The heart of your oil furnace is the firing assembly, as it sprays a mist of oil into the fire chamber along with an air mixture so that the sparks from the electrodes can ignite it. If any part of this system malfunctions, it can light and just not put out the heat that you would expect from your furnace, as one of the elements that are involved could be falling short from a worn or broken part. You can replace the entire assembly, or in many cases, simply clean or replace the spray nozzle mechanism inside it.
Step 1 - Remove the Main Assembly
The first thing to do is to remove the oil line from your oil furnace where it attaches to the unit from the pump to prevent leakage during the operation. Loosen the securing bolts around the assembly and slowly rotate it out, be careful not to hit the electrodes on the sides of the holding chamber as you take it out. If these electrodes get damaged you will not be able to make repairs. If you are replacing the entire unit, go to Step 3.
Step 2 - Cleaning or Replacing the Spray Nozzle
The assembly you removed from the oil furnace has a nozzle on the inside that may or may not have a flash house installed over it. If there is a housing blocking you from removing the nozzle, remove this and set it aside. Place your wrench over the hex nut holding the nozzle into place and remove it. Be sure to hold the oil line connector steady with another wrench to prevent twisting and breaking the oil line as you remove it.
Take these parts and put them in your thinner to clean them up, as clogging can occur in the nozzle which will cause your oil furnace not to fire correctly. Dry the parts off, check and replace any that are worn or broken as your owners manual dictates for your make and model and then reinstall them back into the assembly. Check the rating on the nozzle on the head of the hex bolt, if it is rated at less than 1.5GHP (gallons per hour), you will want to make sure that you replace it. If the nozzle is greater than 1.5GPH, then all you will have to do is clean it during regular maintenance.
Step 3 - Replacing the Assembly Unit
Once you have your replacement assembly, or have finished cleaning the parts in an old one, you can then reattach your oil line, and mount it back into the oil furnace. Be sure not to knock the electrodes against the side of the chamber as you put the assembly back into place. Clamp down the securing bolts to lock the assembly back into place and then reattach the main oil line to the intake. Be sure that is you are replacing the assembly that it meets all the same standards required for your make and model of oil furnace, check your owners manual for details.