CO Detector Alarms After Fireplace Insert Installation: Cross-System Commissioning Decision Tree

Why this matters

A CO detector alarm after a new fireplace insert installation is a safety event, not a nuisance callback. The commissioning technician needs to confirm whether the detector is responding to actual CO spillage or to an unrelated trigger before the appliance is used again. Getting the sequence right protects the occupants and protects the installer from liability if the alarm is real.

Step 1: Confirm the detector is responding to CO

Before opening the flue or inspecting the insert, confirm the alarm type.

• Check the detector display or indicator pattern. CO alarms and smoke alarms are often co-located in combination units. A CO alarm will show a CO-specific code or pattern; a low-battery chirp or smoke detection event has a different signal.
• Note when the alarm sounded: during appliance operation, shortly after shutdown, or with the appliance not running. CO spillage during operation or within 30 minutes of shutdown is the high-concern scenario.
• If the alarm sounded with the appliance not running and the home has other combustion appliances (gas furnace, water heater), the insert may not be the source. Identify and rule out other appliances before focusing on the insert.

If the alarm is confirmed as a CO response during insert operation, continue to Step 2.

Step 2: Check the collar connection for gaps

The single most common commissioning cause of CO spillage from a newly installed insert is an incomplete seal between the insert collar and the liner top or the liner adaptor at the firebox.

Inspect the connection:

• Is the insert collar fully seated and secured to the liner connector? A partial fit leaves a gap where combustion gases bypass the liner and enter the room air.
• Is the liner adaptor at the appliance connection fully sealed with the correct high-temperature sealant? Compression fittings that are hand-tight but not torqued to specification can leak under operating pressure.
• Is the liner hanging correctly and not pulled away from the collar by its own weight? A liner that is too long and coils at the bottom can pull the top connection partially open.

Correct any gaps found. Re-seal with listed high-temperature liner sealant. Test with the appliance operating and a CO detector at the connection point before closing the job.

Step 3: Verify liner sizing for the insert output

An undersized liner cannot carry the full flue gas volume from the insert at rated output. When the liner is undersized, gases back up and spill at the appliance collar or the smoke chamber before reaching the liner.

Check the liner diameter against the insert manufacturer's minimum flue size specification for the appliance's BTU rating:

• A liner that is one nominal size smaller than the minimum specified will spill gases during high-fire operation even if the connection is perfect.
• Spill is most likely at startup and during high-fire operation when gas volume is greatest.
• A liner that is too large for the appliance can also reduce draft velocity enough to allow condensation and poor draw, though CO spillage from an oversized liner is less common.

If the liner is undersized, the insert must either be replaced with a lower-output unit that matches the liner, or the liner must be replaced with the correct diameter. There is no adjustment that compensates for an undersized liner.

Step 4: Evaluate combustion air supply

A tight modern home or a recently weatherized older home may not have adequate makeup air for the insert's combustion air requirement. When the home is depressurized, the insert draws combustion air from the room itself and competes with exhaust fans for the available air.

Test for depressurization:

• Light a small fire in the insert. Open a window or exterior door by 25-50 mm near the appliance. If draft improves and CO spillage stops, the home lacks adequate combustion air for the insert.
• Check whether the insert's combustion air requirement is met by the installation. Many inserts require a dedicated outdoor combustion air supply if the home is below a specified air-leakage rating.

If depressurization is confirmed, a dedicated combustion air inlet sized per the appliance installation instructions is the correct solution. Advise the homeowner that operating kitchen exhaust fans or bath fans simultaneously with the insert will worsen the condition until the inlet is installed.

Decision tree summary

• Alarm during insert operation + gap at collar connection: collar seal fault (Step 2)
• Alarm during high-fire only + no collar gap: liner undersized (Step 3)
• Alarm resolves when window is opened: combustion air deficiency (Step 4)
• Alarm with appliance not running: other combustion appliance source - remove insert from diagnosis

References

• NFPA 211: Standard for Chimneys, Fireplaces, Vents, and Solid Fuel-Burning Appliances - combustion air and liner sizing requirements
• UL 1777: Standard for Chimney Liners - liner performance and installation standards
• CSIA Certified Chimney Sweep field practices for insert installation inspection
• UL 2034: Standard for Single and Multiple Station Carbon Monoxide Alarms - alarm signal pattern interpretation