We don't want to shut off the water when the remodel to the room is finished. We just want to switch the water source . For example after finishing the hall bath, can water coming from the water heater in the attic be switched via the remodel process to the new tankless water heater that will not be in the attic? But still will need the old water heater for rooms not yet remodeled and whose plumbing hasn't been upgraded to copper,etc. Can both water heaters be running at same time? For example keeping the master bath and the kitchen still using the water heater in the attic because plumbing will not be complete yet that routes them to tankless. So we will have both tankless not in the attic and the old water heater in the attic working at the same time but just servicing different rooms. Can we do that? I didn't specify since I'm not a plumber but I can't help thinking that main issue is that you need gas going to both water heaters and water from the outside being routed into both water heaters. Not sure if this is possible to do. Then after 6 months or a year when all rooms are remodeled and have their galvanized pipes replaced with copper, etc. and have plumbing routed to new location of tankless, the old water heater in the attic can be shut down because it won't be used.

There are no energy savings with an electric tankless heater! A typical electric tank heater that is reasonably efficient (2" foam insulation) has an energy factor of .93 The only real exception to that is for a vacation home or guest house where the home is unoccupied for a long period of time -- and then the savings aren't impressive. There is a 25-30% savings with a gas heater tankless over a standard gas tank heater. You need to decide if the space savings is worth it. A whole house electric requires a lot of electrical current to operate and that usually requires expensive electrical service changes. They also tend to have low water flow ratings.

Yes, they do make electric tankless water heaters. You can find some at the link below. Contrary to wild claims, In a standard home usage, there is almost no energy savings with an electric tankless heater. There is about a 25-30 % savings with a gas tankless heater. There might be more of a savings with an electric tankless in your application since hot water isn't being used often. Electric tankless, like electric tank heaters, don't require venting. There is a serious drawback with electric tankless in that they draw very large amounts of current and produce relatively small flow rates (measured in gallons per minute) If I remember right, a tankless electric unit that produces about 3gpm will pull about 100 amps. (the gpm will depend on your water temp going into the heater). Three GPM is a shower or would fill the washing machine slower than normal (washing machines are about 4gpm) If you don't have an extra 100 amp capacity to the barn, I wouldn't go with an electric tankless. I would go with a very well insulated electric tank unit. Pick up a unit with 2" of foam insulation and add a water heater blanket to it. Here is a link that might be useful: Electric tankless

First off let me say that is an insane amount of water, for a residential setting. Really insane for California where there is a shortage of the resource to begin with. It is an interesting idea though. I know nothing about this kind of setup. There are only two people in my home and we get along fine with 40 Gallons of water in a tank with plenty to spare. When the kids were here though we could see issues if everyone decided they wanted to shower at the same time as we do have 3 bathrooms. If I were going to attack this idea, I would put the tankless after tank. In this way (at least in theory) the tankless would only operate at full capacity once the water in the tank was exhausted? Or as the water began to cool the tankless would ramp up. That theory based on how hot the water is flowing into the tankless and whether or not the tankless would operate at a lower setting or not at all if hot water was already flowing through it. I don't know the answer to either one of those questions. As far as those rain shower heads, not a fan. I have a family member who has one, not the best shower experience I have had. To each his/her own though.