Rust on the Furnace or Boiler Flue Pipe: What It Means

What the flue pipe does

The metal pipe running from the top of a furnace, boiler or water heater into the chimney is called the vent connector, smoke pipe or flue pipe. It carries hot exhaust from the appliance into the chimney flue through an opening in the masonry called the thimble. On many systems it is single-wall galvanized steel; some installations use double-wall vent pipe. It must rise toward the chimney, have tight joints fastened as the instructions require, and keep its listed clearance from anything that burns.

Because it sits in the basement where you can see it, the flue pipe is often the first place a venting problem shows itself.

Why flue pipes rust

Natural gas and heating oil both produce water vapor when they burn, and gas produces a great deal of it. While the exhaust stays hot, the vapor stays a gas and leaves through the chimney. When the exhaust cools below its dew point, in a cold basement, a long horizontal run, or a large, cold chimney above, it condenses into water mixed with combustion byproducts, which is acidic. That water attacks galvanized steel from the inside, especially at joints and the lowest points of the run.

In older basements around the region, the usual causes, roughly from most to least common, are a chimney far too large for a modern appliance, a water heater stranded in the old flue after the boiler was replaced, an unlined or deteriorated clay flue, an exterior chimney that runs cold, and a long or poorly sloped connector.

Signs to look for

Most of these can be seen from the basement floor with a flashlight, without touching the pipe, which can be hot.

• Rust spots or streaks on the pipe, especially at joints and elbows
• White, chalky crust on the outside of the pipe or at the draft hood
• Water dripping from joints or pooling at the base of the appliance
• Pinholes or flaking metal you can see light through
• Joints that have separated, or a pipe that sags or slopes downward toward the chimney
• Rust flakes or water at the chimney cleanout

Why it matters

A flue pipe with holes or open joints can leak exhaust, including carbon monoxide, into the basement. A pipe that has rusted heavily can also drop flakes into the appliance's draft hood or the chimney base, and in bad cases the pipe can separate from the chimney. Water dripping back into the appliance can corrode its burner, heat exchanger or controls.

A CO alarm near the appliance and another outside the bedrooms are the backstop. When one sounds or someone feels ill, the household goes outside and calls 911 or the gas company. Do not tape over holes or wrap the pipe; foil tape does not make a rusted vent safe.

• Light surface rust with no holes means occasional condensation; have the venting checked before it worsens
• White crust and drips at joints mean ongoing condensation; the pipe and chimney need inspecting to find the cause
• Pinholes or flaking metal mean advanced corrosion; replace the pipe and fix the cause promptly
• Separated joints or a sagging pipe come from corrosion or poor support; stop and call for service now

Oil systems: smoke pipe corrosion

On oil boilers and furnaces, the smoke pipe faces a different mix. Oil exhaust carries sulfur, and when it condenses it forms a stronger acid than gas condensate. Soot and scale collect inside the pipe and at the barometric damper, holding that acid against the metal. Oil smoke pipes are taken down and cleaned as part of regular chimney and burner service, and that is when thin spots and holes are usually found.

A smoke pipe that has rusted through on an oil system often goes with a chimney base full of scale and fallen tile. Both should be dealt with together, since a blocked base and a leaking pipe each make the other worse.

What a technician checks

The visit covers the pipe and everything it connects. The pipe's diameter, length, slope and joints are checked against the appliance's venting instructions. The thimble is checked for a tight, sealed connection. The chimney flue is measured and inspected for its size, liner condition, debris at the base and signs of water. A spillage check at the draft hood shows whether the system drafts properly with the appliance running and exhaust fans on.

The point is to find why the exhaust is condensing, not just where the rust is.

Support, slope and clearances

Corrosion is not the only reason connectors fail. Pipes that were never supported properly sag over time, which opens joints and creates low spots where condensate collects and rust starts. A connector should slope upward from the appliance to the chimney so exhaust and any water flow the right way, and it should be fastened at each joint as the appliance instructions direct.

Clearances matter as well. Single-wall connectors run hot and need more distance from joists, wood shelving and stored boxes than double-wall pipe. Basements fill up over the years, and items stacked against a hot flue pipe are a fire risk that costs nothing to fix: move them.

How it gets fixed

A rusted connector is replaced with the pipe type and size the appliance instructions call for, sloped up toward the chimney, with joints fastened and supported properly. That part is straightforward. The more important part is the cause. Where the chimney is oversized or unlined, a stainless liner of the right diameter keeps the exhaust warm and moving so it does not condense. Where a water heater has been left alone in a big flue, it gets its own sized liner or is replaced with a model that does not use the chimney.

On long runs through cold basements, a shorter route or double-wall pipe may be part of the answer, within what the appliance instructions allow. Gas venting work often needs a licensed installer, and permits vary by town.

Why the rust comes back

Replacing only the pipe is the most common half-fix. New galvanized pipe on top of the same oversized, cold chimney will start rusting again, sometimes within a season or two. If you have replaced the flue pipe before and see rust again, that is strong evidence the chimney itself needs attention.