This Old House plumbing and heating expert Richard Trethewey explains the extreme importance of maintaining your water heater's temperature-and-pressure relief valve. SUBSCRIBE to This Old House: http://bit.ly/SubscribeThisOldHouse Steps for How and Why to Maintain a Temperature-and-Pressure Valve: 1. Regardless of the type of water heater, whenever water is heated, it expands. And the water-heating system…