June, 2019
Inspection News and Views from the American Society of Home Inspectors

Should a Furnace Leak? ...a Little?


No, furnaces should not leak—but they do. Water and forced-air furnaces just don’t mix. Furnaces in cold climates often have water flowing through humidifiers in the plenum. These humidifiers can leak. A modern high-efficiency furnace (over 90%) works by condensing steam from the products of combustion; this produces water that must be drained from the furnace. 

So, again: A furnace can leak and this leads to anything from a routine maintenance chore to a major defect.

During a typical home inspection, you use normal controls to operate the furnace. You also open the cabinet cover. What if you see signs of water?

Example 1: Major rust and water stains

A leak from a humidifier on the furnace plenum creates a major issue. This defect is easily spotted (Photo 1). In this case, water has been running down the face of the furnace for quite some time. Note the deposits on the top of the furnace housing.

When you remove the cover of the furnace housing, you may be greeted with signs of extensive water damage (Photo 2). You may see water. That water also may be flowing into the blower or fan housing below, and into the circuit board controls.

This is a major defect. Even if it was corrected, this furnace would need a complete evaluation by a heating contractor. Perhaps the humidifier should be relocated to the side of the ductwork.

Example 2: Minor rust and water stains

Let’s take a look at another furnace (Photo 3). You know this is a condensing furnace because of the two white PVC pipes at the right side: an air supply and a combustion vent. A casual look inside the cabinet shows a little white staining. But there is heavy staining on the concrete floor below the unit. You know this needs a closer look. 

What if you see rust and white stains on the metal housing (Photo 4). Is it a problem? You bet—a condensate leak. This becomes even worse if water reaches the circuit boards below. Report this as a major issue or as a maintenance issue that requires full evaluation and repair. 

That leak may have dried before you saw the furnace. Even if you try running the furnace, but don’t see any active condensate leak, it’s still an issue—and you don’t want to pay for a major furnace repair.

So What Should You Do?

Condensing furnaces are prone to leaks, and any leak is an issue that should be reported. Look in the housing, on the floor beneath the unit, below any humidifier and at the air conditioner drain pan. 

To learn more, attend Tom’s technical presentations at educational sessions for ASHI chapters. Tom can also provide his knowledge for your educational event; contact him at Tom@HTOYH.com. 

Tom Feiza has been a professional home inspector since 1992 and has a degree in engineering. Through HowToOperateYourHome.com, he provides high-quality marketing materials that help professional home inspectors educate their customers. Copyright © 2019 by Tom Feiza, Mr. Fix-It, Inc. Reproduced with permission.