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.

This post might be old but id like to respond. HI, Big Bias I'm seeing here... I would like to show some transparency so people know the actual costs. I would also like to note that i'm not including Markup to pay for things such as both types of insurance working and worker, Municipal licencing, Over head for a shop, Gas and insurance for a vehicle, the time it takes to order products as well as picking up/ delivering products. Expect a mark up of 26-37% on everything to pay for these items. If it's a Small one man company out of their home figure on the low end. If it's the largest plumbing company in the district expect to pay more. On the flip side they may get a bigger discount on the unit and hide that mark up in the unit it's self. My location is Alberta Canada, So keep in mind CND$$$. A typical 40-50Gallon Natural Draft hot water tank is 36-50k BTU's . A Rheem or Renai or Navian tankless is typically 140-205K BTU's . The Tankless will need a larger gas line. Usually needing permits. In my Jurisdiction The inspector gives a 4 hour window for inspection. So you'll be paying for a plumber for 4 hours. On top of that Tankless units DO NOT use the same venting as a Natural draft old style water tank. Thus a Venting inspection by the same inspector. Permit needed. So you will be changing two Major components of your homes "gas" and "venting" systems. Typical install time is about 4-6 hours. This includes, Turning off the water supply, Draining the house and tank, Unhooking the venting, Unhooking the old water supply. Then Mounting the new unit to a wall or Optional Metal rack carrier, Running new water supply lines complete with new valves and possibly a new Humidifier valve if the old one was above the hot water tank. Once the new lines are run, then they can Cap the old Vent. In some Jurisdictions this vent may be removed all together with a general contractor removing and capping the roof. Again more permits needed in this case. After the old venting is capped. then running new venting out of the side of the house. which has to be 12" from other venting or 3' from any opening door window or gas meter. If it's an older home you'll need to hire a Coring contractor to Core a hole through the Cement foundation. If the home uses TGI joists you can typically exit through there. Once the venting is complete the contractor can move on to the gas line. Cutting gas to the home for up to a couple hours Hopefully using "lock out tag out procedure" IE locking the riser while they work. They will open the outside of the gas line close to the meter Then Purge. Meaning to leave it open to air for 10 minutes. Some companies use a manual pump from the inside to push the gas outside. Once purged properly the contractor can start to take the gas line apart to add a tee. Remember he was supposed to "Size the gas line" at the time of Estimate to make sure your home has a large enough line for the added BTU's If he doesn't show the inspector this Calculation he could instantly fail inspection even if he did a good job. Most contractors will install a hard Tee then run Flex CSST Gas line or "Gas Tite" similar product. Once the gas is hooked up they do another purge the other direction. removing air from the line from the outside in. once gas is at the fixture it's time for commissioning. another 20 or so minutes till he's done. In my Jurisdiction the venting materials are to be Gas 636 PVC piping minimum. In multi unit dwellings it will need to be a higher spec. Cost of venting materials depends on length but the average is about $210 2" Double that for 3". Parts, correct type of glue Primer and glue. (the inspector checks), 2 45's 4 90's , 2 couplings, TY, 4 lengths of pipe. Seems rare to have an up and out easy unit. but costs obviously for that $60 or so depending on elbows needed. Gas permit $212.65 Materials gas $85.00 15' CSST, CSST adaptors, Nipples, Black 1 1/4 x 3/4 tee, Tape, jet lube, Materials Venting $210 4" B-vent Cap, 636 pvc piping & elbows Optional Rack $310 1/2" Plywood and screws $32.00 6 hours labour $510 Inspection time say we're lucky and only wait 2 hours $170 Owner supplied Tankless install 1219.65 Navian NPE-240A Wholesaler price $3044.22 Including Service kit. Some companies may get a higher discount through volume. Navian NPE-240A Amazon price 2800.00 No service kit https://www.amazon.ca/Navien-NPE-240A-Premium-Condensing-Tankless/dp/B00FAMAB2U/ref=pd_lpo_sbs_60_t_0?_encoding=UTF8&psc=1&refRID=P40Z2FYMVNSYE4DJ1A24 Also don't be fooled when it says 1/2" gas line the actual fitting is 3/4" it's just saying they have a special valve inside that allows for lower gas flow but in turn the burner does not run at full capacity. Total Canadian Dollars $4263.87 Transparent Fair pricing

It would be good to step back a bit. Other than the Bosch unit that had so much trouble (AquaStar?), I don't think that the brand of the unit is the most important issue. You first need to be clear on what your needs are and what venting and other issues you may have. To determine what your needs are, you need to approximate the temp of the inlet water in the winter--In very cold climates that can be around 33 degrees. Then you need to determine what you want to raise that temp to. In most cases the design temp is 105 degrees because that's the temp of a shower (even though the heater will likely be set at 120). The difference is the temperature rise. Then you determine what faucets you are likely to use at the same time and you add the gpm rating of those together. Once you know the temp rise and the flow in gpm, you can find a heater that will do that. They publish charts that show how many gpm they will heat at a given temp rise. What you will find is that the most of the heaters have a similar energy factor and so a 200,000 btu input heater, for example, will heat about 4.5 gal a minute at a 72 degree rise. Once you go over the 200k btu input, there are fewer brands available, but still several. Be aware that the condensing units (that take the flue temps below condensing point) get very expensive and have the issue of the condensate being corrosive. At this time, I wouldn't use one unless there was a compelling reason to do so. (The little additional energy savings isn't a compelling reason, Solving a venting problem because you can use PVC pipe might be) All of that said, I thing the Rheeem/Rudd/Paloma units are a quality unit at a reasonable price. The cost of 200k btu Rheem is ~$1100 with the basic vent kit. If your gas company will install it for $1500, I would do it. That includes the cost of capital, by the way. Most of the units are available as an indoor unit or a outdoor unit. The outdoor units are suitable for warm climates and don't need a vent. Most of the indoor units also offer an outdoor air intake.

If you're really sure you want tankless (and you kinda seem to be), I'm not going to argue with you. I'll just say a couple of things and then I'll bow out. Disclaimer: I'm not an expert on these matters. They just interest me and I hope to be more energy independent in the future, so I read up on them. I always welcome corrections from knowledgeable folk. First point: IMO electric resistive heating of ANY kind basically just doesn't play nice with PV, off grid or on. The only time it really makes sense is when you're off grid and your system produces more energy during the day than you can store in your battery. This can also apply to some extent if you're intertied and your utility pays you wholesale or less when you sell your power, essentially penalizing you for conserving. Either way, dumping your excess PV energy into water heaters is one way to store it for later use. However, it should be obvious that to store that energy, you have to use traditional storage water heaters, not tankless. Second point: Peak demand matters. It matters to your electric utility, and it matters to your bill. You probably know more about CA rates than I do, but it appears to me that right now California is more into time of use (TOU) metering than peak demand metering. However, more and more utilities are adopting peak demand metering for residential customers (they've had it for business for ages). So even if you're not dealing with it now, it may well be in your future. Demand based metering watches your power (not just energy) usage. The higher it is, the more you pay for all your electricity. In one demand scheme, you pay a surcharge based on your peak power usage (in kilowatts), In another, the price they charge you for every kilowatt hour you use depends on your peak usage -- the higher your peak, even if it's only once in the month, the more you pay per kWh that month. Let me give you an example of the second scheme. Let's say your house's big energy hog right now is an electric range.. Most of the time, you just use one or two surface units at a time. So your peak demand is relatively low, and you pay (say) 14 cents per kWh most months. But when you cook Thanksgiving dinner, you have all the surface units going, plus the oven, and a microwave. On that one day, you have a much higher peak demand. Your utility takes note. And that month, you pay 20 cents per kWh for EVERY kWh you use all month. So even though that month's usage is only a little higher than usual, the final bill for the month is MUCH higher. Now imagine what could happen to your bill if you add an appliance that increases your peak demand every day of every month. Guess what: when it comes to peak demand, tankless electric water heaters are MONSTERS. Think about it. With its mild climate, California is a great place for heat pump storage water heaters. That's what I'd get if I were you. Not tankless, and definitely not electric tankless. And that's all I have to say about that.