What is the pipe size for the main gas system from points "C" to "D"?

The main gas line size from points "C" to "D" would typically be determined based on the appliance requirements and the overall gas load demand. A 1/2" pipe size is often adequate for low to moderate gas flow applications, particularly in residential settings where the distance involved is short and the appliances do not require a large volume of gas.

In many heating systems, a 1/2" gas pipe is commonly used for smaller appliances and setups, ensuring safe and efficient operation without the risk of pressure loss or inadequate supply. Smaller pipe sizes like 1/2" can effectively handle such applications under standard conditions, making it a practical choice for main gas distribution in appropriate scenarios.

Larger pipe sizes such as 3/4" or 1" would be used for systems that require higher gas volumes over longer distances. For a main gas line feeding multiple large appliances, those sizes could be more applicable. However, if the context suggests that the gas demand at points "C" to "D" does not necessitate a larger pipe diameter, then the 1/2" option stands out as the most suitable choice.