I attempted to repair a small drip in the hot water output line of our hot water heater. There is a vertical pipe that extends upward about 3 feet to a horizontal pipe. The horizontal pipe leaked at the first 45 degree connector where it was joined to another horizontal pipe. After turning off the water at the main and also the intake water suppy to the hot water heater I drained about 5 gallons of water out of the hot water heater and opened the hot and cold water facets in our utility sink. When I tried to melt the solder joint at the 45 degree coupling I couldn’t because there was still some water in the pipe. I finally cut the connector to drain the water and then sweated a new connector to the pipe. Perhaps a pint of water was still left in the horizontal pipe when I cut the connector. What would prevent the water from draining back into the hot water heater or out of the utility sink faucets – both were lower than the horizontal pipe? Water did drain out of the utilty sink faucets when I first opened them and again when I opened the faucets on the second floor
All you really did was relief pressure to start with. Most likey the line was air locked. If you had opened the drain on the water heater and the T&P valve, you would have put air into the heater which would have eventually gone into the pipes.