The discussion centers on the efficiency of hot water systems in apartment buildings and homes, particularly regarding the waste of water while waiting for it to warm up. A proposed solution involves a mechanical valve above the water tank that redirects water from the faucet back into the tank, reducing waste. However, the effectiveness of this system is debated, as it requires a pressure difference to function properly, and the water in the pipes would need to be pressurized to return to the tank. Some participants mention existing systems that use pumps to achieve similar results but express concerns about energy loss and the practicality of such solutions in residential settings. The conversation also touches on the concept of reusing grey water, with some skepticism about its viability without treatment. Overall, the focus remains on finding efficient ways to manage hot water usage and minimize waste in plumbing systems.