What percentage of water stored in a storage hot water system is considered usable?

In a storage hot water system, the usable percentage of water is typically around 70 percent. This percentage takes into account the practical limitations imposed by water heating systems, such as the stratification of temperatures in the tank and the need to maintain a certain volume of hot water at an optimal temperature for immediate use.

When water is heated in a storage tank, the hottest water rises to the top, while cooler water settles toward the bottom. This stratification means that not all the water in the tank is available for immediate use without risking a drop in temperature or mixing with cooler water. The 70 percent figure reflects a balance between ensuring there is enough hot water available for usage and recognizing that some volume will inevitably be less than optimal for direct draw.

Understanding this concept is crucial for plumbing design, as it influences the sizing and efficiency of water heating systems and directly impacts user comfort and system performance. Knowing this percentage helps designers calculate hot water demands accurately, ensuring that systems are adequately sized to meet user requirements without waste or inefficiency.