Boiler displays FLUE or Flue Temp High; unit may derate or shut down.

Safety first: High flue temperatures can damage venting and pose a safety risk. Turn off power and gas if you suspect vent damage or smell flue gases indoors.

• Check system flow: Ensure all valves are open and circulators are running. Poor heat transfer due to low flow can raise flue temperature.
• Inspect venting: Look for discoloration, warping, or sagging of PVC or polypropylene vent pipes. Any damage requires immediate professional attention.
• Reset and observe: After cooling, reset the boiler and monitor operation. If FLUE returns quickly, stop using the boiler.

When to call a professional: A technician must check combustion, gas input, and inspect/clean the Laars heat exchanger and venting.