Flexible Flue Liner Buckles or Collapses on First Use: Commissioning Fault Decision Tree

Why this matters

A flexible liner that collapses after the first fire is a commissioning failure, not a product failure. The homeowner loses function on day one, the appliance may backdraft, and the callback involves either a full re-pull or a repair - both of which are significantly more expensive than getting installation right the first time. Understanding which of three installation errors caused the collapse tells the technician exactly what to fix and whether the liner can be salvaged.

What collapse looks like on inspection video

Before diagnosing cause, confirm that a collapse is actually present. Lower a camera from the top of the liner after a cool-down (never enter the flue during or immediately after a fire).

• Buckle: a single inward fold at one point in the liner wall. The corrugated wall has creased inward on one side, reducing the effective diameter at that point. The liner is otherwise intact above and below.
• Kink: a sharp angle in the liner path where the liner has folded at a bend, typically at a horizontal offset or elbow point. Distinguished from a buckle by the location (always at a directional change) and the severity (often reduces effective diameter by more than 50%).
• Accordion compression: multiple adjacent corrugations have compressed together along a straight vertical section, like a compressed spring. This is the signature of insufficient tension during installation.

All three produce a visible restriction on the camera. Document the location (measured from the top in feet), the type, and the estimated remaining diameter at the restriction point before proceeding to root cause.

The three causes

Cause 1 - Insufficient tension during installation (most common)

Flexible liner is manufactured in a compressed state for easier handling. During installation, the liner must be pulled taut from the top while a second technician feeds it from below. If the liner is simply dropped or fed without continuous tension, it rests at its compressed length inside the flue. The first thermal expansion cycle heats the corrugated walls, the liner tries to expand longitudinally, and sections that had slack compress further into accordion folds.

Tells: the restriction is in a straight section, not at a bend. Multiple soft compressions visible on camera rather than one sharp crease.

Cause 2 - Undersized liner diameter

Each appliance has a minimum liner area requirement based on its BTU output, fuel type, and height of the flue. If the liner installed is smaller than the minimum required, the volume of flue gases generated exceeds the liner's capacity. The resulting pressure differential can draw the flexible liner wall inward at the weakest corrugation - typically a point where the wall was already slightly deformed in handling.

Tells: collapse occurs near the bottom third of the liner, closest to the appliance connection where gas volume and velocity are highest. The liner may appear intact on camera but significantly reduced in diameter.

Cause 3 - Bend radius exceeded

Every flexible liner has a minimum bend radius specified by the manufacturer. At horizontal offsets or where the liner must transition from a vertical to an angled path to reach an offset flue, the liner can be bent sharper than this minimum. The corrugated wall at the outer radius of a too-sharp bend deforms during installation, and the deformation collapses inward on the first heat cycle.

Tells: the restriction is at a direction change. The camera will show a single sharp kink at the bend point. The angle of the kink matches the angle of the flue offset.

Diagnostic decision tree

Step 1 - Locate the restriction

Camera inspection from the top. Record distance from the top in feet and type (buckle, kink, accordion).

• Restriction in a straight section with multiple soft folds -> Cause 1 (slack). Go to Step 2.
• Restriction in a straight section with a single inward crease, near the bottom -> Cause 2 (undersized). Go to Step 3.
• Restriction at a bend or direction change -> Cause 3 (bend radius). Go to Step 4.

Step 2 - Verify liner length vs. flue height

Measure the flue height from crown to appliance connection. Compare to the installed liner length.

• Liner length exceeds flue height by more than 150 mm? -> Excess liner has compressed. The liner was not tensioned and the excess bunched at the bottom or mid-point.
• Liner length matches flue height? -> Tension was lost during installation without excess length. Liner must be re-pulled with proper tension technique.

In both cases, the repair is a re-pull with correct tension. If the liner is undamaged (no sharp creases), it can typically be re-used. If creases are present, inspect for wall cracking before re-use.

Step 3 - Verify liner size vs. appliance requirement

Obtain the appliance BTU input rating and the manufacturer's minimum flue area or diameter specification. Compare to the installed liner diameter.

• Installed liner is smaller than the appliance minimum? -> Undersized liner. The liner must be replaced with the correct diameter. The undersized liner cannot be re-used on this appliance.
• Installed liner meets the minimum? -> A diameter-pressure collapse from a correctly sized liner is unusual. Re-check for Cause 1 (slack) or Cause 3 (bend) as contributing factors.

Step 4 - Verify bend radius

Identify the flue offset location. Measure or estimate the bend angle. Compare to the liner's minimum bend radius (expressed as minimum bend diameter or minimum angle in product specifications).

• Bend is tighter than the minimum spec? -> The liner kinked during installation. If the liner is intact at the kink (no cracking), a rigid offset connector section can be inserted at the bend point to relieve the radius demand on the flexible liner. If the liner wall is cracked at the kink, full replacement is required.
• Bend is within spec? -> The kink was caused by handling or installation force, not geometry. Inspect the liner wall at the kink for cracking. Intact wall: re-pull with a guide to maintain bend radius. Cracked wall: replace the liner.

Repair vs. replace decision

Documentation

Record the following before repair:

• Liner diameter installed vs. appliance minimum required
• Flue height and installed liner length
• Camera footage showing restriction location and type
• Bend angles at any offset points
• Whether liner was re-used or replaced

References

• National Fire Protection Association NFPA 211 - liner sizing requirements for solid fuel and gas appliances
• CSIA technical bulletin on flexible liner installation procedures
• Liner manufacturer installation instructions (minimum bend radius and tensioning procedures vary by product)