The oil is first pumped into the furnace from a storage tank. It is turned into a mist, sprayed out through the blast tube and ignited with a spark.
Because it is mixed in fine droplets with the air, it burns very efficiently and makes a powerful flame. The flame heats a heat exchanger and then the combusted gasses exit the oil burner through a chimney.
Air from the house is drawn through air exchanges, past the heat exchanger and back out of the vents in the house. In the heat exchanger, it picks up heat from the combusted gasses without mixing with them directly.
In most modern systems, the air also flows through a filter which takes any impurities out, making it healthier to breathe.
Controlling the Furnace
Oil furnaces are controlled in the same way as gas furnaces. Typically, a thermostat in the house is set to a certain desired temperature.
The furnace turns on until the house reaches that temperature, then turns off again. Like gas furnaces, oil furnaces also have safety devices to turn off the flow of fuel if the furnace won't light.
Modern furnaces use a type of photo-detector called a Cadmium, or Cad cell. The Cad cell is at the back of the blast tube.
When the flame is burning, the Cad cell detects light and sends an electrical signal to a switch called a relay. If the burner is on, but the Cad cell doesn't sense light, the relay turns off the burner until it is manually reset.