1) Fill pipe: This is a 2 inch galvanized steel pipe that has either a lockable flip cap or a brass cap that screws into a galvanized steel collar. These pipes may be flush with the ground or they may stick up several inches. Fill pipes are typically within twelve feet of the house foundation and go directly down into the tank. Because fill pipes commonly stick up above the ground surface, many property owners have removed or cut off these pipes below the ground surface when the tank is taken out-of-use.
2) Vent pipe: This is a 1 to 2 inch galvanized pipe that has a mushroom shaped cap on the open end (top) of the pipe. Usually the vent pipe is plumbed away from the tank and set next to the house foundation. Vent pipes typically stick up 6 inches to a foot and a half above the ground surface. Vent pipes were not always installed on underground tanks. In rare occasions the vent pipe is constructed of PVC.
3) An old oil furnace or parts of an oil furnace in or under the house: Glass and metal oil filter housings are commonly left usually attached to the fuel supply lines (copper)
4) Two Copper Tubing Lines (¼ inch) in the crawl space or basement: After the tank is taken out of service, the old fuel supply lines are often left under the house even if the old oil furnace is removed.
5) A furnace chimney: Many 1950's houses have a small (1 foot by 1 foot) furnace chimney. Occasionally, a furnace chimney is routed into a fire place chimney. If so, there will be two flues coming out of the top of the brick chimney. Examine the top of your chimney.