you say one bathroom in house, I will assume its a small house and never will expand on the house to two bedrooms. the tk-jr will be able to feed one shower for ever, you will run out of gas or water. its the best unit for one shower house. on the back of the flyer there is a chart, it will tell you exactly what it can handle.

Okay... Size your heater for the worst case, about 35 degree inlet and 5 gpm. I normally design for 105 degree shower, but you say you like it hot, so 110. The manufacturer will provide the table that shows the temp rise at a given flow rate. Ignore the "efficiency rating"... it's bs. Stick with the energy factor. I think they need an energy factor greater than .80 to qualify for the tax credit. Don't bother with one less than that. You will not see a significant savings between the .80 and the condensing unit. Only go condensing if you need to vent with PVC pipe to get the vent cost down.

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.

This is very frustrating: The cost to operate involves only 3 things: The load (how much heat output is needed), the efficiency (how much heat is wasted - usually up the flue), and the cost of the fuel. Period. Propane is one of the most expensive fuels to use - so if you are using it go with condensing (92%+) heaters. That leaves the load as the only other place to make improvements - and I'm guessing you could make a lot of improvements there. It's not about judgement, it's about math. Within reason, the high btu input on a condensing heater doesn't translate into much difference in operating costs. Neither does the tank vs tankless. The least expensive way to heat hot water is a hybrid electric tank heater - that's especially true compared to propane. It's not the answer for your floor though or if you have uninsulated circulation lines that run all the time. If you absolutely want the ability to heat the floor on occasion then the answer is a 200k btu hour condensing tankless heater or a condensing tank heater. The condensing tank or tankless will cost about the same to operate. Lastly, are your circulation lines insulated? Can you run your circulation pump intermittently (timer, motion detector, push button) to limit heat loss. Regarding your question about keeping up over a long period of time (an hour) - Tankless heaters can deliver 184k btu/hr continuously until your propane tank runs out. That amounts to about 8.5 gpm of water heated from 60 to 105 (that's more than 3 showers at 2.5 gpm - a normal shower head). The tank type can deliver about 70k btu/hr continuously until your propane tank runs out (about 3 gpm - or about 1 normal shower head of water heated from 60-105) To answer your question about keeping up with demand we need to do some math. How many gallons per minute is one of your shower heads? How many showers at the same time? How cold is the inlet water? That's the only way the question can be answered.