What must valves, pipes, and fittings be installed in relation to?

Valves, pipes, and fittings must be installed in relation to the direction of the flow because proper installation ensures that the fluid flows through the system as intended. This is critical to maintain system efficiency and prevent issues such as backflow, which can lead to contamination or other operational failures.

When valves are installed incorrectly, they may obstruct the intended flow or fail to function properly, which can cause damage to the plumbing system and result in leaks or other malfunctions. Understanding the flow direction helps in adhering to best practices as outlined in the International Residential Code (IRC), ensuring that the plumbing system operates effectively and safely.

The other options, while important factors in plumbing design and installation, do not directly relate to the critical need for aligning components with the flow direction. Temperature can affect materials and their longevity, material type is important for compatibility but does not influence flow direction, and the type of application may dictate certain choices but does not override the necessity of proper flow direction for valve and pipe installation.