There are a few common reasons why your furnace is leaking.

First, however, what kind of furnace do you have?

Look for the yellow AFUE label on your furnace.

AFUE is the acronym for Annual Fuel Utilization Efficiency, which tells you how efficient the furnace is.

High-efficiency furnaces have an AFUE rating of 90 percent or more, meaning that 10 percent or less of the energy is lost – to air leaks or the chimney or other openings.

Conventional furnaces are less efficient. They get an efficiency rating of 80 percent, meaning they lose twice as much energy as more advanced furnaces.

High efficiency furnace leaks

One common reason for high efficiency furnace leaks: The condensate drain or line is clogged.

The condensation forms when the furnace’s heat exchangers absorb so much heat that the gas turns to water as it’s sent through the exhaust pipe. The water can be blocked by a clog. That’s why it’s leaking.

Sometimes you can clear the line by pouring a 50-50 blend of white vinegar and hot water through the line.

But if that method doesn’t work, you may need a new condensate pump to push the water along the line. Condensate pumps can wear out every few years.

Conventional furnace leaks

You may have a poorly fitted exhaust pipe or a problem with the humidifier built into the system.

A loose pipe will cause water to leak rather than exit via the drain line.

A furnace humidifier can develop tiny leaks or cracks in its water line.