What causes resistance to flow in a hot water system?

Resistance to flow in a hot water system is influenced significantly by the condition of the piping internal surfaces. Rough or corroded internal surfaces can create turbulence and friction, impeding the smooth flow of water through the pipes. The smoother the interior surfaces, the less resistance there is to flow, allowing water to move more freely and efficiently.

While the length of the piping can also contribute to the overall resistance, it is the condition of the internal surfaces that has a direct impact on friction and flow dynamics. The temperature of the water primarily affects its viscosity—hotter water is less viscous and flows more easily—but this does not compare to the impact that surface condition has. Ambient pressure can influence boiling points and other properties in some contexts, but it does not directly relate to the resistance to flow in the system.

Overall, the internal surface condition plays a crucial role in determining how smoothly water can travel through a hot water system, making it the key factor in resistance to flow.