First shut off the gas supply line to the water heater. There should be a valve near by. Secondly you will also need to turn the water off completely. Open up the faucets in the house and let the water drain. You may need to drain off a little of the water in the old heater.
If your existing heater has a shut off valve on the water supply line, Cut off the supply line and leave the valve for reuse. Replace the valve if it’s corroded or filled with lime deposits. Disconnect the gas supply line at the union joint. The union joint is where you’ll join your new heaters supply line.
Remove the old water heater and position you new heater in place. Make sure that the gas line, lines directly up with your hew heaters thermostat. You may want to use a flexible gas line in some applications. Attach the gas line to your new heater and tighten the union.
Next you’ll need to put the new water connectors on your water inlet and outlet. Screw on the female connectors using thread tape and tighten down securely. Then you’ll have to connect the supply line and water outlet to the new connectors. You can solder the connections together or you may be able to get some compression fittings.
Turn the water back on and check for leaks. If there aren’t any leaks continue to turn the gas supply valve back on.Check for leaks by using a dish soap and water mixture. Put some on the connection and if you don’t see any bubbles the you don’t have a leak.
Light the pilot on your heater according to the instructions and set your thermostat to your desired temperature. Warmer in winter and cooler in the summer months. Your water should begin to warm shortly. Go back through the house and open up all of your faucets to let the air out of the lines.