What does a 'hot water return line' do?

A hot water return line is essential in a plumbing system designed for hot water usage, particularly in systems that utilize recirculation. Its primary function is to allow unused hot water to return to the water heater or boiler for reuse. This mechanism helps to maintain a steady supply of hot water at faucets and fixtures throughout the house, minimizing the time it takes for hot water to reach the tap.

In a typical setup, when hot water is drawn, the cold water that replaces it in the hot water line can create a delay before hot water is available. The return line mitigates this wait by ensuring that any unused hot water is cycled back into the heating system instead of being left in the pipes to cool down. This system not only enhances comfort by providing immediate hot water but also improves energy efficiency by recycling the hot water that might otherwise be wasted.

The other options describe unrelated functions. Supplying cold water to the system pertains to the inlet supply line rather than the return line. The release of pressure is typically managed through different components like pressure relief valves. Preventing water from freezing is a concern for cold water pipes and unheated areas, rather than a function associated with a hot water return line.