What Makes Ice Form On a Furnace Flue?

Cracked Vent

• If your vents are cracked or broken, water can get stuck inside the vent. During cold weather, the water freezes over and turns to ice. Similarly, if there are bird droppings, leaves or other debris inside the vent, water can get trapped inside. The trapped water can't drain properly and will freeze over during cold spells.

Furnace Off At Night

• When the furnace is running, ice generally doesn't form on the flue. The furnace emits hot gases while it is running, which melts any ice forming and stops new ice from being able to freeze over. However, if you turn your furnace off at night to save energy, it doesn't emit any gas. Thus, ice can form on the flue overnight. To solve this problem, keep the furnace running at a lower temperature when you go to bed at night.

Water Entering Home

• If water enters the home through a vent, it can freeze over on your furnace flue while the furnace isn't running. Water can come in even if the vent isn't cracked or damaged. Vents need to point downward to reduce this possibility. You can also plant evergreen hedges around the sides of your home to reduce the risk that water can enter the home through a side vent.

Conversion of Furnace

• If you convert from an older furnace to a new one, ice may form on the top of your chimney and potentially block the flue. Older furnaces usually run at higher temperatures than newer furnaces; thus, as exhaust from the furnace travels up the chimney, it may condense into water and eventually into ice. Some chimney liners are made out of galvanized metal, which may rust or corrode over time. If this happens, water may seep through the chimney liner and freeze elsewhere.