F6 displayed, water heater will not fire, fan may not start

Safety: Disconnect power before removing the fan or pressure tubing. Do not run the unit with the fan housing open.

• Inspect flue and air intake: Check the concentric flue terminal outside for obstructions. Ensure intake and exhaust pipes are not crushed or disconnected.
• Check fan operation: With the front cover removed and power restored, call for hot water and observe if the fan attempts to start. If the fan hums or does not spin, power off and check for debris in the impeller. Spin the fan by hand; if stiff or noisy, replace the fan assembly.
• Inspect pressure tubing: Locate the air pressure switch or differential pressure sensor connected to the fan housing. Ensure silicone tubes are not cracked, blocked with condensate, or disconnected. Clear any moisture and reattach firmly.
• Test pressure switch: With a multimeter, verify that the pressure switch contacts change state when the fan runs (for models using a mechanical switch). Replace the switch if it does not respond.
• Reset and test: After repairs, restore power, clear the F6 code, and run the heater. If the fault persists, the fan control circuit on the PCB may be defective and requires professional diagnosis.