The pipe in question is the discharge pipe that connects to the pressure release valve. This valve will open or leak for the following reasons:
Water temperature gets too hot
Thermal expansion (most common)
First check the water temperature at any faucet. Put a thermometer in a glass and place it under the tap. Now let the hot water run for several minutes. If the temperature is in excess of 160 degrees, it's too hot. In this case the gas valve # F145-1278 will need to be replaced. If the water temperature is ok, the problem will either be thermal expansion, a defective valve or water pressure. To check the valve, open a faucet and allow approximately 4 - 5 gallons run. Then shut off the faucet and manually open then close the relief valve. If it does not completely shut off, the valve # 9000071 will need to be replaced. When replacing this valve, make sure the main water valve to the water heater is shut off and drain the tank below this point. Also use teflon tape on the valve threads to make a good seal. If it does shut off, the problem will be thermal expansion or the water pressure is too high. The pressure should not exceed 80 psi (a tech may be needed to check this). If the pressure is ok, the problem will be thermal expansion. In this case an expansion tank # 33102 will need to be installed. I'm also attaching an explanation of thermal expansion. All homes now, are supposed to have the backflow preventer valve. The parts listed are available at
. If an expansion tank needs to be installed, I recommend having a plumber or technician install it. If you do not feel confident repairing this problem yourself, then you can have it repaired at your home by a Sears technician. Here is a link for the website:
Sears Home Services
. If you need more assistance, please resubmit your question, with additional details.