How do you determine nozzle size for oil furnance?

This is determined by the BTU output of your furnace. There is a tag of the furnace normally by the burner assembly that will tell you the BTU. Here is a list of nozzle sizes for BTU. You can get more information on oil furnaces from my book, Forced Hot Air Furnace Troubleshooting and Repair at Amazon: BTU Nozzle size (gallon per hour) 50,000 0.35 55,000 .040 60,000 .045 65,000 .050 70,000 .050 75,000 .055 80,000 .060 85,000 .060 90,000 .065 95,000 .070 100,000 .070 110,000 .080 115,000 .085 120,000 .085 125,000 .090 130,000 .090 135,000 1.0 140,000 1.0 You also need to consider the angle of the cone at the end of the burner assembly.This is the angle of spray for the nozzle. The nozzle will have a marking on it such as 1.0/80 which means 1.0 GPH with an angle of 80 degrees. Be sure to match the nozzle spray angle to your furnace.