Propane to natural gas

Natural gas piping is usually black steel pipe with malleable iron fittings, not cast iron fittings. The same holds true for propane, although many propane companies will use copper tubing, which is usually discouraged by the natural gas utility.

The fuel gas code does not require only one specific piping material for residential gas piping. It lists several approved materials, including steel pipe, CSST, and other approved materials depending on the application. However, traditional residential natural gas piping is commonly black steel pipe with malleable iron fittings. Propane systems may also use steel pipe, CSST, or copper tubing where permitted by the code, the gas supplier, and the local authority having jurisdiction.

The fact that you have some existing black steel piping does not automatically mean it can or cannot be reused. That depends on how much fuel the pipe needs to carry, the pipe size, pipe length, number of fittings, allowable pressure drop, and the delivery pressure of the gas system.

LP gas is often delivered at a higher pressure inside residential buildings than natural gas. Because of that, a smaller-diameter pipe can sometimes carry the same appliance BTU load when using LP gas. Natural gas systems are often operated at a lower pressure, so the same BTU load may require a larger-diameter pipe.

The piping should be sized using the proper fuel gas sizing tables for the specific gas, pressure, pipe material, developed length, and appliance load.

This is generally correct in most areas, but not absolute. Some residential natural gas systems are also medium-pressure systems with regulators at the appliances, and some LP systems are low-pressure after the second-stage regulator. The code does not size pipe by “fuel type only”; it sizes by gas input, pressure, allowable pressure drop, pipe length, and material. The IFGC and NFPA 54 covers design and sizing for gas piping installations, including materials, pressure requirements, and sizing.

So the short answer is MAYBE!

If it black pipe you can use it for natural gas if it is the right size. Propane is usually 10-11" wc inside a house while NG may be at a lower pressure.

Also for the equal BTU rating NG requires larger pipe than propane. propane is about 2500BTU/cubic foot . NG is about 1000btu/cubic foot

2 different animals

I don’t see a problem with switching gas in a black steel piping system.

If a gas supplier is doing a connection and meter set, they may require an observed pressure test on the complete piping system.