Tankless sizing, Noritz NRC98-DV versus NRC111-DV

I am having my house re-piped, and plan to install a tankless water heater. I had planned on getting a Noritz for a few reasons: thick copper in the exchanger, "dual combustion" burner, and the wide range of flame modulation (BTUs).

I am contemplating the sizing, NRC98 vs NRC111. It is common for us to use two showers simultaneously (5 GPM estimated) or even one shower and one bath (7.5 GPM estimated). Our groundwater is not that cold, probably no worse than 55 degrees F even in the winter. I would like to maintain a 115F output temperature under most conditions, although dropping to 110F would be acceptable under maximum load given the thermostatic shower valves.

At first I thought the NRC98 would be appropriate, given that I have heard it is best not to oversize a tankless HWH. However, looking at the temperature rise, flow rate, and especially the pressure drop, it appears the NRC111-DV would be the way to go, especially given the lower pressure drop. And this is their largest model!

Is my thinking correct? Do I really want the largest heater? Will it still be efficient under low-flow conditions? The minimum BTU rating is actually lower for the NRC111-DV (11,000) vs the NRC98-DV (16,000), although I don't know if this reflects the reality of their performance under low-flow conditions.

Lastly, I intend to use a Ready Temp hot water recirculator for the kitchen sink loop 50' away with a dedicated 1/2" return line. I am contemplating timing it to be on 10 hours a day, or pushbutton. The pipes will be insulated, of course. Would this cause unreasonable wear and/or excessive operation of the tankless?

No matter what you set the tankless output temp to, the showerhead output is going to be ~105F, give or take a couple of degrees. At 115F it would be scalding hot. If you set it that high you'll have a lower gpm/higher delta-T through the tankless than at the shower head, and mixing in some from the cold side at the shower.

With 55F groundwater and a 105F shower output you have a 50F delta-T. At 5 gpm of shower head flow that's about 2500lbs/hr, so your peak draw is really (50F rise x 2500lbs/hr=) ~125,000BTU/hr.

The NRC98 takes in 180,000BTU/hr, so even if you assumed only 80% combustion efficiency at max-fire (it's actually higher than that) you'd have 144,000BTU/hr of output to support a 125,000BTU/hr load, which is PLENTY of margin. Then when you figure 2.5gpm showerheads really run only ~2 gpm unless you have unusually high water pressure, the only time you'll be bumping the thing up to anywhere near full fire is with 2 showers AND another significant simultaneous draw, or when filling a tub.

With a recirc you won't get very high net efficiency out of the recirc burns, since the incoming water temps are high and the volumes are low- it's short-cycle. If you go that route the push-button versions offer the least wear & tear, and higher efficiency, since you'll be feeding it cooler tepid-water from the recirc path, and you'll be using the hot water immediately rather than making it a separate burn.

The pressure drop issues are largely overstated as a problem. Where & when it's an issue there are relatively easy fixes that don't involve bigger burners.

The difference of min-fire output isn't enough to be of concern. It's primarily an issue of temperature regulation at very low flows, but anything under 20K won't have flame-out issues at low flow in summer the way some old-school 30K-min tankless units can. Most of them are pretty efficient at low flow min-fire, but the sweet-spot is usually at a somewhat higher fire than that where there's a bit more turbulence in the gases on the fire-side of the heat exchanger. Where you should worry about efficiency is at flows that are sustained and high volume, such as showers or tub fills, since that's what will show up in fuel use. Most low-flow draws are short-cycles anyway, often throwing away as much heat in the ignition cycle & flue purges as went into heating those 1-2 cups of hot water that you actually used. But most of the fuel is burned for the volume draws.

Thanks for the reply. My comments were based on Noritz specs. The pressure drop at 7 GPM for the NRC111 is 12 PSI, 7.4 GPM at 50 degree rise. Pressure drop at 7 GPM for the NRC98 is 22 psi, 6.7 GPM at 50 degree rise. I am going to some effort to use properly sized pipes in the re-pipe and assure good water pressure throughout the house regardless of simultaneous fixture use. Perhaps I am overreacting from living in a house with bad pipes for the past 6 years, but if there is not a significant penalty in cost or efficiency I would prefer to design in some margin for pressure drop, temperature rise, etc.

I understand hotter than 105 degrees is not needed for the shower, but I think it is not a bad idea to have water entering the valve at a temperature slightly higher than desired. The shower valve can always mix in a little cold.

If my read on the specs is correct, the NRC98 can only reach 100F at 7.5 GPM flow, with significant pressure drop. Yes, the shower might flow less than 2.5 GPM. Or I might get a proper head and "ensure" the flow is 2.5 GPM.

It sounds like you think the NRC98 is enough? What about margin for a better showerhead, tub fills, or who knows what?

With my RedyTemp unit, at the setting I have it (it's adjustable via a front-panel knob), in the winter, it runs for about 90-seconds, about 4x an hour. At this setting, the water is warm immediately, and hot shortly after. Since my shower and everything else in the townhouse are closer to the WH, they get hot essentially immediately. Many of the recirc system run the pump continuously, and use a valve to open and close to maintain the desired temp. The RedyTemp (at least with the model I have), it only runs the pump when it needs it. This does mean two things, though, the pump is likely to wear out sooner, and the check valve will too. Now, they say they test the check valve at a minimum of 1M operations, which is a lot of years. Mine's been going fine for about 7-years now with no service and, if you have power where you can plug it in, it takes all of a few minutes to get it up and running. I've got mine on a timer that only turns it off while I'm normally sleeping. Before I retired, I had it shut off during the weekday as well.

It is my understanding the positive action of the Redytemp solenoid valve precludes siphoning or leakage of the hot water into the return line, making it more efficient. I will probably use a pushbutton timer so that one event keeps the pipes hot for 30 minutes or so, nearly eliminating any extra firing events from the recirc.

They used (and probably still do) a Deldrin pop-it valve to open/close the cross-over so when the pump is not running, there should be no possibility of leakage into the cold water system. Now, if you had a dedicated return line, it would still help some.

It's the only one that easily lets you set the desired minimum temp at the end point (where the pump is). I've got mine set so it is just warm, but everything else is closer to the WH, so they're hot. Warm at the vanity sink is all I need and my shower is hot immediately along with everything else in the house.

I finally got my new NRC-1111DVNG installed. It is almost completely silent and has a Redytemp hot water recirc pump with dedicated return to supply the kitchen at the opposite end of the house. I also installed Fleck 7000 SXT Centaur carbon filter tank and water softener tank, both 1.5 cubic feet, to protect the wifes skin and the hot water heater. Gas pipe starts out at 1-1/4" from a larger meter that I had the gas company install, dropping down to 1" about 50' from the heater. It does not adapt to the 3/4" inlet pipe untill 6" from the heater. Same thing with the water in/out - 1" copper adapting down to 3/4" right at the heater. Hard pipe only with unions and isolation valves for service, no flex.

I really like the lower temperature allowed by the tankless HWH, which does not require excessively hot water just to retain capacity. Makes it much easier to adjust the temperature at the faucets. Not to mention the Toto thermostatic shower valves.