Bosch Flue Gas Spillage / Incomplete Combustion

Bosch water heater causes carbon monoxide alarm to sound when operating

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Safety Warning

This repair may involve working with high voltage components or water connections. Always unplug the appliance before removing any panels.

If you are not confident in your ability to perform this repair safely, we strongly recommend contacting a professional technician.

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Possible Causes

Blocked or disconnected vent, Negative building pressure causing backdraft, Improper venting materials or slope, Incorrect gas/air mixture

How to Fix / Troubleshooting

Safety first: Turn OFF the water heater immediately and ventilate the area. Do not use the heater until the cause is identified and corrected. Carbon monoxide is deadly.

  • Inspect venting: Check that the vent is securely connected to the heater and intact along its entire length. Look for disconnections, holes, or corrosion.
  • Check termination: Ensure the vent termination is not blocked by debris, snow, or other obstructions.
  • Building pressure: In tight homes, exhaust fans can cause backdrafting. A professional should evaluate combustion air supply and building pressure.
  • Professional combustion test: A qualified technician must perform a combustion analysis and verify proper gas pressure, air supply, and venting configuration.
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Repair Difficulty

Professional Required 5/5

Required Part

Vent Pipe / Flue System
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