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Should a Furnace Leak? … A Little?

By Tom Feiza, Mr. Fix-It, Inc.

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?

Major rust and water stains

A leak from a humidifier on the furnace plenum creates a major issue. This defect is easily spotted (photo – Furnace Humidifier Leak). 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 – Leak Inside Furnace Cabinet). You may see water. That water may also be flowing into the blower/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.

Minor rust and water stains

Let’s take a look at another furnace (photo – Furnace with Minor Stains in Cabinet). 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. 

You see rust and white stains on the metal housing (photo – Furnace with Water and Rust). 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 to be reported. Look in the housing, on the floor beneath the unit, below any humidifier  and at the air conditioner drain pan.