Masonry Chimney Relining — IRC 2024 M1805.3 Requirements for Mechanical Equipment

Quick Answer

Under IRC 2024 Section M1805.3, when an existing masonry chimney is reused for new mechanical equipment, the chimney must be inspected and, if the existing liner is defective or the flue dimensions do not match the appliance requirements, a new liner must be installed. The most common relining method for gas appliances is a flexible stainless steel liner pulled through the existing masonry flue, sized per NFPA 54, with insulation in the annular space for cold climates. A cleanout tee at the base and a listed rain cap at the top are required, and the work typically requires a mechanical permit.

What IRC 2024 Actually Requires

Section M1805.3 requires that chimneys used with mechanical appliances have an approved lining system that is in serviceable condition and sized for the appliance being connected. The IRC defines three acceptable liner types for existing masonry chimneys: (1) the existing terra cotta or fired-clay tile liner if it is in good condition and the flue dimensions are correct; (2) a cast-in-place poured refractory liner system; and (3) a listed factory-built chimney liner inserted into the existing masonry shell.

For residential gas appliance relining, the flexible stainless steel liner — typically Type 316L or Type 904L stainless corrugated tubing listed to UL 1777 — is by far the most common method because it can navigate minor chimney offsets that would prevent installation of rigid liner sections, and because it is available in continuous lengths that avoid intermediate joints. The liner must be continuous from the cleanout tee connection at the base to the termination cap at the top, with no splices.

The liner diameter must be selected from NFPA 54 vent capacity tables based on the appliance BTU input, fuel type (natural gas or propane), and total chimney height from the bottom connection point to the top of the termination cap. Typical residential water heater relining with a single appliance of 40,000 to 120,000 BTU input in a 20- to 30-foot chimney requires a 3-inch to 5-inch liner, far smaller than the typical 8-inch by 12-inch original masonry flue.

At the base of the liner, a cleanout tee — a factory-manufactured tee fitting with a removable plug at the bottom — must be installed to allow access for annual chimney cleaning and to provide a connection point for the appliance vent connector. The cleanout plug must be accessible and must not be sealed permanently.

At the top of the chimney, the liner must be secured with a stainless steel liner clamp at the chimney crown and covered with a listed rain cap that provides at least 12 inches of clearance between the chimney crown and the underside of the cap. The annular space between the liner and the existing masonry should be filled with poured insulation (vermiculite or calcium silicate granules) to maintain liner temperature and improve draft, particularly for exterior chimneys in cold climates where the masonry acts as a heat sink.

Why This Rule Exists

Masonry chimney relining is required because the original terra cotta tile liner in most pre-1990 chimneys was designed for the combined BTU input of multiple appliances that may no longer be connected, or for oil-fired appliances that have since been converted to gas. An oversized or damaged liner allows flue gas to cool below the dew point, producing acidic condensate that penetrates tile joints, saturates mortar, and eventually causes structural chimney failure — a process that takes 5 to 15 years but is essentially irreversible without significant masonry repair.

Terra cotta tile liners also crack under repeated thermal cycling, particularly in climates with freeze-thaw cycles, and are attacked by sulfuric acid condensate from gas combustion. Cracked or missing tile sections allow flue gas to migrate through the masonry and into the building through mortar joints in the chimney breast.

The relining requirement ensures that the appliance operates with a correctly sized, chemically resistant, and structurally sound flue system throughout the appliance’s service life, typically 20 to 25 years. A properly installed stainless liner outlasts the appliance and does not require replacement at the next appliance change if it remains correctly sized.

What the Inspector Checks at Rough and Final

Before a relining permit is signed off, many jurisdictions require a chimney inspection report from a CSIA-certified chimney professional documenting the condition of the existing masonry, the dimensions of the existing flue, and the presence or condition of any existing liner. This report is submitted with the permit application to demonstrate that relining is necessary and that the masonry is structurally sound enough to support a liner installation.

At final inspection, the inspector checks that the liner material and diameter are documented and match the NFPA 54 sizing for the connected appliance. They check that the cleanout tee is accessible, that the top cap is a listed cap (not a generic galvanized cap), and that the liner is secured at the top. If annular space insulation was specified as part of the installation, the inspector may check that it was installed and that the top is sealed with a liner plate against rain intrusion.

What Contractors Need to Know

Before pulling a liner, have a chimney camera inspection performed to identify any offsets greater than 30 degrees, collapsed sections, or debris blockages. A flexible liner can typically navigate offsets up to 30 degrees, but tighter bends may require a rigid offset section or may preclude liner installation in that chimney. Finding a severe offset after purchasing the liner is an expensive discovery; the camera inspection prevents it.

When the liner is dropped from the top, attach the pull cone to the leading end and attach a pull rope. The liner must be dropped top-first (top connection end leading) so that the corrugated ribs face in the correct direction for condensate drainage. Inserting the liner backward reverses the direction of the ribs and creates condensate traps at every corrugation peak, dramatically accelerating corrosion.

For two-appliance common vent systems where both a furnace and water heater were on the original chimney, and the furnace is being converted to a condensing model with its own side-wall vent, the water heater liner is sized only for the water heater BTU input. The liner must be sized individually for this reduced load — using the combined BTU sizing from the original system will result in an oversized liner for the orphaned appliance.

What Homeowners Get Wrong

Many homeowners attempt to skip the relining step when replacing a water heater, assuming that if the old water heater was connected to the chimney, the new one can be too. However, if the original chimney was shared with a furnace that has since been replaced with a condensing model, the chimney is now oversized for the water heater alone. The absence of the old furnace means the new water heater is operating in the orphaned condition described in M1805, and relining is required even if the chimney appears to be in good condition.

Another misconception is that a larger chimney provides better draft and is therefore safer. In reality, an oversized chimney cools the flue gas faster than a correctly sized one, reducing the thermal buoyancy that drives draft. The physically larger flue is counterproductive — smaller is better in a correctly sized vent system. This is counterintuitive and is a frequent source of homeowner resistance to the relining requirement.

State and Local Amendments

New York City requires a signed letter from a registered design professional (PE or architect) certifying the chimney relining design whenever the work involves a Category III or IV appliance or a masonry chimney with any structural deficiencies noted in the inspection report. California requires the chimney inspection report as part of the permit package, and some Bay Area jurisdictions require a licensed contractor (not a handyman or self-install) for all chimney relining work.

In Colorado and other high-altitude states, NFPA 54 BTU de-rating for altitude must be applied to the appliance input before selecting the liner diameter. At 5,000 feet above sea level, gas appliances typically operate at 88 percent of their sea-level BTU rating, which affects the appliance output but also affects the flue gas temperature and draft characteristics. The liner sizing calculation must account for this de-rating.

When to Hire a Professional

Chimney relining is a professional trade skill. The work requires safe access to the roof, knowledge of NFPA 54 sizing, chimney inspection experience, and familiarity with liner installation hardware and techniques. Most jurisdictions require a mechanical permit for chimney relining, and the permit holder must be a licensed contractor in most states.

Even if DIY is technically permitted in your jurisdiction (some states have no contractor licensing requirement for this work), the consequences of an incorrectly installed liner — carbon monoxide exposure from a failed joint, structural chimney damage from an oversized liner causing condensate, or draft failure from an undersized liner — are severe enough to justify professional installation and inspection. Hire a contractor who specializes in chimney liner installation and can provide a sizing calculation documented to NFPA 54.

Common Violations Found at Inspection

• Liner installed in the wrong direction (bottom-first instead of top-first), reversing the corrugation ribs and creating condensate traps that accelerate corrosion
• Liner diameter selected from original combined-BTU sizing rather than resized for the single orphaned appliance, resulting in an oversized liner that perpetuates the condensation problem the relining was supposed to solve
• No cleanout tee at the base of the liner — annual cleaning is impossible and blockage accumulates unseen
• Generic galvanized rain cap used at the top instead of a listed stainless cap compatible with the liner material — galvanized caps corrode rapidly from acidic flue gas condensate
• Liner not secured at the chimney crown with a liner clamp, allowing the liner to shift or descend over time under its own weight in tall chimneys
• No annular space insulation in an exterior chimney in a cold climate, resulting in poor draft performance and condensate problems during cold weather startup
• Liner spliced with overlapping sections rather than installed as a single continuous piece, creating a potential leakage point at the lap joint
• No permit pulled for the relining work, so no NFPA 54 sizing verification was performed and no inspection confirmed proper installation