Tankless WH...Mixed reactions.
Tankless systems for S&B can be a great way to be eco & budget friendly provided you can recoup the costs when used as a retrofit. Much less expensive in a new build. There are units for S&Bs that are very sophisticated and can provide a very stable water temp output.
That being said I'm a tad concerned about how they will work consistently well in RV applications.
Perhaps the most annoying thing is called SLUGGING. That is caused when the water flow rate of the is reduced below the point that it deactivates the WH and you get slugged with COLD water.
So, as an example, if you are using a low flow shower head. After a bit you think the water is too hot and you turn the handle to the cold side. This could reduce the hot water flow rate just enough to cause it to shut off and you now get slugged because of either a flow rate cut off or high temp cut off.
Keep in mind that most simple tankless heaters raise inlet water temps X degrees. X maybe be a single value for simple units used under a vanity or a variable value if there are adjustments available to the user. As a safety the unit should have a max temp limit but that can be tricky if it is simply and ON/OFF limit.
Looking at the Girard User's Manual
, bottom of page 12, item 6 & NOTE you get some valuable information.
1. User needs a minimum of 1 GPM to activate the unit. Below that, you get "limited" AKA slugged.
2. Adding cold water (but not too much) could keep the water flow near 1 GPM but, reading between the lines, the slow rate could raise water temp to the cut off of 131*. If that happens, you get slugged until the water get to 110* and then gets heated again...repeat and rinse.
3. The user had a Gas Modulator Control (GMC) and if not properly adjusted you could get slugged. See bottom of page 11.
For all practical purposes this unit is kinda "dumb". Its proper operation depends on the user fully understanding how it works and how to use the GMC. Smart units will actually measure inlet water temps, water flow rate and outlet water temps and adjust automatically to keep the water temps consistent.
The trick is to have a good feel for the actual temperature of the water entering the WH. That can be wildly variable from between the temp of water in the ground pipes to the water in the hose/filters/water softener and what ever before it gets to the WH. This can also change when you move from one CG to the next depending on outside and soil temps and thus finding the proper GMC setting again.
For those reasons I would suggest you enough water before getting in the shower. The idea is to get water from a stable temp source to your shower head. It could take a lot of water when using CG water with long water hoses, filters and softeners exposed to outside temps or not too much if using onboard water tanks.
BTW...be careful when taking a shower while running things such as a dish washer or washing machine. When they start cycling use of water off and on...your shower could get very interesting. LOL
I'm just responding from my previous experiences with tankless systems and reading the literature for one particular RV brand and model. Researching all available units might you find a better/smarter unit. However, low flow rates seem to bedevil many good units so look closely for low minimum flow rates to be one of the most important things to start with. Again, this is very important with RVers using low flow rate shower heads and/or using low flow rates simply to preserve internal water tank levels during dry camping situation.