An oil-fired warm air furnace uses number 2 fuel oil, which is delivered to homeowners by an oil company. Keeping an eye on oil levels is the homeowner's responsibility. Coming home and finding out that the heat is not working is not ideal. There are several reasons why the heat would be off, but one common cause is that the oil tank is empty. As a homeowner, you have several easy ways of testing the furnace and checking if indeed you need to call for an oil refill.
Locate the oil tank gauge; basement tanks have a gauge on the top while in-ground tanks have a remote readout device mounted on a wall.
Shine a flashlight on the gauge to get a better visual. Read the gauge (it is similar to a vehicle gas gauge and has marks from empty to full). Confirm if the oil is on empty, if the reader needle is located anywhere at the zero or "E" mark.
Push the reset button on the furnace only once. It should be a red button with "Reset" or "Restart" button written on or above it. Locate it either on the primary control at the stack (on older models) or on the burner motor (on newer appliances). Wait 60 seconds for the furnace to start up.
Listen for the oil passing through the tank. Listen for a loud bang, as that is a sign that no oil is available to pass through the tank nozzle and the furnace is unable to fire.
Go upstairs and confirm that the heat is not functioning. Touch the registers to see if they are warming up--if cold to the touch there is no oil to be pulled and burned for heat.