Why Is My Boiler or Chimney Emitting Smoke?

Smoke emission from your boiler or chimney usually indicates an issue with air flow or draft. If air intake (aeration) is stronger than the chimney draught, smoke may be pushed into the room instead of flowing outside.

Common Causes

• ❌ Inappropriate chimney design (e.g., holes or connections to other ducts)
• 📏 Chimney height or cross-section is too small
• 🧊 Lack of insulation on the exterior part of the chimney
• 🔗 Other appliances connected to the same chimney duct
• 🧹 Dirty or blocked chimney or boiler
• 🌀 Improper elbow construction (sharp angles, too many bends)
• ⬆️ Flue duct lacks upward slope (exhaust gases should rise slightly after leaving the boiler)
• 📉 Flue duct inserted too deeply into the chimney
• 💨 Ventilation openings are too large, causing reverse airflow

For Boilers with Automatic Aeration

If your boiler uses automatic air intake, the airflow may need to be limited. You can do this by:

• 🛠️ Using a metal flap on the ventilator
• ⚙️ Adjusting airflow parameters in the boiler control system