A furnace problem becomes an emergency when waiting could put people or property at risk or leave the home without adequate heat during dangerously cold weather. A complete heating failure during freezing temperatures is much more urgent than a furnace that is still operating but making a minor unusual noise.
Combustion-related warning signs deserve extra attention. Gas odors, carbon monoxide alarms, smoke, overheating equipment, or a suspected heat exchanger problem need to be addressed as soon as possible. If there is a gas leak, fire, or another immediate danger, leave the property and contact emergency services or your gas utility before calling for furnace repair.