Okay... There are lot's of variables, you need to be systematic about ruling them out.

To start with, I have a question about scalding water from the on-demand water heater. There's simply no reason to have a water temp higher than about 110 degrees on a tankless. Showers are typically 105 which allows for some heat loss in the pipes and for some who might like it a little hotter.

You need to find out at what temp the heater is set, and see if it's maintaining that temp over a 15 minute shower. If you have to, put a digital thermometer on the outlet pipe and wrap it with something that will insulate it. You will get a reasonably accurate reading.

I suspect that you will find the problem is with the heater. Bosch heaters are crap, I'm sorry to say.

The other area that can be a problem are the anti-scald mixing valves. I would eliminate the heater as a source of the problem first though.

the heater temp is set at 106
I will check your suggestions and let you know
Thank you

Have you cleaned the water filter on the Bosch?

First, I took a shower and it was a 2 minute cicle of rollercoster hot/cold water effect. Jake - then I did the test on the outlet pipe. The temp went up to 142 F. The max temp listed in the manual is 140 with +/- 2F Stability. Also the manual says the LCD screen will blink until the unit reaches the set/desired temp - well , it keeps blinking all the time, even as the water is scolding. When it got to 142F, it started to drop. So, that tells me the temp limiter is working, but the hot temp sensor is not working. I am going to call Bosch and see if they can send replacement part.
Anyone knows how hard it would be to do labor ourself? the warranty does not cover the labor and may not cover the little part.

have not cleaned the water filter. Sounds like taking the cover is next in line for me.
Thanks

My question is why in the world would you install a PRV one well system? PRV's are only installed where the supply pressure exceeds or is likely to exceed 80psi.

On a well system the well pump determines the supply pressure and there are no static head variances such as can occur from a municipal storage tank. The well pumps cut out pressure is generally set at 60psi so there is absolutely no reason to install a PRV.

well, we thought that was the problem b/c of the air pockets in water, and b/c winter time and sprint time situation were slightly different, and it was suggested in the troubleshooting section. So, it's in the line now. The PSI at the water tank is 50 but I know it fluctuates through the house.

The Bosch rep said to clean the water filter and if the problem continues, then call them back with multimeter in hand and they will walk us through some testing. Will report later.

Lazypup,

Often PRV's are recommended by Tankless companies for systems on a well system. The changes in pressure affects flow which can equate to a roller coaster of output temps as the unit attempts to adjust fan/gas input with rising and falling input water pressures.

Usually the PRV is set at or near cut-in pressure or in the middle of cut-in and out to split the difference and semi regulate inlet flow to the Tankless.

This is often used as a bandaid for jet pump systems or submersible well pumps with less than desireable tank, control and distribution.

A PRV has absolutely no regulating effect ofter than to limit the max pressure to 80psi. When the line pressure is less than 80psi the PRV act as a length of pipe and has no effect. On the other hand, all PRV's are required to have a screen on the input side of the PRV and generally that screen is built into the PRV body.

While the PRV will serve no practical purpose, the screen in the PRV can and will introduce a repetitive problem from sediments in well water.

Complete disagreement there, since the majority of PRV's are adjustable

Take the Watts 25AUB,
Adjustable Reduced Pressure Range: 25 to 75psi (172 to 517 kPa), Standard Pressure Setting: 50psi (344kPa).

So if it needed to regulate 40-60 PSI well output it could be dialed to 40 PSI (minimum most tankless will allow) and deliver constant pressure to the tankless.

Plugged screens from a well is a whole other issue.

here is the text from Bosch's instruction:

Water pressure must stay above 30-50 psi during heater operation. For installation on a private well system with the use of a pressure tank, the lowest pressure range setting recommended is 30-50 psi (2.07-3.45 bar). The use of a pressure reducing/regulating valve directly after the pressure tank is an effective way to maintain constant water pressure to the water heater.
Watts brand 25AUB-¾" or N35B-¾" pressure reducing/regulating valvesor equivalent is suggested.

****

I am hoping it is just the dirt in the water accumulated over 3 years, we never had the problem 3 years ago, then it became slightly noticeable a year ago and now it is what it is. We have a water filter in the line immediately AFTER the PRV, - can it be placed after the water tank but before the PRV?

Yes you should move it, PRV screens are small and meant to just protect inner workings.

Many PRV's have a 1/4" TAP with plug for purpose of adding a gauge to adjust, some such as Honeywell/Sparcos, actually include one with many models

The Bosch instructions you posted are similar to all tankless instructions or troubleshooting.

update - did all the checks from Bosch instructions and checked filters on all fixtures in the house. The two fixtures that had most dirty filter were kitchen sink and guest bath sink. The inlet water filter in the heater was a MESS - lots of clay dirt, tiny pieces of grass, and its physical condition is not good so it has to be replaced. We took it out and continue to use the heater as normal until we get replacement. It costs $7, and $7 shipping, so I am going to buy two - for the same cost of shipment - to make it worth the cost. Now that we have big filter in main line , this little one should not have too much work to do.
I also changed a rubber washer in our showerhead to open water flow. I know these reducers are designed for city people to reduce their water bill, and we had it in the city, but now we dont need it.
All this really opened up water flow and reduced problem but not completely. Right now temp setting is at 104, but the outlet hot water pipe temp reads at 115F. We should still do the multimeter check of the water temp sensors.

But I was able to enjoy shower for the first time in longest time. Definately a noticeable improvement so thank you all!

also, our PRV does not have a tap to check the pressure, so we will install a gauge in the line, and while we are at it - we will re-arrange the order of filter and PRV.