Test your smoke and carbon monoxide detectors every 30 days to make sure they are working properly.
Older, low-efficiency furnaces have an AFUE rating between 50% and 70%. Mid-range efficiency furnaces around 80% to 85%, while high-efficiency furnaces have AFUE rating above 90%, sometimes as high as 98.5%.
When there is a problem with your furnace, it is usually the thermostat (control system), gas valve/thermocouple/heat exchanger (heat source), or the blower/air handler/ductwork (distribution system). In any case, a trained and qualified HVAC technician will be able to fix the problem quickly and effectively.
Thermostat and Gas Valve