The draft regulator, located on the stack, is closed when the burner is off but opens automatically to let air into the chimney when the burner is turned on. Accumulated soot and rattling are signs that the draft regulator needs to be adjusted. Too much air in the chimney wastes heat; too little air wastes fuel by failing to burn it completely. To increase the airflow, screw the counterweight inward. To decrease airflow, turn the counterweight outward. The draft regulator should be adjusted by a professional service person as part of regular annual maintenance.
The limit switch is a safety control switch and is located on the furnace just below the plenum. If the plenum gets too hot, the limit switch shuts off the burner. It also shuts off the blower when the temperature drops to a certain level after the burner has shut off. If the blower runs continuously, either the blower control on the thermostat has been set to the ON position, or the limit control switch needs adjustment.
To determine the problem, check the thermostat. If the blower control has been set to ON, change it to AUTO; if the blower control is already on AUTO, the limit switch needs to be adjusted. To do this, remove the control's cover and find the toothed dial underneath. One side is marked LIMIT; don't touch this side. The other side is marked FAN. There are two pointers on the fan side; the blower turns on at the upper pointer setting and shuts off at the lower pointer setting. Pointers should be set about 25 degrees apart. Set the upper pointer at about 115 degrees Fahrenheit (46.1 degrees Celsius) and the lower one at about 90 degrees Fahrenheit (32.2 degrees Celsius).
Do not try to adjust the burner of an oil furnace; call a professional service person.
Finally, we have electric furnaces and heaters. Electric heat is in some ways much simpler than either gas or oil, but it can still have its fair share of problems. Keep reading to the next section to learn how to maximize the efficiency of your electric furnace.