F15 displayed, furnace fan runs continuously, no heating

Safety first: Turn off power before accessing control terminals.

• Step 1 – Check thermostat settings: Ensure the room thermostat is set to heat mode and above room temperature.
• Step 2 – Inspect wiring: At the furnace, open the control terminal cover and verify that the thermostat wires are firmly connected to the correct Protherm terminals (e.g., TA, RT, or eBus as per model).
• Step 3 – Bypass test (low‑voltage only): If using a simple on/off thermostat, briefly bridge the thermostat terminals with a jumper wire. If the furnace starts, the thermostat or its cable is faulty.
• Step 4 – Integrated sensor: If the furnace uses an internal room sensor or Protherm digital controller, check its location (not in a draft or direct sun) and wiring. Replace the controller if it does not respond.