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Consider replacing your gravity furnace when it's over 30 years old, when you experience frequent system failures, or when your heating bills continue rising despite maintenance. Many older gravity systems operate at substantially lower efficiency rates compared to modern alternatives, often 50% to 60% versus the 80% to 98% efficiency of contemporary furnaces. 

Replacement also makes sense when your existing system shows signs of deterioration that could affect safety or when professional inspection reveals significant issues.

Most gravity furnace replacements require between six and 12 hours of professional labor, spanning one to two working days, depending on scheduling and complexity. Basic installations with minimal complications may be completed in a single day, while projects requiring extensive ductwork modifications or asbestos remediation will require additional time. 

The timeline includes removal of the old system, preparation of the installation area, placement and connection of the new furnace, and thorough testing.

Watch for warning signs, including inconsistent heating throughout your home, unusual noises during operation, visible corrosion or damage to the heat exchanger, and steadily increasing energy consumption. If your furnace requires frequent repairs or if you notice a yellow or flickering pilot light instead of a steady blue flame, these may indicate serious issues. 

Age remains a primary consideration—systems over 30 years old warrant replacement rather than continued repair.

Well-maintained gravity furnaces that pass professional safety inspections can continue operating, but they inherently carry more risk factors than modern systems. Potential concerns include less efficient combustion that may produce carbon monoxide, deteriorating components that could create fire hazards, and the possible presence of asbestos in insulation materials. 

Modern furnaces incorporate numerous safety improvements, including automatic shutdown features, sealed combustion chambers, and electronic ignition systems that eliminate standing pilot lights.