While sweat solderibng copper tubing, be careful not to start a fire. Wet any wood nearby. Use metal shielding. Make sure that no gas leaks. Have a fire extinguisher at hand.
The cold water gate valve must be left open all the while a water heater is in use.
Check the relief valve's action periodically to make sure it opens and closes again. Remember that water coming from the relief pipe may be very hot.