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?

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

Guys if you read the post, the reason the tank water heater was so expensive was due to the construction required to get his house up to code. I personally like my tank heater. If your changing from a tank to tankless there is alot of work to do. Moving water lines, moving gas lines (possibly upgrading gas line size too) and running a new intake/exhaust if using gas, upgrading the main service amps if using electric. Some things to consider Tank Heater Pros Costs less to replace in the future Easier to maintain (Tank water heaters are easy to flush out every year) Easier to install (dont have to upgrade gas line or main service amps) Gas water heater will still operate in power outage (electric will still have some hot water to use for a while till power kicks back on). Cons Not as efficient as tankless No Tax Credits Hot water limited by FHR (first hour rating) Tankless Heater Pros Unlimited hot water More efficient Saves space Cons Will cost more in the future to replace (there is no guarantee the tankless will last longer than a tank version, the heat exchanger can go bad and that is the most expensive part of the tankless) , Harder to maintain (you need to try to clean those tankless heaters out every year with a descaler so they stay working efficiently) Harder to install (upgrade gas line or main service amps) Tankless heaters have a limited throughoutput. While it is unlimited hot water, it can only supply so many gallons of hot water at a time Will not operate in a power outage. A tankless water heater is controlled by electronics whether the heater is gas or electric does not matter. Requires minimum gallons per minute from city water supply, some people have not been able to install due to that problem. Consumer reports note that users complain of inconsistant water temperatures. http://www.consumerreports.org/cro/appliances/heating-cooling-and-air/water-heaters/tankless-water-heaters/overview/tankless-water-heaters-ov.htm Another good site to look at: http://www.waterheaterrescue.com/pages/WHRpages/English/Longevity/tankless-water-heaters.html