Furnace runs but produces no heat to the home; blower or pump operates but radiators or air outlets stay cold.

Safety first: Turn off power before touching pumps or valves. Allow hot surfaces to cool.

• Check thermostat and demand: Ensure the room thermostat is calling for heat and set above room temperature.
• Verify valves: Confirm that the flow and return isolation valves beneath the Sime furnace are fully open. Check any external zone valves for correct position.
• Bleed pump: On pumps with a bleed screw, switch off power, place a cloth under the screw, and open it slightly to release trapped air until water flows steadily. Retighten and restore power.
• Check for circulation: Feel the flow and return pipes. If one is hot and the other cold, circulation is poor and the pump or valves may be faulty, requiring technician replacement.