Do not close the hot water tap until all the air is exhausted from the tank and water flows from the tap. Heat Traps Metal is an excellent conductor of heat, and both hot and cold pipes offer a thermal shortcut for escaping heat. This phenomenon is easily observed by placing your hand on the cold and hot water pipes leading from your water heater at varying distances from the tank. Both pipes are usually warm, a sure sign of unwanted heat loss. Heat is carried out of the tank by the movement of hot water, even when none is being drawn out of the taps.
Hot water is buoyant, so it tends to rise in any vertical pipe, such as the hot water feed line.
That hot water then releases its heat to the air surrounding the pipe until it cools and sinks back down into the tank. This is called a convection loop and it should be eliminated whenever possible. The best way to prevent convection loops is to install heat traps and pipe insulation on the first few feet leading out of the tank. The simplest heat trap is a small one-way valve that can easily be inserted where the pipes enter the tank. The one-way valve prevents the cooling water from settling down again into the tank and being replaced by hot water.