Sorry, I cannot comment on the Eternal line, but I can tell you I installed a Navien 240A (recirculating) tankless in my house nearly 2 years ago. I live in zone 5, Utah -- the temps range in the winter from 5 degrees at night to highs in the upper 20s. I'm very happy with it and would do it again. I have 2.5 baths in the house.

Tankless water heaters are more expensive to buy and operate than tank models, and most require annual maintenance (probably $100-200 per year). The model you're looking at (as per its website) would struggle to do more than one thing at a time - a typical winter temp rise (cold water of 45 deg to hot at 120) may only be done at a rate of less than 5 gal per minute. So if filling the tub, no one showers. Or if showering, no dishwasher or clothes washer activity without having the shower's hot water throttled back.

If you're set on tankless, I'd go the multiple unit route or connect it to the tub only and use conventional water heaters for everywhere else.

Usually people that don't have tankless or a quality tankless misunderstand tankless.
I'm from NY, and 5GPM is plenty for a 2.5 bath house. Quality tankless regulate flow, so everyone gets hot water. Yes you can shower and do wash or dishes, since a typical shower uses 2 GPM of hot water mixed with cold. If you have 3 showers running at once they just split the flow, no one gets a cold shower, just less pressure.

In the end it may take some occasional fixture use planning, like tub filling but at least you don't run out of hot water

Try doing that with a 40 gallon tank, at 5 GPM, it will be running cold in 5 minutes.

I would stay away from the eternal units.

zl700, I'm glad you used the word "usually".

Usually people plan the systems of a house to meet expected use requirements. It's not usually necessary to plan use to meet the requirements of the systems. Most people would be calling their plumber for help after taking a shower with one third of 5 gpm of hot water.

A 4.5 bathroom house is not going to have one 40 gallon unit. I have 4.5 bathrooms, two tanks, and enough hot water to satisfy multiple and any uses at any time. It's done much less expensively than if I had tankless heaters.

To each his own.

This is the old "speed costs money, how fast do you want to go" discussion. The Eternal is a good unit, but once you deplete the tank, which is quite small, it goes back to standard tankless operation. Navien makes a good unit as well. I was the Rinnai rep for 20 years so, I'm biased that way and because I have the numbers on them will reference them.

A good shower head is 2.5 rpm. With a 70*rise (say 50-120the RU98i (199kbtu, .95 EF) will make 5.5gpm. A RL94i (my favorite, 199kbtu, .82EF) will make 4.8 gpm. At a 50* rise the units will make 7.7 and 6.8 respectively. this is important when considering tub fills. The reason I say this is my Rinnai has the optional bath controller. With this I can program the volume of water necessary and the desired temperature and the unit will provide that amount of water at the set-point temp, shut itself off and beep to tell you your tub is ready. It needs 18-2 wire to the water heater. BTW, if you sit in a tub over 105* you are no longer bathing, you are par-boiling.

BIG tub tells me nothing. You are building a mechanical system and numbers are important. What is the tub volume? Figure the cu ft of the tub. There are 7.48 gal/cu ft. The manuf web site will give you that figure also, but be careful as you need to figure how deep you will actually fill the tub.

Get a thorough water analysis done prior to purchase. Not knowing the lay-out it is difficult to advise as to whether it is best to locate multiple units together and manifold them for max capacity or to break it up and get them closest to the point of use. I prefer to break them up, but the tub fill rate can become an issue. One way to bring flows down is to put 1.5 gpm shower heads in the secondary bathrooms and 2.5 gpm in the master. In my house with 50-55* and 120* supply water at my desired shower temp it took 2.1 gpm of hot mixed with .4 of cold to feed my shower. You can read those flows on the touch pads. Care must be taken in sizing gas lines! Care must be taken in gas line sizing>

Very high quality tankless water heaters have been available in the US for 13 yrs now. Training up to the level of complete tear down service is offered by all the manufacturers.

When installed properly all that is necessary to start the unit is to open the tap. Out of the box reliability is outstanding...when properly installed, and it ain't rocket science. If the contractor doesn't do his part on the install, then they may well be on the line with tech service. Once it is run for 10 minutes or so, turn it off, turn off the hot and cold service valves and clean the inlet water filter. That eliminates the "new construction dirt and debris" that moves to the unit in the new piping.

Another thing about tankless, at least the Rinnai's, is that they are designed to guarantee temp at the expense of pressure, meaning that if you are in a shower and other multiple uses occur which exceed the rated output capacity of the unit the flow control valve will begin to close restricting the vol but guaranteeing temp. You would see a reduction of pressure in the system. As load is shed the pressure will come back up.

One of the things I like most about tankless is that you will not run out of hot water. We had a crew at the house some time back. The house was full, and people were camping out with us. On Sunday morning we ran two showers continuously with 18 people showering. Everyone had as long and hot a shower as they wanted and we had no issues. Pretty cool! Try that with a tank(s).

4.5 baths is a number. Will you have 4 people bathing at same time? Even a wife/ hubby rarely shower together. And if they do, they use 1 shower.