Chimney thermodynamics & air flow – how log burners cause damp

A chimney naturally ventilates a heritage building through the stack effect, where warmer indoor air rises through the chimney due to its lower density compared to cooler outdoor air. This airflow helps regulate indoor humidity, remove pollutants, and prevent dampness. The effectiveness of this natural ventilation varies with the season, influenced by indoor heat sources like heating systems, appliances, and even body heat.

How It Works

Heat Generation: Heat generated inside the building—from heating systems, appliances, lighting, and occupants—warms the indoor air, making it less dense. This warm air rises through the chimney, creating a low-pressure zone at the base.

Rising Warm Air: As the warm air rises, cooler air is drawn into the building to replace it, maintaining a continuous cycle of airflow that ventilates the building.

Ventilation: This airflow removes moisture, excess heat, and indoor pollutants, preventing dampness and condensation, which are particularly damaging to heritage buildings.

Impact of Wood Burner Installations

When a wood burner is installed, a register plate, flue liner, and capping chimney with a flue are often added to improve the efficiency of the burner and prevent heat loss. However, these modifications can significantly disrupt the natural airflow through the chimney:

Register Plate: Installed above the wood burner, this plate seals off the chimney, reducing the amount of warm air that can rise through it.

Flue Liner: While a flue liner is essential for safety and efficiency, it reduces the diameter of the chimney, limiting the volume of air that can move through it.

Capping with Flue: Capping the chimney and installing a flue restricts airflow further, effectively sealing the chimney when the wood burner is not in use.

Consequences

These modifications completely block the natural ventilation provided by the chimney, leading to several issues:

Condensation: Without adequate airflow, moisture generated inside the building can become trapped within the chimney. This moisture can condense on the cooler internal surfaces of the chimney, especially in winter, when the temperature difference is more pronounced.

Mobilisation of Hygroscopic Salts: The trapped moisture can activate hygroscopic salts within the masonry. These salts, which absorb moisture from the air, can migrate through the plaster and masonry of the chimney breast. As they crystallise on the surface, they cause visible salt damp, often presenting as white powdery deposits or damp patches on internal walls.

Airflow Volume Data

The airflow through a chimney without these modifications is affected by the height of the chimney, the temperature difference between indoor and outdoor air, and the cross-sectional area of the flue.

Winter Scenario (Indoor 20°C, Outdoor 5°C):
Temperature difference: 15°C
Estimated airflow: 0.085 m³/s (306 m³/h)

Summer Scenario (Indoor 15°C, Outdoor 20°C):
Temperature difference: -5°C
Estimated airflow: 0.025 m³/s (90 m³/h)

When the wood burner setup is installed, these airflow volumes can be drastically reduced, leading to the issues described.

Summary
Winter (20°C indoor, 5°C outdoor): 0.085 m³/s or 306 m³/h airflow under normal conditions, significantly reduced by wood burner modifications.
Summer (15°C indoor, 20°C outdoor): 0.025 m³/s or 90 m³/h, with potential reverse airflow, further diminished by a capped chimney.

The installation of a wood burner with a register plate, flue liner, and capping can stop or reduce the natural airflow through the chimney, leading to condensation within the chimney. This trapped moisture can activate hygroscopic salts, resulting in salt damp on internal chimney breasts. Proper ventilation management and moisture control are crucial to preserving the structural integrity of heritage buildings, especially when modern heating systems are added.