"Also have mixing valves at each zone with out door reset. So you can control the temp of the water by the outside temp." This. The boiler needs to be sized for the design limits, and the rest of the time (actually almost all the time) is oversized. Using a boiler wit an indirect tank and a reset thermostat allows the tank to serve as the buffer between system size for the coldest hours and the actual needs of the house when temperatures are milder. The boiler heats the tank and runs hot enough to not be damaged. The temperature in the tank is controlled by a reset thermostat based on the outdoor temperature. The zone valve allow final tempering for each zone based on their actual requirements.

Is 15A 120V breaker all that is needed for the electrical requirement on a gas tankless? Or does it need to be 20A? I believe there is some open slots in my panel for a new breaker, and the panel is only about 15 feet from where the unit would go, so running a new one isn't too big of a deal. The unit would be going on the exterior wall that is part of my garage, so a new electrical run could be ran through the attic, down on the interior of the wall inside the garage, and then drilled through the wall and an electrical box placed on the exterior. Or, it can be ran through the attic and down through the soffit and ran through exterior grade conduit on the exterior wall of the house. Which one of these would be approriate? If running down the exterior wall in conduit, where does the conduit need to start? Just somewhere up in the attic, but it doesn't matter where? I know that code in this area does not require full length of electrical runs to be in conduit Lastly, is the electrical connection from the gas tankless hardwired in, or is it an actual plug? The reason I ask these questions is that I have 2 quotes and one of the plumbers doesn't do electrical so I have to source it out, and the other plumber does, but he is the one who said he'd tap the GFCI. They also wanted to charge me $300 to tap the GFCI, run 2 ft of cable, drill a hole, and mount a box, and that is just absurd, so I'd probably do the electrical myself or have my handyman do, it is far cheaper.

update from original poster: here is more info and quote for your review from plumber #1: i have original galvanized plumbing. eventually i will upgrade but not planning to right now (tapped out from the kitchen remodel). i live in northern CA (SF Bay Area), it rarely freezes here, winter is mid 30's, usually warmer than that. the unit will be mounted in my sub-basement under my house, it will not be outside. it is about 10' to the side wall of the house where it will need to be vented. to clarify for the poster who suggested i stick with a tank, i can't due to the need to remove the flue in kitchen and headroom in sub-basement will not accommodate the power vent tanked kind (wish it did as i would buy one and case closed..) it is just me in the house so max usage at one time is one shower and the washing machine. one quote so far: he recommended the TK-3, quoted $3800 ($2660 after tax credit) which included unit, materials and labor: install unit, run a 1" gas line from meter to unit, install earthquake auto shutoff valve at meter, provide and install new 3/4" copper water supply lines from existing lines in sub-basement, insulate hot water lines from unit to tie point, install SS flue pipe to sidewall of house, patch exterior, provide and install T&P discharge line, provide electrical to unit (not sure what that means, have question in to him), permit included in bid. does this sound about right and are there any questions i should be asking? what kind of maintenance is required for this device? frankly i like my existing tank that works just fine and i can ignore it. don;t have a choice so want to understand what i am signing up for with this new model. the other model he quoted was the eternal on demand hybrid which is more expensive and unproved track record since it is so new on the market. for my needs he suggested i stick with the TK-3. the quote for the eternal was $4700, same install details as above plus the condensing line.

Okay, I'm listening to what you are saying. Evidently 1.5 gpm is standard for an energy saving showerhead. A normal showerhead is flow restricted to 2.5 gpm. The next time I'm telling someone how to size a tankless, I'll include that information. And I'm all for using less water - but the 1.5 gpm number hasn't commonly been used in sizing. That said, when we are talking a shower with a rain showerhead, it's not going to be less than 2.5. I'm very comfortable with the 5 - 8 gpm number. What I don't know is the water temps in S. Louisiana in the winter. The average is 67... so is 60 a good winter time number? At 60F water inlet, the NRC98 will do 7.5 gpm - which is about right. Kudzu - Regarding "...almost 20 gpm of hot water..." this is an important thing I want to drive home with the OP and anyone else: You cannot look at that "best possible situation" gpm number on a tankless heater and think it will produce that. The only GPM number that's applicable is the amount of GPM it will produce given the temp rise needed to bring the groundwater in your area up to 105 F in the winter. Note: I made a mistake on the temp rise and GPM on my original example. I quoted the numbers for a 60 degree temp rise when it should have been 50 degree temp rise. At that rate a single heater would provide the needed GPM. I have edited the entry to correct it.