Furnace shuts down and displays an overheat/limit error; casing may feel unusually hot and burner will not restart until cooled.

Safety first: Turn off power and allow the furnace to cool completely. Do not bypass safety thermostats.

• Check air and water circulation: For air furnaces, ensure all supply and return vents are open and filters are clean. For Sime hydronic/boiler‑type furnaces, verify that system pressure is within the range on the gauge and that radiators or underfloor loops are not shut off excessively.
• Inspect circulation pump (hydronic): Listen for pump operation when the unit calls for heat. If silent or excessively noisy, the pump may be seized or air‑locked. Bleed air from the pump vent screw if present, with the power off and a cloth ready for small water drips.
• Clean or replace filters: If your Sime furnace is part of an air‑handling unit, replace or clean the air filter to restore airflow.
• Check overheat thermostat: Locate the manual‑reset overheat thermostat on the heat exchanger. Once cooled, press the reset button if fitted. If it trips repeatedly, do not continue to reset; find and correct the underlying cause.
• Descale heat exchanger (advanced): Heavy scale inside a Sime primary heat exchanger can cause localized overheating. This requires chemical descaling and should be performed only by a qualified technician.