Rusted Metal Flue Pipe: The Science Behind Your Inspection

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Rusted Metal Flue Pipe: The Science Behind Your Inspection

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

Holes in metal flue pipes that extend from gas appliances to a chimney can create a hazard if they fail. They may also indicate significant gas appliance and home performance issues. What is the science behind the stained, rusty metal flue pipes? Why did they fail? Why does a home inspector care?

Problems Galore
Figure 1 shows telltale signs of moisture issues on two metal flue pipes from gas appliances to a masonry chimney. You can see stains and drips from moisture. There are pinholes in the metal flue and extensive rust. Pinholes show up on the outside of a metal flue pipe as slightly tan, white, or rust-colored.

These flue pipes are failing. If you look closely, you can see the lower pipe is newer but still showing signs of stains and rust. I believe a severely corroded metal flue pipe was replaced but the draft and condensation problem causing the corrosion was not corrected.

The science? The system is not properly removing the products of combustion with adequate flow. The natural “draft” of the chimney and hot combustion gas is not working. It is not venting properly. The flow of combustion gas is slowed and it cools. Combustion products contain lots of moisture. As the combustion gas is cooled below the dew point temperature, moisture forms inside the metal flue. The moisture is mildly acidic from the products of combustion. The acidic water stains and rusts through the galvanized coating and through the metal; this is a hazard all inspectors should be concerned with—a failing flue pipe and possible backdrafting of combustion products into a home.

Signs of Condensation
The products of gas combustion should stay warm enough to flow up the chimney without condensation. Watch for signs of poor draft of combustion gas, such as condensation and rust on the metal flue pipe from the appliance to the chimney (See Figure 2). Burn marks, rust, melted plastic, or melted foam insulation on pipes at a draft diverter are sure signs of backdrafting (See Figure 3).

Although it is hard to see on the outside of the metal pipe, Figure 2 is a severely rusted flue pipe on the inside. If you look closely, you can see rust holes through the elbow joints and along the horizontal run. The little tan spots on the exterior surface are actually holes through the metal flue. This metal pipe has failed and is about to fall apart. When it leaks or falls apart, combustion products are dumped in the home.

Figure 1

Figure 1: Signs of Condensation


Figure 2

Figure 2: Burn marks, rust, melted plastic, or melted foam insulation


Figure 3

Figure 3: Gas Water Heater with Signs of Backdrafting

A Prime Candidate for Rust
Look at the gas water heater in Figure 5. A long, low-slope flue pipe extends back from the water heater, over the crawl space, and all the way to the masonry chimney. Immediately you’ll see red flags: Does the flue pipe have the proper pitch? Is the flue pipe too long for the height of the chimney? Could the chimney be blocked? There are also deposits below the draft diverter on top of the water heater.

I photographed Figure 6 of that same water heater flue pipe, near the masonry chimney in that crawl space. Notice a dip in the slope of the flue pipe, patched with duct tape. (Too often we see duct tape, “the handyman’s secret weapon,” at the site of a defect). Condensation has also created rust holes and metal failure.

For this water heater, the metal flue pipe run was too long, the slope was inadequate, and the chimney was not high enough for the horizontal run. The products of gas combustion did not flow up the chimney. Instead, these moisture-laden products cooled in the metal flue, causing water condensation and damage to the metal.

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Related Issues due to Poor Design
Even without a complete evaluation of the chimney, flue pipe, and BTU of a gas appliance, we can still understand the problem. Where combustion gases don’t flow properly, they cool in the flue pipe, and condensation and rust occur over time. Figure 8 shows holes in the back of a boiler combustion gas venting system. This boiler has a powered flue damper that may contribute to trapping combustion gas.

Burn marks and signs of backdrafting at the draft diverter are related issues in addition to a rusted flue pipe (See Figure 3) shows related issues. In addition to a rusted flue pipe, you may see signs of backdrafting at the draft diverter and burn marks near the gas burner. A lack of combustion air or negative pressure in a home can also cause backdrafting and rust. The negative pressure may be due to a large-volume kitchen fan or even a clothes dryer in a very tight home. Oversized chimney flues are another potential source of draft issues.

Figure 4

Figure 4: Chimney—Oversized Flue, Water Heater

Figure 5

Figure 5: Long Low Slope Flue Pipe

Figure 6

Figure 6: Extensive Corrosion

Oversized Chimneys:
Often a gas water heater is “orphaned” in a masonry chimney when a naturally vented gas furnace is updated to a direct vent furnace. The flue gas/heat from the gas furnace is removed from the chimney making the water heater an orphan.

The gas water heater combustion products do not contain

enough heat to warm the masonry chimney and poor draft or backdrafting occurs; this is shown in Figure 4. Condensed moisture will damage the inside of the masonry chimney, damage the chimney brick structure, and rust the metal flue pipe.

An oversized chimney is corrected by adding a liner to the masonry chimney, as shown Chimney—Oversized Flue Repair (See Figure 7). The liner is typically a thin flexible metal that can be easily heated by the products of combustion from a gas water heater. Once the air inside the thin metal is warmed, a draft occurs. With a small metal liner, the full masonry chimney tile does not need to be heated.

Mid-Efficiency Furnaces (85%) Issues:
Rusted metal flue pipes can also be a problem in 85% furnaces if they are not venting properly. The fan on an 85% furnace is designed to deliver the products of combustion to a chimney where they should flow naturally up the chimney. Figure 9 shows a close-up of the stains on the draft fan housing. Figure 10 shows an 85% furnace with leaks/stains from the metal flue pipe running down and over the draft fan. The furnace is not drafting properly. There should never be stains or rust on the metal flue of an 85% furnace.

You should note the metal flue pipe just above the cabinet/fan is new—someone replaced the rusty flue pipe but may not have corrected the draft issue. The issue should be noted as a possible failure to draft properly that needs further investigation.

Figure 7

Figure 7: Chimney—Oversized Flue Repair

Figure 8

Figure 8: Holes in Boiler Flue

Figure 9

Figure 9: 85% Stains, Leaks From Flue

Figure 10

Figure 10: 85% Furnace Leaks

The Takeaway:
Rust and condensation on the metal flue pipes of gas appliances indicate improper drafting of combustion products out of a home and up the chimney. You should describe this as a safety defect requiring further evaluation because the products of combustion leak into the home. Combustion products contain moisture, contaminants and possibly carbon monoxide.

A rusted, failing metal flue pipe is also a safety hazard because it may leak products of combustion into a home. While the signs of failure are readily visible to a good inspector, you don’t need to identify all the potential causes for poor drafting. Just warn your customer of the potential safety hazard.

About the Author
Tom Feiza has been a professional home inspector since 1992 and has a degree in engineering. Through, he provides high-quality marketing materials, books for homeowners, and illustrations that help professional home inspectors educate their customers. Copyright © 2023 by Tom Feiza, Mr. Fix-It, Inc. Reproduced with permission.

Visit for more information about building science, books, articles, marketing, and illustrations for home inspectors. E-mail Tom at with questions and comments, or phone (262) 303-4884.

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