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The recommended temperature setting for a temperature and pressure (T and P) relief valve on a domestic hot water tank is typically around 210°F. This setting is crucial because it helps to ensure the safety of the water heating system. At this temperature, the valve is designed to release excess pressure that can occur if the water temperature exceeds safe levels, thus preventing potential hazards such as explosions or ruptures.

The reasoning behind this specific temperature is that it allows for the safe operation of the water heater while accommodating normal variations in temperature and pressure that can occur during operation. Setting the T and P relief valve too low might not effectively manage pressure buildup, while setting it too high could allow water to reach unsafe temperatures before the valve activates.

In practical terms, maintaining the T and P relief valve at 210°F helps ensure that the system operates within safe limits and provides hot water effectively without risking damage or accidents.