Derating is the practice of installing a lower gallon per hour (GPH) rated nozzle in an oil fired combustion appliance to reduce fuel use. This often requires reconfiguring the shape/size of the combustion chamber and/or adjusting the fuel pump pressure, which is a job for a trained professional. The practice is also sometimes referred to a "down-firing."
Many boilers and furnaces in today's homes are oversized, particularly if you've upgraded the envelope of your home with new windows and added insulation. It is simple to reduce the heating capacity of your oil boiler or furnace to make it operate more efficiently by having a technician install a smaller nozzle. The cost is minimal and it could cut fuel bills by as much as 10%.
If you have an old, inefficient burner, though, you may want to replace the whole burner. A flame retention burner will block airflow up the chimney when the unit isn't running, saving up to 20% on fuel costs at a cost of about $500. Note that steam boilers should only be derated if the steam system is also modified to remove excess radiators, which is a difficult procedure.
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