Lennox GCWB95 28-29 — Flue NTC Heat Exchanger Sensor Fault
28-29
Flue NTC heat exchanger sensor fault. The flue NTC heat exchanger sensor faulted; the manual directs calling an authorized service technician. The table classifies it as Error.
If a carbon-monoxide alarm sounds
- Leave the building immediately and get fresh air.
- Call 911. Do not go back inside until emergency services say it is safe.
- Have every fuel-burning appliance inspected before use.
Only after that, call ROMA about the boiler. Official guidance: Health Canada on carbon monoxide.
Applies to
This venting or air-supply fault code is displayed by the GCWB95 control (numeric error display with RESET) on GCWB95 models (GCWB95W-150, GCWB95W-205 wall hung combi and GCWB95F-150 floor mount combi), as documented in Lennox GCWB95 User's Information Manual, PN 240012894 Rev. A [04/15/2020], Section 3.6 error messages (models GCWB95W-150, GCWB95W-205 wall hung combi and GCWB95F-150 floor mount combi) (240012894 Rev. A). Codes can differ between controller generations, so always check the manual that matches your unit.
Controller: GCWB95 control (numeric error display with RESET). Source manual: 240012894 Rev. A (2020-04-15).
Why a code is not a diagnosis
A venting or air-supply code points at the combustion-air path: a blocked terminal, a restricted flue, a failed fan, or a condensate restriction. It does not say which, and running an appliance with a blocked flue is unsafe.
Safe things to observe
- If a carbon-monoxide alarm sounds anywhere in the building, leave immediately and call 911 from outside.
- From a safe distance, check that the vent terminal and combustion-air inlet are not blocked.
- Do not run the appliance while a blockage is suspected.
Do not
- Do not put anything into the vent or air-intake openings.
- Do not remove or modify vent pipe.
How professional service helps
- A technician inspects the full vent and combustion-air path, clears blockages and checks the fan.
- Combustion and safety devices are verified with an analyser and the manufacturer's procedure after any repair.
Where ROMA provides on-site service
On-site service is currently offered in Greater Vancouver, BC, including Vancouver, Burnaby, Richmond, Surrey, Coquitlam, North Vancouver, West Vancouver, New Westminster, Port Coquitlam, Port Moody, Delta, Langley, Maple Ridge, Abbotsford, Chilliwack, White Rock, Pitt Meadows, Tsawwassen, Anmore, Lions Bay, Belcarra, Bowen Island, Mission. Everything else on this site is reference information you can use anywhere in Canada.
Parts and service
Parts commonly involved
These parts appear in the documented text for this alert. A code reports a condition, not a failed part — a technician confirms the cause by testing before anything is replaced.
Heat exchanger
The heat exchanger is where a boiler transfers heat from the burner flame to the water that heats your home.
What a technician checks →Boiler venting (flue and air intake)
A condensing boiler vents cool, wet flue gases through dedicated plastic pipe that must meet ULC S636 in Canada and the boiler manufacturer's own…
What a technician checks →Thermistor / NTC sensor
A thermistor is the small temperature probe — technically a negative temperature coefficient, or NTC, resistor — that tells a boiler's control how…
What a technician checks →Boiler sensors (thermistors)
Boiler sensors are small temperature probes, usually thermistors, that tell the control how hot the water and flue gases are so it can fire safely…
What a technician checks →
Service for this alert
- Gas Boiler Repair
Licensed gas technicians from ROMA Heating & Cooling diagnose and repair residential and light-commercial gas boilers across…
- Gas Boiler Replacement
ROMA replaces old, failed or inefficient gas boilers in Greater Vancouver, handling removal, venting and gas-supply changes,…
Every boiler part explained — what it does, how it fails and what a technician checks.
Documented for Lennox Lennox GCWB95 (gas-fired condensing combination water boiler).
Frequently asked questions
Is this code a carbon-monoxide danger?
The code relates to combustion or venting, so treat any CO alarm as an emergency and call 911 from outside. The appliance itself should be checked by a technician before it runs again.
Can I clear this code myself?
Clearing a code without fixing its cause usually just brings it back. Note the code, check the safe items on this page, then call a licensed technician. If your appliance manual describes a user reset for this exact code and it stays away, no call may be needed.
Boiler locked out? Call with the alert text ready.
The brand, model and this alert are passed along when you request service online, so the technician arrives prepared.
On-site service in Greater Vancouver, BC. ROMA lists “Open 7 days” on its site; call to confirm same-day availability.
Sources and verification
- Lennox GCWB95 User's Information Manual, PN 240012894 Rev. A [04/15/2020], Section 3.6 error messages (models GCWB95W-150, GCWB95W-205 wall hung combi and GCWB95F-150 floor mount combi) · 240012894 Rev. A · accessed September 9, 2026
Facts checked September 9, 2026. Last updated September 9, 2026. Technical reviewer: not yet assigned.