F5 displayed, furnace will not start, fan may not run

Safety first: Isolate power. For hydronic systems, allow water to cool before touching pipework.

• Step 1 – Locate flow sensor: On Protherm hydronic furnaces, the flow NTC is usually clipped to or inserted into the flow pipe leaving the heat exchanger. Follow the two‑core cable from the PCB to find it.
• Step 2 – Inspect wiring: Check the cable for cuts, crushed sections, or signs of moisture. Reseat the connector at both the sensor and the PCB.
• Step 3 – Basic sensor check: With power off, disconnect the sensor and measure its resistance using a multimeter. Compare with the temperature/resistance table in the service manual. An open circuit or near‑zero ohms indicates a faulty sensor.
• Step 4 – Replace sensor if faulty: Remove the old NTC (unscrew or unclip). If it is a wet‑pocket sensor, be prepared for some water loss and close isolation valves if available. Install the new Protherm‑compatible NTC with thermal paste if required.
• Step 5 – Reset and test: Restore power and reset the fault. If F5 persists with a new sensor and good wiring, the PCB input may be defective and should be checked by a technician.