To answer the title question: No.
From the Rinnai manual:
- RE199i, RE180i, RE199e, RE180e - temperature setting 98 F - 140 F
- RE160i, RE140i, RE160e, RE140e - temperature setting 120 F - 140 F
Searching a bit more, found some Rheem gas 100 F 140 F and a Titan electric 105 F - 120 F.
And now I found Rheem electric 18 kW 3.51 GPM which according to the manual can be set between 80 F and 140 F.
I suspect that electric actually gives finer control here than gas because it can run at a low level and/or be modulated (similar to dimming a light fixture) which is not so easy to do with gas.
80 F is the lowest I found. Is 70 F possible? Yes. But since the primary use case is hot water, that isn't going to be a terribly popular option.
And now back to my regularly scheduled tankless
Let's see if I can understand this:
- Tankless water heater is unable to produce hot enough water. Meaning, it is undersized for the intended usage.
- Rather than look at a different solution, just go ahead and add another tankless water heater.
That doesn't make much sense to me. If "plan a" doesn't work, why not try "plan b" instead?
In fact, "plan b" is the older, proven, technology - tank water heater. With one exception - extreme space constraints - it actually works very, very well.
Tanked and tankless water heaters do the same thing: heat up water. Tankless has two possible advantages:
- Unlimited hot water, as long as you stay within the limits (flow rate and temperature combination).
- No wasted energy due to loss of heat from already heated water.
The first one has already turned out to not be correct in your particular installation. That may be due to poor specification - i.e., not planning for the flow rate actually desired. Or it may be due to serious limits - particularly if this is an electric tankless heater. Electric tankless water heating uses HUGE amounts of electricity - often equivalent to everything else in the home put together, sometimes 2 - 3 times as much as everything else in the home put together. Really. Adding more electric tankless may not even be an option if your service is already maxed out, and that may be the reason why you have too small a tankless heater for your needs. Or you may have natural gas tankless and a little more capacity (maybe) to add an additional tankless heater.
The second reason is actually highly overstated, particularly by vendors of tankless systems. Touch the outside of a modern (last few decades) tank water heater (electric or gas, doesn't matter) anywhere except for at the hot water pipe, the exhaust duct (for a natural gas heater) or near the burner (for a natural gas heater). It doesn't feel warm, does it? That's because these tanks are extremely well insulated. Is some heat lost over time - of course. Can't escape thermodynamics. But very little. And in the winter it is 100% OK because the heat goes into the rest of your house. It is only a problem (a very minor problem) in the summer.
So the tankless advantages really aren't, at least not in your setup.
A tank water heater takes a few square feet of space, but other than that installs very similar to a tankless water heater. In fact, if it is electric then the installation is much easier because it only needs (typically) a single 30A circuit rather than as much as 3 x 40A circuits. A tank can typically hold 40 to 50 gallons. As you get larger it gets complicated due to government rules designed to push energy efficiency. But the basic tanks are extremely simple and reliable. They can also provide a considerable amount of hot water. Looking things up, I quickly found a 50 gallon electric heater that is rated at 61 gallons in the first hour and 21 gallons per hour recovery with a 90 F rise (e.g., 40 F in, 130 F out).