Does My Furnace or Water Heater Chimney Need a Liner?
Why a gas furnace or water heater venting into an old masonry chimney can damage it, the signs, and what a stainless liner costs in Northwest Indiana.
NWI Chimney · October 4, 2026
The short answer
Often, yes. Gas exhaust is full of moisture. In an old, unlined or oversized brick chimney, it cools, condenses and soaks the masonry from the inside — especially after a high-efficiency furnace is installed and the water heater is left alone in the chimney. A stainless liner sized to the appliance fixes it, typically $1,200–$2,500, after a $199 camera inspection.
Why it happens
Older furnaces put out enough hot exhaust to keep the chimney warm. When the furnace is replaced with a high-efficiency model that vents through a plastic pipe out the side of the house, the water heater is often left venting into a chimney sized for two appliances. Its small, cool exhaust can't heat the flue, so water condenses inside — and in a Region winter, that water freezes in the brick.
Signs to look for
- White staining or crust on the chimney, inside or out
- Damp spots or peeling paint on walls around the chimney
- Crumbling mortar or bits of tile at the clean-out door in the basement
- Rust on the vent pipe or water heater draft hood
- Spalling bricks near the top of the chimney
The fix
We start with a Level 2 camera inspection ($199) to see the flue. A stainless liner sized to the water heater or furnace runs $1,200–$2,500 for a typical single flue; insulated liners for tall or offset flues $2,000–$3,500. A cap goes on top to keep rain out.
If you smell gas
Leave the house, don't touch switches, and call 911 and NIPSCO at 1-800-634-3524 from outside. Book a repair only after the utility has made it safe.
More answers
Who connects the gas appliance to the liner?
The appliance and gas connection are for a licensed HVAC or plumbing contractor; we install the liner and cap in the chimney.
Does a gas chimney need sweeping?
Gas doesn't make creosote, but the flue should be inspected — debris, nests and liner damage can block exhaust.
Is a carbon monoxide alarm enough?
Every home with fuel-burning appliances should have working CO alarms, but they warn you after the fact. A proper liner keeps the exhaust going out.
Is a Prefab Fireplace Worth Repairing?Factory-built fireplaces fail differently from brick ones. What usually breaks, what's worth fixing, and when a Northwest Indiana prefab unit should be replaced.
What Are the White Stains on My Chimney?White, chalky deposits on chimney brick are efflorescence — a sign water is moving through the masonry. What causes it and how to stop it in Northwest Indiana.Should I Get a Chimney Inspection Before Buying a Home?Why a general home inspection doesn't cover the inside of the chimney, what a Level 2 inspection shows, and what it costs in Northwest Indiana.Ready for a safer, cleaner chimney?
Same-week appointments, honest flat pricing, no scare tactics — and same-day emergency service when it can't wait. Licensed and insured.