What's In This Article
Most People Don't Know This
In a great many St. Louis homes — particularly anything built before the last decade or two — the gas water heater does not vent out through a wall. It vents upward, into the chimney.
Usually not into the same flue as the fireplace. Chimneys often contain more than one flue: one for the fireplace, one for the furnace and water heater. From outside you see one chimney. Inside, it's two or three separate vertical shafts.
So your water heater has a flue. That flue has a liner. And that liner has a service life.
Why the Liner Corrodes
A wood fire is hot and dry. Its flue gases rise fast and stay hot enough that they don't condense on the way up.
A modern gas water heater is different. It's efficient, which means less waste heat, which means the exhaust is cooler. Cool exhaust condenses on the flue wall on its way up. And that condensate is not clean water — it's mildly acidic, carrying combustion by-products.
Now run that, every day, for twenty years:
- Clay tile liners — the acidic condensate attacks the mortar joints between tiles first, then the tiles themselves. Joints open. Tiles crack and spall.
- Older metal liners — thin aluminum or galvanized corrodes through, usually at the joints and at the bottom where condensate collects.
It's slow. It's invisible. And it works from the inside out, which is why nothing about it is visible from the ground or from the basement.
What Actually Goes Wrong
The liner's job is to give the exhaust a sealed, continuous, correctly-sized path from the appliance to the outside air. Two things go wrong when it fails.
The exhaust leaves the flue
Once there are gaps — open tile joints, corroded-through metal — the exhaust can escape into the masonry of the chimney, into the wall cavity, or back into the house. That exhaust contains carbon monoxide. This is the reason the item matters.
The flue stops drawing properly
A liner that has partially collapsed, or that is oversized for the appliance, doesn't establish good draft. The exhaust rises slowly, cools further, condenses more, and can spill back at the appliance — a condition called backdrafting.
Whatever else you take from this article: if you have any gas appliance venting through a chimney, you should have working CO detectors on every level of the home. That's not a substitute for a sound liner. It's the thing that tells you when the liner has failed.
Why This Item Gets Missed by Everyone
It's a jurisdictional gap, not negligence.
- Your plumber services the water heater. They look at the tank, the valve, the anode, the connections. They do not go up on the roof.
- Your chimney sweep — if you have a fireplace — cleans and inspects the fireplace flue, because that's what you called them about.
- Your HVAC tech services the furnace, and may glance at the flue connection, but is not inspecting the liner along its full height.
- Your home inspector at purchase notes that the water heater is chimney-vented and moves on.
Everyone touches something adjacent to it. Nobody's job description includes it.
Which is why, when we ask a homeowner "when was your water heater's flue liner last inspected," the honest answer is very often: never.
Chesterfield: New Liner as Part of a Restoration
The homeowner called us about a cracked crown and failing mortar. The liner wasn't on their list — they didn't know it was a thing. We found it during the inspection, and because we were already on the roof for the crown and the tuckpointing, adding it cost a fraction of what a separate visit would have.
That's the practical argument for handling it as part of a restoration package: the expensive part of a liner job is getting access, and if the access is already paid for, the liner is comparatively cheap.
Signs Your Water Heater Liner May Be Failing
- Rust or corrosion on the top of the water heater, particularly around the draft hood
- Moisture, staining, or white mineral deposits on the chimney masonry in the basement or attic
- A CO detector that alarms intermittently, especially when the water heater fires
- Visible spalling or staining on the exterior masonry near the water heater flue
- A back-puff of exhaust at the draft hood when the burner lights
- The water heater is over 15 years old and the chimney has never been relined
AIO Pro Chimney — Liner Pricing
If your water heater vents into the chimney and nobody has ever looked at that liner, put it on the list. It's one visit, and then you know.
Frequently Asked Questions
Does a water heater really vent through the chimney?
In many older St. Louis homes, yes. Gas water heaters and furnaces commonly vent upward into a dedicated flue within the chimney stack — separate from the fireplace flue. From the outside it looks like one chimney; inside there are often two or three separate shafts.
How long does a water heater flue liner last?
Roughly 20 years is a common working figure. Water heater exhaust is relatively cool, so it condenses inside the flue, and that condensate is mildly acidic. It attacks clay tile joints and corrodes thin metal liners from the inside over time.
What happens if the water heater liner fails?
Two things. Exhaust — which contains carbon monoxide — can escape through gaps into the masonry, the wall cavity, or back into the home. And draft can deteriorate, causing exhaust to spill at the appliance rather than rising. Working CO detectors on every level are essential with any chimney-vented gas appliance.
Why doesn't my plumber check the chimney liner?
It falls between trades. Plumbers service the appliance, chimney sweeps are usually called about the fireplace flue, HVAC techs service the furnace, and home inspectors note the venting arrangement without inspecting the liner's full height. Nobody's scope includes it by default.
How much does it cost to reline a water heater flue?
Chimney relining with AIO Pro Chimney starts at $2,000, and costs less when done alongside other chimney work because the roof access and setup are shared. Inspection and written estimate are free.
Chimney-Vented Water Heater? Get the Liner Checked.
We check it on every inspection, free. If it's sound, we'll tell you so and you can stop thinking about it.
