Condensation From a Furnace to the Chimney Pipe

Normal Condensation

• Sometimes, the appearance of condensation on a furnace chimney pipe simply indicates a temperature difference between the gases traveling through the chimney pipe and the air outside of the pipe. This is a commonly reported problem by homeowners that upgrade an old, inefficient furnace to a newer, more efficient model without upgrading their single-walled ventilation to double-walled ventilation. Newer, more efficient furnaces produce gas byproducts at a much lower temperature than their older counterparts, and when the cooler gas reacts with warmer air outside the pipe, condensation forms.

Acidic Condensation

• A more nefarious type of furnace chimney condensation is acidic condensation produced by mid-efficiency furnaces -- those with an efficiency rating of around 80 percent. This acidic condensation can corrode chimney lining, which in turn creates the greater problem of some of the acidic condensation making its way out of the chimney and putting your family at undue risk.

What to Do

• If you determine that the condensation you notice is simply due to the temperature differential inside and outside the chimney pipe, simply replace your single-walled ventilation with double-walled ventilation designed to mitigate temperature differences that make condensation form on chimney pipes. The formation of acidic condensation is usually due to a furnace's being ventilated through an oversized chimney. Use chimney liners to decrease the size of the chimney down to a size that is ideal for your furnace or have the furnace vent directly outside the home.

Other Considerations

• Due to the problems associated with higher-efficiency furnaces and condensation, some may consider it not worth the cost to upgrade their old furnaces to newer, more energy-efficient models. But it is almost always more cost efficient to replace your old, inefficient furnaces with newer ones. Just remember to upgrade your chimney and chimney lining if your current chimney is not well suited for your upgraded furnace. If necessary, consult a contractor or furnace expert for advice on retrofitting your chimney for your new furnace.