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.

Be careful with the first hour ratings. On a 100 gal tank you will get about 70-75 gallons of hot on that first load. You then get the balance of the first hr rating over that first hr. For the kind of lay-out you suggest I think you should split the house up into two systems and try to centralize each tank to its load. That minimized pipe losses with the recirc, and yours will be substantial. The very best recirc system is the Metlund Design system. It complies best with all the new Title 24 regs. You will need 3/4" insulation on your hot piping. Again T24. Be very careful with the pex sizing. I re-piped with Uponor and the brass fitting here in the Foothills of N CA. There is a bushing affect on the fittings. The ID of 1/2" is 3/8, 3/4=9/16 and 1" is 13/16. It does make a difference. I have a couple faucets I was surprised how poor the pressure is on my current well 40-60 psi set-up. I had intended to go to a constant pressure well pump but now will definitely do so. Everything is good about the 50 psi range. Not so at 40# I am a fan of tankless. I represented a major manuf of tankless for 20 yrs. As far as your suggested products I will only suggest that you avoid Bosch. They, imho, do not have the right technology. Your first step regardless of what you do for equipment is a thorough water test. Manuf of tankless have to list annual cleanings for maintenance, but I had one unit for 12 years and never touched it. It was a pre-production model and I had to replace it prior to selling the house. I cut it in half and it was as clean as a whistle. Many of your plumbing questions and answers begin with the compete water test. Tankless (2) could handle your loads until you bring the tubs into play. They represent a massive load, even for your Vertex's. To handle your tub flows you should have two units on each side of the house. The thing about the tankless is they will each make your 7 or so gpm 24 hrs per day, but 135/7=about a 18-20 min fill. Will you wait that long for your bath. I have a bath fill controller on my tub. I set it at 104 and 35 gal (the vol of my tub) and it will deliver 35 gal of 104* water and shut off the hot and beep to tell me my bath is ready. Pretty cool actually. Your shower should be okay with one tankless, but your shower head(s) must fall within the flow capacity of the water heater. I can go on and on with this topic. Get your water tested and report back. That should be step one regardless of what your are doing.

Wow....sounds like my house. We live in a rural area and have well water and propane. Our propane water heater died last month it was impossible to replace it because it was a side vent water heater and we couldn’t find one anywhere. We ended up going to Home Depot and getting a tankless propane water heater. I think the cost of it was around $1500. Our home is about 2500 square feet, two stories. My husband was able to install it in 2 days. Of course it’s in the basement and had to have a side vent. They make two models one for inside installation and one for outside. We have of course the inside. According to the information on this water heater four people could take 4 showers at the same time and not run out of hot water. Since there’s just the two of us that’s not going to happen. I do however run the dishwasher and washing machine at the same time. So far so good. One thing I noticed is that it takes longer for the hot water to get where it’s called for than the old conventional propane water heater. It’s off until the hot water is called for, than it fires up and you run the water in the hot water tap waiting to flush out the cold water waiting for hot. It’s nice that it’s not on when not needed but if I could have found a water heater to replace the old one I would have stayed with the conventional. Time will tell as to whether or not we are saving on propane. The water heater we purchased was a Rheem. There is also a water filter screen on this tankless unit that should be taken out and cleaned each month. Since we are on a well this might be required more often. Our old conventional heater was set at 130 degrees. It was happy, we were happy. This tankless one needed to be set at 140 degrees. We were just not getting the hot water we needed at 130 degrees. Now it colder here than where you are we live in western Washington state, and the basement is not heated. Maybe the pipes down there are colder even though they are wrapped. Sorry this is so long I wish you the best of luck.

We installed a gas tankless in 2013. No problems at all. The size & number needed was decided by the company installing our Rinnae brand, which was purchased through Ferguson. Our choice was determined by space considerations. As mentioned, it is helpful to place it close to your kitchen/bathrooms. Our location was not ideal. The size of the gas line had to be increased, adding to the cost. We have no regrets, but moving yours to the garage sounds like a much cheaper option. As previously mentioned, it does not work in a power disruption. It also did not improve the look of the laundry room. There are pipes running above & below. I wish I had known what the finished installation would look like, as I would have changed the placement.