Liners for Furnace, Boiler and Water Heater Flues

Chicago heats with gas. In the city and suburbs we serve, 75 to 91 percent of homes use utility gas as their main heating fuel (Census ACS 2020–24), against about 47 percent nationally, and Berwyn is at 91.3 percent. In the city alone about 495,700 homes were built before 1940. In a lot of them, the furnace or boiler and the water heater beside it still vent into a brick chimney with clay tile flues built for coal or wood.

Gas exhaust carries a lot of water vapor. In a big, cold masonry flue it cools before reaching the top, condenses, and the acidic water works on clay tile and mortar from the inside. Over years the tiles crack, mortar drops to the base and debris can partly block the flue. A liner sized to the appliances keeps the exhaust warm and moving, and keeps carbon monoxide inside a sealed path to the outdoors.

Signs you need this

• A new high-efficiency furnace venting out the side, and the water heater still in the chimney
• Rust, water stains or white crust on the chimney wall in the basement
• Crumbled tile or mortar at the cleanout door
• A CO alarm going off near the furnace, boiler or water heater
• Moisture dripping where the vent pipe enters the chimney

How we do it

1. Free flue checkWe look at every vent connector, the cleanout and the top of the chimney. If hidden damage is suspected, a camera scan is priced at the inspection.
2. Size the liner to the appliancesThe liner diameter comes from the furnace, boiler and water heater it will serve, because an oversized flue is often the very problem.
3. Install from the topThe liner goes down the flue, connects to the appliance at the base with a sealed fitting, and gets a cap at the top.
4. Fire and check the draftWe run each appliance and confirm the exhaust is pulling up the new liner before we leave.

The orphaned water heater

This is the most common liner job in Chicago basements. The old furnace and the water heater shared one big clay flue. Then a high-efficiency furnace went in, venting through the wall in plastic pipe, and the water heater was left alone in a flue sized for both. Its small, cool stream of exhaust can't warm that much masonry, so it condenses, drafts weakly and can spill back into the basement. If your furnace was replaced and the water heater still goes into the chimney, have the flue checked.

Chicago's carbon monoxide rules go further than the state's

Illinois law (430 ILCS 135) requires a working CO alarm within 15 feet of every room used for sleeping in homes with a fossil-fuel heater, a fireplace or an attached garage. The owner installs the alarms and the tenant tests them and replaces batteries.

The City of Chicago adds two more places: inside any bedroom that has a fuel-burning appliance or fireplace, and in the same room as every fuel-burning furnace, boiler and water heater. According to the Department of Buildings, owners face fines of $500 to $2,000 per day for each violation. In a two-flat, that means each unit's furnace and water heater room needs its own alarm, and the owner provides them.

Aluminum or stainless steel?

Aluminum liners are made for gas appliances only and cost less, typically $625 to $2,250 installed in 2026. Stainless steel liners typically run $900 to $3,800, last longer and suit more kinds of appliances. We suggest aluminum only when the flue will never serve anything but gas equipment. Where both fit, both go on the written quote.

If you smell gas, leave the house first and call from outside: Peoples Gas at 866-556-6002 in Chicago, or Nicor Gas at 888-642-6748 in most suburbs.