Water – Elec hot water tank

Some taps and other fittings and appliances are rated for either high-pressure supply (direct mains) or low pressure supply (from a header tank in the building).

If any occupants have any fittings or appliances that are only rated for low-pressure, converting the building to high pressure could cause problems.

As others have noted, converting completely from low-pressure to high pressure means isolating and emptying the old header tank.

Have you considered adding a drain and rigid metal drain pan to the water heater in its current location. That's likely to be a much saner approach?

To move an electric water heater, the new location needs the following:

1. 220v outlet on its own circuit with heavy-gauge wiring rated for an electric water heater. This is doable. If there isn't already such a circuit, you can hire an electrician to put in a new one, and you said the panel is pretty close by.

2. An incoming cold water line, which will be harder, but if worse comes to worst, you can run a PEX line from the incoming cold water line on the second floor to the new location in the garage.

3. An outgoing hot water line that serves as a trunk feeding the rest of the house with branch lines going to the fixtures. This is going to be the deal-breaker. The previous location probably has the correct trunkline hot water plumbing. If you seal that off and make a new input in your garage, you'll be supplying hot water to your whole house through a former branch line, which is narrow. Hot water pressure may decrease, and the time it takes to get hot water to distance fixtures will rise quite a lot. And you may be able to run fewer hot water outputs at once.