A furnace problem is considered an emergency if it poses a safety risk or significantly affects your comfort during extreme weather. Indicators of an emergency include strange smells like burning or gas, which could signal a fire hazard or gas leak; any signs of carbon monoxide presence, such as a headache, dizziness, or a CO detector alarm; unusual loud noises that could precede a breakdown; or complete loss of heating when the outdoor temperature is very low. If you encounter any of these issues, turn off your furnace and contact a professional immediately.