Thank you Jake the Wonderdog.

We are trying to avoid combustion exhaust by eliminating a gas log fireplace and a gas range. We were hoping to avoid having to install a gas tank and supply just for one tankless water heater. Our current home has two propane tankless heaters, installed on opposite exterior sides of the house, an arrangement that works very well for us and our guests.

There were many things that had to be considered in designing the new house the views, the lot size and topography, natural light, and the floor plan for example. Unfortunately priority could not be given to consolidating the plumbing, we knew we would have to live with the consequences of having a linear, shallow home, both good and bad.

Your idea of the point of use electric tank heater in the master is interesting. Could this be a small capacity electric tankless rather than a tank heater?

I hadn't considered that the hot water pipes would be in the slab. This could shorten the run to the master to approximately 75 feet.

I will look into the Rheem. Thank you again.

Sure, I understand about not running gas for just one appliance (the tankless water heaters) that makes a lot of sense.

Regarding tankless electric for POU - don't do it..

Here's the thing: In your master bath you need to be able to supply 1.5 gpm for the sink (an electric tankless POU could do that no problem) but you also need to be able to provide 2.5 GPM for the shower or 4+ GPM in the tub... those higher flow rates would be problematic for an electric tankless POU . You would be getting into running larger electrical service and such. And it wouldn't save you anything.

A simple, cheap, 6-10 gal POU tank heater right in the bathroom will get you hot water almost instantly and cost very little to operate. A 100' run of 3/4" pipe would be about 2.5 gal of water... so that would half empty a 6 gal tank - which probably has about 5 usable gal of hot water. That would all work out about right.

There's nothing magic about the Rheem 80 gal hybrid water heater. I think it's a high quality heater. The point is that it has the full set of resistance heating elements in addition to the heat pump (that's why the 30 amp circuit - which is standard for a standard electric tank heater). The efficiency is as good or better than the Steibel as long as it's running in heat pump mode. Of course the efficiency drops when it goes into high-demand mode. Keep in mind that an 80 gal tank is going to provide a lot of showers before it resorts to the electric heating elements.

If you figure 2.5 gpm for a shower x 10 minutes x 0.7 (70% hot +30% cold water mix) you get about 4.5 showers out of a full tank (75 gal) with no recovery time between or an 89 gal first hour rating which would get you 5 full showers.

Recovery is 29 GPH with a 90F rise with the 30 amp version verses 16 GPH with the 15 amp version like the Steibel (Rheem sells both).

The first hour rating with the Steibel Accelera 80 is 74 gal. and a heat up time of almost 10 hours with 60F ambient air temp. That's just too long for full recovery if you have a full house.

Re: Pipe in the slab: Be sure the water line is well insulated. One person had an installation that wasn't insulated and the hot water would completely cool down by the time it got to the bathroom. It was heating up all of the concrete between the two points though.

The Rheem heat pump water heater has a control screen where you can change how it operates so it makes hot water faster, a high demand mode. When you have your 8 guests visiting then you could change it to that setting so it makes hot water faster. After the guests leave then you can change it back to the heat pump only mode so it goes back to saving more energy.

Here's how Rheem explains it on page 18 of the owner's manual on the Home Depot website, "High Demand - This mode provides the highest recovery while still providing good energy savings. Water heater operates Heat Pump and electric heat simultaneously."

I think it's important to point out that even the 80 gal Rheem with fast recovery will not allow 8 guests to bathe / shower in the same hour.

As was shown with the math, it would allow about 5, 10 minute showers (not fill up a bath tub) in an hour and then take about 2 hours to recover.

To really get 8 showers x 10 minutes = 140 gal of hot water in the first hour you would need 2 of the 65 gal units. They each have a first hour rating of 75 gal.

You would potentially just shut one off during the time when you weren't running a hotel.

Thank you so much for taking the time to give all this help. It is great to have something logical to work on. It has been difficult to find any unbiased information or sensible solutions to our specific issues. Now we have to digest all you have said and do some research into specific heaters. I may return with more questions! I am so glad I posted.