Is an Aluminum Chimney Liner Good Enough?

The narrow job aluminum was designed for

A flexible aluminum liner is a thin, corrugated tube lowered down a masonry flue to carry exhaust from a gas appliance. It is light, it bends around offsets, and the material costs less than stainless. Those strengths come with tight limits, and the liner's listing, printed in its maker's instructions, spells them out: which appliances, which fuel, what diameter and what clearances.

In practice, aluminum belongs on gas appliances that vent by natural draft: water heaters with a draft hood, and many standard and mid-efficiency furnaces and boilers. The appliance has a say too. Its own installation manual and the gas venting tables decide whether it may vent into a lined masonry chimney at all, and at what liner size.

• Gas water heaters with a draft hood, when the liner is listed for them
• Standard and mid-efficiency gas furnaces and boilers, when listed and sized from the venting tables
• Not wood fireplaces, wood stoves, inserts or coal stoves
• Not oil boilers, oil furnaces or oil-fired water heaters
• Not high-efficiency condensing units, which vent through their own specified piping

Why wood, coal and oil are off the table

Heat is the first reason. Aluminum melts at a temperature far below what a chimney fire reaches, and a hot wood fire alone can push flue temperatures into the range where thin aluminum loses strength. A creosote fire inside an aluminum-lined flue can leave a sagging, holed liner with flames and gases reaching the masonry behind it.

Chemistry is the second. Oil and coal exhaust carry sulfur and soot that settle as an acidic scale, and aluminum has little resistance to it. Gas exhaust is far cleaner, but it is wet, and that brings its own problem.

A common trap is a flue that once served a gas water heater and is later reused for a basement wood stove or a fireplace insert. The aluminum liner left in place does not suit the new appliance and has to come out before the first fire.

What shortens an aluminum liner's life

Water is the usual culprit. Burning natural gas or propane produces a lot of water vapor. In a warm flue that vapor leaves as a gas. In a cold one it condenses on the liner wall, picks up acidic combustion byproducts and slowly eats the thin metal. The coldest flues tend to be the ones on an outside wall, like the gable-end chimney on many New Jersey and Delaware colonials, and the tall, wind-chilled stacks above three- and four-story rowhouses.

Several conditions speed that up, and an older house often has more than one of them.

• A liner larger than the appliance needs, so exhaust moves slowly and cools
• A water heater left alone in the flue after the boiler was replaced with a sidewall-vented unit
• Short burner cycles that end before the liner has warmed up
• A missing cap or failed top seal that lets rain run down the inside
• Bleach, laundry products, paint strippers or softener salt stored near the burner, whose fumes form corrosive acids in the flame

Aluminum and stainless compared

Both materials can be correct for a given flue. The table shows where they part ways.

Signs the liner in your flue is corroding

Failures tend to show at the bottom first. A grey or white powder, sometimes in crusty flakes, on the floor of the cleanout or inside the connector tee is aluminum oxide from a liner breaking down. Small holes, a crushed section seen from the top, or a liner that has slid down and pulled away from its connector are further signs.

Around the appliance, watch for soot or rust streaks at the draft hood, water dripping from the vent pipe, and warped or melted plastic fittings on top of a water heater. Each one points to exhaust spilling into the room instead of rising. Carbon monoxide from spillage is dangerous and has no smell. Keep CO alarms near the appliance and on every sleeping floor, and if one sounds, leave the house and phone 911 or your gas company once you are outdoors.

How long one lasts

No single lifespan can honestly be quoted, because the liner's surroundings decide it. A correctly sized, capped aluminum liner serving one listed water heater in a warm interior chimney can give long service. The same liner in an oversized exterior flue, short-cycling through a hard winter, can corrode through in a fraction of that time.

NFPA 211 calls for chimneys and vents to be inspected once a year. On an aluminum-lined flue that yearly look earns its keep: the cleanout and the top show how the metal is holding up well before a hole opens.

What we check on an aluminum-lined flue

The visit starts at the appliance. The technician confirms what the liner serves, reads the data plate and the venting section of the manual, and checks the connector pipe for slope, tight joints and rust. With the burner running, the draft hood is tested for spillage, because a liner that is too big, partly blocked or collapsed often gives itself away there first.

• The cleanout: powder, flakes, water or fallen debris at the base
• The connector tee, its cap and the seal where the liner enters it
• The full height of the liner from the top: dents, kinks, holes and slippage
• The top plate, the cap and the crown around them
• Whether another appliance or a fireplace has been tied into the same flue

Deciding which liner fits your flue

Plan for what the flue will serve in coming years, not only today. Many homes in the region, from Long Island capes to Philadelphia twins, are mid-change: an oil boiler converted to gas, a boiler swapped for a sidewall-vented model that leaves the water heater on its own, or a plan to move to heat pumps. An aluminum liner sized for today's appliance can be the wrong size, or the wrong material, after the next change.

We check the appliance's venting instructions, the chimney height, which wall it sits on and how cold the flue runs. Where both materials are reasonable, aluminum and stainless go side by side in a free written estimate so the choice is yours.