Furnace and water heater chimney liners explained

Ask a Rockford homeowner what the chimney is for and the answer is often “the fireplace.” In a gas-heated house it may be the furnace and water heater exhaust, with a fireplace attached or none at all. That flue has a job every day of the heating season. The chimney liner service page covers installation; this guide explains why a gas flue gets one.

Why this is a Rockford question

83.5% of Winnebago County homes heat with gas, and only 169 homes in the county heat with wood (Census ACS, 2020 to 2024). Rockford's median home was built in 1963; 22.4% of Rockford homes were built before 1940 and 46.5% before 1960. Put those together: a large number of Rockford chimneys are old masonry stacks built for coal or wood that now carry gas exhaust.

What the flue does for a gas appliance

A gas furnace or water heater makes exhaust gases that must leave the house. Carbon monoxide is one of them. The flue has to carry that exhaust out reliably and not leak it into the walls or the room. Nicor Gas, the area utility, says a yellow or orange flame can mean poor combustion or venting, and that blocked vents can let carbon monoxide into the home. A cracked clay tile, a gap between tiles or a nest in the flue can all do that.

The Consumer Product Safety Commission names furnaces, water heaters, chimneys, flues and vents in its call for a yearly professional inspection. The Illinois CO alarm law, covered in the carbon monoxide alarm guide, applies to homes that burn natural gas to heat or make hot water.

What Rockford and Loves Park require

Rockford's Property Standards say all fuel-burning appliances must connect to an approved chimney or vent. They also say a fireplace or wood-burning appliance cannot be a building's primary heat source. Loves Park's property maintenance amendment (Ord. 4090-16) requires every fuel-burning appliance to connect to an approved chimney or vent. That is the local code reason an old chimney gets a liner when an appliance changes. The wording is about the connection; neither city, in what we read, spells out liner specifications, so ask the building department or the installer what they will inspect.

Rockford's permit page says installing or replacing a heating appliance needs a mechanical permit, and the Loves Park Building Department lists mechanical work among the work that needs a permit. Chimney relining is not named either way in either city's pages. Call Rockford Construction and Development Services at (779) 348-7158 or the Loves Park Building Department at (815) 654-5033 before treating a new liner as permit-free.

When the appliance changes

CSIA recommends a Level 2 inspection when the appliance is replaced with a dissimilar one, such as a different fuel, efficiency or type. A new furnace or water heater is exactly that moment. The scan shows the existing liner's condition, and the installer can tell whether it suits the new appliance. A liner is sized for what it serves, and we found no Winnebago County source that sets a size rule, so rely on the installer and the appliance maker's instructions.

Liner types and national prices

No cost guide publishes a Rockford price. Angi puts a liner of any type at $1,500 to $5,000 (Angi, chimney liner cost, national, updated September 21, 2026), with labor $400 to $1,250. HomeGuide puts it at $1,500 to $4,000 (HomeGuide, chimney liner cost, national, updated February 9, 2026), a stainless steel reline at $900 to $3,800 (HomeGuide, chimney liner cost, national, updated February 9, 2026) and a clay or cast-in-place liner at $2,000 to $7,000 (HomeGuide, chimney liner cost, national, updated February 9, 2026). A job over $1,000 falls under the Illinois Home Repair and Remodeling Act, which names chimney liners and requires a signed written contract. See the chimney cost page for every range, and the liner guide for the other reasons to reline.

The simple version

• Find out what your chimney vents. Look at the flue pipe from the furnace and water heater to see whether it enters the chimney.
• Have the flue inspected every year, and with a camera when an appliance changes.
• Fix cracks and gaps before the heating season, not during it.
• Keep working CO alarms near the bedrooms.