The Temperature and Pressure Relief Valve (T&P valve) is a safety device to relieve pressure from a hot water cylinder when they reach extremes. As water expands when it is heated, the T&P valve protects the cylinder by discharging water through the waste pipework when the cylinder temperature and pressure reaches excessive levels. There are several reasons your system could be discharging the water, including but not limited to the following: thermal expansion, excess system pressure, low temperature relief, setting the water heater too high, or some other factor within the heater itself.