Can't isolate my water heater with shutoff valve
Single handle faucets can allow water to flow from the hot side to the cold if turned on at all. Not sure if some of them will allow the flow when turned full off. Fix would be to turn off the hot water shutoff valve under your sinks. If the flow is coming thru a bathtub or shower valve, that would be more difficult.  Might be easiest to simply remove the flex pipe on the hot side where it connects to the galvanized pipe and cap it off for the duration of your project. You'll probably have to replace the flex pipe as the gaskets tend not to be very reusable.