I get asked about an aluminum chimney liner for a furnace a few times a season, usually because another company quoted one or the furnace installer said that is what the job needs. The homeowner is not wrong to ask. It is a real product, it is sold for this, and in certain situations it is allowed.
My answer is still no, almost every time, and I would rather explain why than just say it.
What an Aluminum Liner Is and Where It Came From
A flexible aluminum liner is a thin, lightweight corrugated tube that drops down a masonry chimney to give a gas appliance a properly sized path to the outside. It is light enough to handle alone, it bends around an offset easily, and it is the least demanding liner to put in. Those are real advantages on installation day.
It exists because the codes that govern gas venting allow it in a narrow set of conditions, for natural draft gas appliances only. That is the category of appliance that pushes its exhaust up the flue on buoyancy alone, with no fan forcing it, and that does not deliberately condense its exhaust.
Gas only, and I mean only. Not wood. Not coal. Not oil. Oil burns hotter than gas and leaves a harsher residue behind it, and a solid fuel fire reaches temperatures that will destroy aluminum outright, never mind what a chimney fire would do to it. It has no business in a fireplace flue either. A fireplace is a wood appliance even when there is a gas log set sitting in it today, because the set can come out and somebody can light a real fire in there. If anyone offers you aluminum for a fireplace, an oil burner or a wood stove, that is the end of the conversation.
Here is the part I want to be plain about. Allowed and will last are two different claims. A code is a floor, not a promise. It tells you the minimum that is permitted to be installed. It does not tell you what that material will look like after eight winters in a cold brick chimney on the North Side.
The Problem: Flue Gas Condensation Eats Aluminum
Burning natural gas makes water. That is not a defect, it is just what the chemistry does. Hydrogen in the fuel combines with oxygen and goes up the flue as water vapor, along with the carbon dioxide and the heat.
As long as that vapor stays hot, it stays a gas and it leaves the chimney. The trouble starts when it cools below its dew point before it gets out. Then it turns back into liquid water on the inside wall of the flue, and that water is not clean. It picks up carbon dioxide and whatever trace sulfur came along with the fuel, and it comes out mildly acidic.
Now put that in a Pittsburgh chimney. A lot of our brick homes were built around a flue sized for a coal furnace or an old oil burner, and that flue is enormous compared to what a modern gas appliance needs. A small volume of exhaust going up a big cold masonry shaft loses its heat fast. It does not have the mass to keep the flue warm. So the gases drop below the dew point partway up, and the condensation is not a rare event. It happens on cycle after cycle, all winter.
Aluminum does not hold up to that. It is thin to begin with, and the acidic condensate works on it steadily. It pits, the pitting turns to flaking, the flaking thins the wall further, and eventually the liner opens up or breaks apart. Stainless steel made for gas venting is built to take that same condensate. Aluminum is not.
There is a detail here that catches people out, because it runs opposite to what you would expect. A newer mid-efficiency furnace, the 80 percent kind that still vents up the chimney, is better at pulling heat out of the fire than the furnace it replaced. Better for your gas bill. But the heat it keeps is heat that is no longer going up the flue, so the exhaust leaves the appliance cooler than the old one's did. Cooler exhaust condenses sooner. Putting a more efficient furnace on the same old chimney can make the condensation problem worse, not better.
One more thing, said once and plainly. A liner that has corroded through or collapsed is not just a worn out part. It can block the flue, or it can let exhaust escape into the chimney cavity instead of carrying it outside, and that exhaust contains carbon monoxide. That is the reason this matters more than a normal materials argument.
The Orphaned Water Heater Makes It Worse
There is one setup where everything above gets worse at once. The furnace gets replaced with a high-efficiency unit that vents out the side wall through plastic pipe, and the water heater is left behind on the big masonry chimney by itself.
A water heater fires in short bursts and puts out a fraction of the exhaust a furnace did. On its own it has no chance of warming that flue. The gases cool almost immediately, condensation runs heavy, and an aluminum liner in that chimney is in the worst conditions it could be asked to work in. This is also the most common liner situation I see in Pittsburgh, which is why I keep coming back to it. I wrote about the orphaned water heater problem separately if you want the full picture, and the gas furnace and water heater liner page covers how I handle the fix.
What a Failed Aluminum Liner Looks Like
I am not going to describe a job I have not written down, so let me stick to what this kind of failure looks like as a condition rather than as a story.
Corrosion on an aluminum liner does not show up as one neat hole. It shows as pitting first, small dull craters in a surface that should be bright. Around the pits the metal goes chalky and starts shedding. Because the liner is corrugated, the low points of the corrugations hold condensate longest, so that is where it goes first. By the time a section has perforated there is usually loose material down at the base of the chimney, and the thin wall near the damage will crumble under light pressure rather than bending the way sound metal does.
None of that is visible from inside the house. The liner runs inside the masonry, and the only parts anyone normally sees are the termination at the top and the connector down at the appliance. A liner can be well through this process while everything a homeowner can look at still appears ordinary.
What I Install Instead
Stainless steel, rated for gas appliances, sized to the appliance it is actually serving.
That sizing part matters more than people expect. A liner is not fitted to whatever diameter happens to slide down the chimney. It gets sized from the appliance manufacturer's instructions and the code sizing tables, against the input rating, the height of the chimney and the length of the connector. Too large and you are back to the same cooling and condensation problem the oversized masonry flue caused. Too small and the appliance cannot vent. A chimney carrying both a furnace and a water heater is a different calculation again.
Insulation goes on where the situation calls for it. Wrapping the liner keeps the flue gases hotter on the way up, which answers the condensation problem rather than tolerating it. On a tall exterior chimney on a cold wall, it earns its place.
The top gets a proper termination and a cap, sealed at the crown so water running across the masonry cannot get down between the liner and the old tile.
On alloy, I will tell you what I am putting in and why before the work starts, and any installer should be able to do the same. Different alloys are made for different fuels and appliance types, and the right one depends on what is venting into the chimney. If someone cannot answer which alloy they are installing and what it is suited for, that is worth noticing.
When Aluminum Is Fine
Aluminum is permitted for certain natural draft gas appliances, it is sold legitimately, and there are installations that have gone the distance without trouble. A properly sized flue in a warm interior chimney, serving an appliance it was listed for, is a much kinder environment than the one I described above.
My argument is not that the material is fraudulent. It is that our housing stock stacks the conditions against it. Oversized flues built for other fuels, exterior chimneys on cold walls, and orphaned water heaters are the normal case here, not the exception. In those conditions I do not think it is worth gambling on, and the difference between the two materials is not a good reason to take that bet when the failure mode involves carbon monoxide.
How to Tell What You Have
Four things you can do without climbing anything.
Look at the top of the chimney from the ground, with binoculars if you have them. If a liner was installed you can often see the termination at the crown, a metal collar or plate where the liner comes out, rather than bare clay tile.
Look at the connector pipe in the basement where the furnace or water heater meets the chimney. You cannot see the liner itself from there, but you can see whether the connection looks deliberate and sealed or improvised.
Find the paperwork from the furnace installation. If a liner went in with a new appliance it should be on the invoice, and the material and size should be named.
If you cannot answer it any of those ways, that is what an inspection is for. A Level 2 chimney inspection is the one that applies when the appliance venting into the flue has changed or the house is changing hands, and it is the right call whenever nobody can say for certain what is inside the chimney.