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 blower control is already on AUTO, the limit switch needs adjusting. 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 and the lower one at about 90 degrees Fahrenheit.
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.
Do not try to adjust the burner of an oil furnace; call a professional service person.
Although you cannot fix major problems with an oil furnace, with the information in this article you now have the know-how to handle most of the problems that will come your way.
For more articles on home repair, check out the following links.
- Furnace Maintenance: Save yourself time and money by learning the steps to keep youor furnace in prime condition.
- Major Appliance Repair: If the furnace isn't the only thing in your house on the fritz, you can learn how to fix other machines in this article.
- Small Appliance Repair: Once you've tackled the furnace, a toaster or blender seems like child's play. Find out how to fix them here.
- Thermostat Maintenance: To make sure there's actually a problem with your heating system, you may want to check the thermostat, too. Learn how to calibrate a thermostat.