosion-resistant materials. Regulations checked 04.11.2026; from 2 January 2027 they will be replaced by new fire safety regulations, which we will check as soon as they are adopted. Source: likumi.lv, checked 04.11.2026.
How we deal with condensation in a chimney
- Cleaning We remove soot and tar so the flue walls can be seen and no dirt hides anything.
- Camera We inspect the whole flue: cracks, damp spots, crumbled joints, the cross-section. About this job: camera inspection.
- Cause We compare the flue size with the appliance's requirements, assess where the chimney cools down and check the draught.
- Solution Most often: a liner of suitable diameter, insulation in the cold part and a condensate collector at the bottom. For a gas boiler: liner for a gas boiler. For the cold part: chimney insulation.
- Drying the masonry After lining, the damp masonry gradually dries out. A damaged head is usually restored once it has dried and the weather allows work with mortar.
What you can do yourself
- Burn dry firewood that has been dried under a roof for at least one summer.
- Do not burn for hours on a small, smouldering flame with the air supply closed.
- When you notice the first signs, watch when the patches grow: in rain or when the fire is lit. This will help find the cause.
- Do not climb onto the roof and do not try to dry the chimney out with hard firing: overfiring an appliance is prohibited, and it can set the tar alight.
If the patches disappear with a masonry stove and dry firewood, you do not need a specialist. If they remain or the masonry crumbles, describe the situation in the form.
How much does fixing a condensation problem cost?
The price depends on what we find. Sometimes cleaning and advice on firing are enough; other times a full-length liner, insulation and restoring the head are needed. It is affected by the length and diameter of the flue, the height of the chimney and access to the roof, the fuel and type of appliance, the amount of tar and the materials (liner, insulation, cap). Cleaning and the camera inspection can be done on the same visit. The specialist will name the price after you submit the form and after the inspection, before the work. All prices: prices.
Sources
- Prohibited to use a damaged flue (Cabinet Regulation No. 238, 89.1); prohibited to clean by burning out (89.11); prohibited to overfire (89.2); Cabinet Regulation No. 238 loses force on 02.01.2027 checked 4 November 2026
- Flues made of non-combustible, heat-resistant and corrosion-resistant materials (LBN 231-15, 50) checked 4 November 2026
- Soot forms from wet firewood; burning soot damages the flue (VUGD 19.01.2026) checked 4 November 2026