Boiler shows F5 and locks out, fan may run but no ignition

Safety: Isolate power and gas. Flue and combustion air components must be serviced by qualified personnel.

Diagnostic steps:

• Visually inspect the flue terminal outside for obstructions such as leaves, nests, or ice. Clear any blockage.
• Open the boiler casing and locate the air pressure switch and its silicone tubes connected to the fan or flue hood.
• Check that the tubes are not kinked, split, or blocked with condensate or debris. Clean or replace tubing as necessary.
• Verify that the fan spins freely by hand (with power off) and is not noisy or stiff. A stiff fan indicates bearing failure and requires replacement.
• With a multimeter, test the air pressure switch contacts while running the fan (qualified technician only). If the switch does not change state when the fan runs, replace the air pressure switch.
• If the fan does not receive power during start‑up, the PCB or wiring may be faulty and should be tested and replaced by a technician.

After repair: Reassemble the sealed chamber, restore power and gas, reset the boiler, and confirm that the fan starts, the pressure switch operates, and F5 does not recur.