Furnace Condensation Problems


Furnaces that generate heat through combustion of gas or oil will produce condensation, or moisture. This condensation isn’t usually a problem because the chimney and the furnace are designed to expel this moist air before it condenses inside your chimney. However, when there is too much moisture in the air, the resulting condensation in your chimney and on the flue can lead to damage to your chimney and to the interior of your home.

Excess Condensation Causes

  • If you’ve converted from an older furnace to a new, energy-efficient model, your chimney may be too large. High-efficiency furnaces require smaller chimneys. The combustion gases rise more slowly in oversized chimneys due to the amount of cooler air around them. Because these gases have already been cooled slightly from the heat exchanger in your furnace, they may already be the same temperature as the water vapor contained in them by the time they reach the flue. The resulting condensate that forms then soaks into the brick of the chimney. High humidity levels in your home caused by other appliances or increased water usage in bathrooms, kitchens or laundry rooms can also cause your furnace to form excessive condensation, particularly if it is not a newer, high-efficiency furnace.


  • Water leaking into the brick of your chimney creates several problems. The water combined with the flue gases makes acids that are corrosive. These acids can eat away at the brick, causing the chimney to deteriorate from the inside out. The frigid temperatures that come with winter pose another problem as the moisture inside your chimney freezes and thaws. The freezing and thawing process breaks away the bricks and the mortar of the chimney, and this can lead to leaks inside your home. These leaks from your chimney can damage drywall near the chimney, and can also cause corrosion as they run back into your furnace. The flue inside your chimney vents harmful gases formed through the combustion of gas or oil outside. If condensation is forming on the flue, this can cause problems within your furnace by damaging the flue tile or preventing the gases from being safely carried through the chimney.


  • Air leaks within your furnace can contribute to excess condensation inside your chimney and flue. When the air from outside the furnace is allowed to enter the flue, it cools the flue and lowers the temperature of the air leaving the furnace. This causes the air to lose moisture rapidly as it collects on the flue and inside the chimney. Leaks can also allow harmful combustion gases to escape from the flue, which presents a risk for carbon monoxide to leak into your home. If your furnace is creating excessive condensation, call a technician to inspect the flue and the furnace for air leaks to prevent both damage to your chimney and carbon monoxide poisoning.

Signs of Condensation Problems

  • Wet spots on drywall behind or near the chimney or rust marks on the back of the furnace indicate a chimney that is leaking due to moisture. Outside, you may see efflorescence, which is a white powdery substance, forming on the outside of your chimney. In more severe situations where damage has already occurred, you may also notice flaked bricks, crumbling joints or pieces of flue tile at the bottom of the chimney.

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