Propane Lines
A propane line is the line that runs between the house and the gas tank. Black iron pipe threads are used because of their high heat resistance. They are cut with a slight taper and are sometimes known as national pipe taper.
Water Pipes
Black iron pipes are resistant to water damage, making them a popular choice for water pipe lines. Black iron pipes are less widely used than they were a few decades ago, as they have now been replaced by regular steel pipes which are less prone to leaking and are more durable. However they are still less expensive than regular steel pipes and, with the right maintenance, can be used effectively.
Heating
Black iron pipes can be used as a heat transporter for a house heating system. They are both heat resistant and conductive and can therefore transport heat well.