I have a large, three story house. There is a 60 gal gas water heater in the basement and a 40 gal electric in the upstairs closet. I always assumed they operated separately for different parts of the house. Well last June the upstairs unit stared leaking. I examined it and found shutoffs for both the supply and return and secured them both. Surprisingly enough, I did not lose water supply or hot water to any fixture or appliance in the home. Never the less, I replaced the unit. The original installer plainly marked the copper risers "hot" and "cold" so it was pretty straightforward. After installing the new unit, I opened the supply and return valves, let the air out of the unit, and flipped the breaker. Then the most interesting thing happened. I now have hot water for a minute or so from the cold water side of all the faucets in the house. Baffled, I disconnected the power to the unit, but the condition persisted. I then closed the supply and return valves to the unit and the problem stopped. I am guessing that this is some sort of a recirculation system with the electric unit in a loop and serving as a booster. Still, I don't understand why a simple switch-out went wrong. Help.