Why does a new furnace make the old chimney flue a problem?

Chicago Chimney & Fireplace Repair · November 1, 2026

How common this is around Chicago

Census data from the 2020–2024 American Community Survey show that 77.5% of homes in Chicago heat with utility gas, and the share runs from 75% to over 90% across our suburbs. Berwyn sits at 91.3%. Most of those houses were built with the furnace and water heater sharing one flue inside a brick chimney.

Many of the same houses are old. The same survey puts 38.8% of Chicago's housing units, about 495,700 homes, in buildings from before 1940, along with 60.5% in Berwyn and 59.3% in Oak Park. When an old furnace gets replaced, the chimney rarely gets a second thought.

What changes when the furnace leaves the chimney

An older furnace sends a large amount of hot exhaust up the flue, which warms the clay tiles and keeps a steady draft going. A high-efficiency furnace pulls so much heat out of its exhaust that it vents through plastic pipe, usually out the side of the house, and the chimney loses that heat source.

The water heater on its own sends a much smaller, cooler stream of exhaust into a flue built for two appliances. In cold weather it can take longer for the draft to start, and the water vapor in the exhaust can condense on the cold tile walls before it reaches the top.

• Condensation is mildly acidic and wears at mortar joints and clay tile over time.
• Moisture can show up as stains, flaking or white deposits on the chimney walls or around the cleanout.
• A slow draft can let exhaust spill at the water heater's draft hood, especially when the house is closed up in January.
• Pieces of tile and mortar can fall to the base of the flue and partly block it.

Why the CO rules make this worth checking

The City of Chicago requires a CO alarm in the same room as every fuel-burning furnace, boiler and water heater, on top of Illinois' rule for alarms within 15 feet of sleeping rooms. That alarm is the last line of defense. A flue that drafts properly is the first.

Nicor Gas recommends having gas appliances inspected at least once a year and warns that snow or ice over a gas vent can cause carbon monoxide to build up. A new furnace's side-wall vent needs to stay clear in winter, and the water heater's chimney flue needs to stay open.

The fix: a liner sized for what is left

A liner gives the water heater a smooth flue of the right size from the basement to the top of the chimney. For gas appliances, an aluminum liner typically costs $625 to $2,250, and a stainless steel liner runs $900 to $3,800. If broken clay tiles have to come out first, that work usually adds $500 to $2,500.

Which liner fits depends on the appliance, the run of the flue and what is left of the tile. We have not found a City of Chicago page that names liners in its permit rules, so we check what your city requires for each job.

What we look at during the free inspection

We trace which appliance vents into which flue, look down the flue with a light from the top, check the base and cleanout for fallen tile, and look at the draft hood and connector pipe in the basement. If something hidden is suspected, a camera scan of the flue is priced at the inspection. You get photos and a written price either way.