The two-line fuel system on an oil-fired furnace consists of a supply line and a return line. The supply line delivers fuel oil from the holding tank to the fuel pump. The return line sends excess oil not used by the furnace back to the oil tank. The two-line system is most commonly used in underground oil tanks or tanks located lower than the furnace's oil burner. You can hook up a two-line system with a few tools and basic mechanical skills.
Things You'll Need
2 copper oil lines, 3/8-inch
Install a threaded duplex bushing into the fuel inlet hole at the top of the oil tank. The bushing is designed to accommodate two fuel lines. Tighten the fitting snug by turning it clockwise by hand.
Insert the open end of a 3/8-inch copper oil supply line into either of the two duplex openings at the top of the tank. Push the line down into the tank until it stops at the bottom. Mark a line where it meets the bushing using a marker.
Raise the line up from the tank bushing 4 inches, measuring from the top of the tank (not the bushing) to the mark on the oil line to verify the distance. The line remaining in the tank will now be set at the correct depth.
Connect the flared end of the supply line to the inlet port fitting on the furnace's oil pump. The port is identified with "In" cast into the body of the pump. Tighten the nut on the flared end clockwise using an adjustable wrench.
Insert the open end of the return line into the remaining opening in the duplex fitting. Push the line in approximately 3 inches, using the measuring tape to gauge the depth.
Tighten the duplex bushing clockwise using the adjustable wrench. Tightening the bushing into the tank inlet also seals the two lines within the bushing.
Connect the flared end of the return line to the "Out" port fitting on the oil pump. Tighten the line nut with the adjustable wrench.
Overtightening of the line nuts may distort the flaring, creating leaks at the pump fittings.