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.

The disadvantage of tankless is that you may not have enough hot water to supply multiple simultaneous uses (running 3 showers at a time, for example). So what I'm thinking is a tankless and a tank heater feeding into a mixing valve such as a Watts LFMMV. Set the tankless to deliver 110F and the tank to deliver 120F and set the mixing valve to supply 110F. Then under normal conditions the the mixing valve calls for "cold water" of 110F from the tankless but if the tankless can't keep up and falls below 110F then the mixing valve mixes in hot water from the tank to maintain the 110F. Would this work?

There is a LOT of disinformation on these forums about on-demand tankless units, ESPECIALLY the electric units. First, energy cost savings is NOT a reason in itself to make the switch. No reason alone is sufficient. It is the total package, in light of your particular lifestyle, that will dictate the pro-con outcome of the decision making process. First, tankless units save space -- a lot of it, especially electric units that do not need to be vented. In Texas we have no basements, so that can be an important issue. Second, an electric unit is unlikely to blow up, unlike anything using natural gas. Third, there is an endless supply of hot water, which in a successive-morning-shower household like mine, is a really important issue. And finally, yes there is some documented comp fuel savings (electric tankless over electric tanks, or gas over gas), that is really offset by the added initial cost of the unit plus upgraded electrical service or gas supply line sizing. Moreover, there ARE full-house electric units out there that provide plenty of water, unless you plan on taking two showers simultaneously while also doing your laundry and running the dishwasher! And if that's your bag, you can run 2 or more electicals together. Or get a really big gas unit. Bottom line, nearly every nay-sayer of tankless units ignore the pros and focus on a single con - cost, cost, cost. Well, we all don't drive a Ford Focus for a reason. Some of us enjoy our Buicks, Porsches or Camrys even though another car would get us from point A to point B cheaper while using less fuel etc etc etc. Full disclosure: I have a Steibel Eltron Tempra 36 whole-house electric unit. Love it. My electric bill went up nary at all.

We don't have much info to go on. We don't know what your needs are, what your wants are or what your budget is. All we really know is that you want gas in the event that the electricity is out. A tankless requires a small amount of electricity to operate - it would not operate if the power was out. A generator would solve that problem and provide power for other things as well. I don't completely agree with Dan, you can size a tankless to run more than one thing at a time. I sized mine so that in the coldest part of the year I could run the shower and the bathroom faucet - in the summer I can run about twice that amount. It works fine in my house, but if I had a larger family I'd go with a larger unit. There are also 50 gal Hybrid units - They store the water but have high BTU input and very high efficiency. They can produce a certain amount of continuous hot water as well. These meet the 30% tax credit requirements and vent with PVC pipe - making them cheap to install where long vent runs are needed. I have included an example - I'm not endorsing a particular brand. Note that these would not run without a small amount of electrical power. Here is a link that might be useful: State Hybrid