Fireplaces and flues
The old brick chimney that vents your furnace or water heater
We camera-inspect, sweep, reline and cap the masonry flue that serves a gas furnace and water heater in older Portland houses, including the flue a new sidewall-vented furnace left behind with only the water heater on it.
Updated
What a furnace and water heater flue is and how it vents
A furnace flue is the passage in a brick chimney, often a second flue beside the fireplace flue or a separate stack in the middle of the house, that carries exhaust from a basement furnace and water heater up past the roof.
It vents by natural draft: hot exhaust rises, pulls air in behind it at the draft hoods and keeps the tile warm and dry. Inside is clay tile set in brick, with a cleanout door at the base and thimbles where the vent connectors enter; the earliest Portland stacks have bare brick.
Nearly half of Portland housing predates 1960 by Census count, the decades when a basement furnace vented into a brick flue, and some of those furnaces burned oil before a gas conversion. It matters today because a condensing furnace vents out a side wall and leaves the chimney to the water heater, or to nothing.
How to tell what you have
Four things tell you which flue you have and what still uses it.
- Sheet metal pipe rising from the water heater, and from an older furnace, into a thimble in the brick is the vent connector that feeds this flue.
- White plastic pipes leaving the furnace through a side wall mean a condensing unit that skips the chimney; whatever still enters the brick is the only load the flue has.
- A small iron door low on the chimney in the basement is the cleanout; soot, sand, tile chips or a puddle behind it tells the story of the flue.
- From the sidewalk, a slender stack through the middle of the roof with a small tile or a bare rectangular opening on top is the furnace flue in many of these houses.
What you will find in Portland homes
These are the furnace and water heater flues we meet in Portland basements.
| Type or model | Era | Notes |
|---|---|---|
| Furnace and water heater sharing one tile flue | Older houses | Two connectors, one flue, both appliances warming it; the yearly check is the connectors, the cleanout and the tile joints. |
| The orphaned flue after a condensing furnace | After a furnace upgrade | A water heater alone cannot warm a flue sized for two; the exhaust condenses and flakes the tile, and a liner sized to the heater is the fix. |
| The oil-era flue converted to gas | Former oil-heated houses | Oil soot still coats the tile or brick, sometimes with a barometric damper left on the connector; it is swept before the camera goes up. |
| The unlined furnace flue in an older brick house | Older brick houses | Bare brick, often in one stack with the fireplace flue; acidic condensate eats the joints from inside, and a liner is the answer whatever vents into it. |
What goes wrong and what fixes it
Each flue problem below comes with the service that answers it.
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A new furnace went in, or nobody knows what the chimney still vents
A Level 2 camera inspection from the cleanout to the top, with photos and a plain opinion Chimney Inspection, from $129
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Flaking tile, white crust and a damp cleanout under a water heater venting alone
A liner sized to the water heater, connected at the thimble and sealed at the top Chimney Liner Installation, from $1999
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An open flue top letting rain and animals into a working or dead flue
A cap on a working flue; a cap plus a sealed thimble on a retired one Chimney Cap Installation, from $299
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Oil soot, mortar sand and tile chips packing the base and the thimble
The flue swept down to the cleanout and the connector path cleared Chimney Sweep, from $249
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Efflorescence and spalling on the outside of the stack from a wet flue
Joints repointed and spalled brick replaced once the flue stops wetting them Tuckpointing and Masonry Repair, from $449
What happens to a brick furnace flue when the furnace stops using it
The flue goes cold, and a cold masonry flue with one small appliance on it runs wet. The water heater alone cannot warm a passage built for a furnace, so its exhaust condenses on the tile instead of leaving.
Gas exhaust carries a lot of water vapor and the condensate is acidic. Season by season it softens the mortar between tiles, flakes the tile face into the cleanout, rusts the draft hood and streaks the outside of the stack white.
The quiet risk is draft. A cold, oversized flue can fail to pull, and a spilling draft hood sends water heater exhaust into the basement. A liner sized to the water heater restores the draft and keeps the condensate in metal.
What we check on a furnace and water heater flue
We check the draft hoods and connectors, the cleanout and full flue on camera, and the tile, crown and cap on top.
At the appliances
Draft hoods for rust and spillage marks, connectors for slope, joints and screws, the thimble for a tight fit, and a draft test at the hood once the water heater fires.
The cleanout and the flue
The cleanout is opened and read first: tile chips for a flaking liner, sand for failing mortar, soot for an oil past. Then the camera goes from the cleanout to daylight looking for open joints, offsets, a thin wall into the fireplace flue and nests.
The top and the write-up
The tile, crown, cap and staining are photographed, and the write-up says whether the flue is fine as it is, needs a liner sized to what is left on it, or should be retired.
Reline it for the water heater, retire it, or leave it alone
There are three honest answers after the camera run: a liner sized to the water heater, a power-vent water heater with the flue capped, or nothing when the tile is sound and the draft proves out.
The liner is the usual answer when the water heater stays. It is sized to that one appliance so the exhaust stays warm and moving, and it belongs in the furnace installer's quote, so ask whether it was there.
Retiring the flue means a cap on top, a plate over the thimble and the cleanout shut; on a tall unbraced stack it can also mean taking the brick down to the roofline. Portland requires a mechanical permit for a change to what vents into a chimney.
Furnace and water heater flue questions
Why is there white staining and flaking tile at the base of my furnace flue?
Because the flue is running wet. Gas exhaust that cools inside an oversized or orphaned flue condenses, and the acidic water flakes the tile face, turns mortar to sand and carries minerals out through the brick as a white crust. A camera inspection shows how far up it goes.
My new furnace vents out the side wall. Does the old chimney flue still need anything?
Yes, a look, because the water heater is now alone on a flue built for two appliances. That flue runs cold and wet from the first winter, and the usual fix is a liner sized to the water heater, or a power-vent water heater and a capped flue.
Can a dead furnace flue in a Portland house just be capped and forgotten?
Capped, yes; forgotten, not quite. A flue with nothing on it needs a cap on the tile, a plate over the open thimble and the cleanout closed against rain, cold air and animals. The brick above the roof still needs a crown and sound joints, and a tall unbraced stack is still a quake hazard.
Is old oil furnace soot in the chimney flue a problem after a gas conversion?
It is a reason to sweep before anything else. Oil soot is fine, oily and acidic, it holds moisture against the tile and hides the joints from the camera, so the flue is brushed to the cleanout and vacuumed before the inspection. The oil tank is a separate job.
How do I know whether the furnace flue is backdrafting into the basement?
Rust, scorch marks or melted plastic on the water heater's draft hood are the signs. A damp sour smell when it fires and moisture on cold pipes nearby point the same way. If a carbon monoxide alarm sounds or anyone feels dizzy or sick down there, leave the house and call 911 first, then keep the water heater off until the flue is inspected.
Related pages
Furnace and water heater flue calls near you
Furnace and water heater flue work has taken us to Sellwood-Moreland, Wilsonville, Bridlemile and the rest of the metro, and the posted price is the same at every address.
Other fireplace and flue types
Ready to book a sweep?
Call with what the fireplace is doing, or text a photo of the opening and one of the chimney top, and we will name the service and its posted price before we come out.