I installed a Rinnai tankless water heater approx. 8 months ago. It worked flawlessly until this past week. I have noticed that halfway through a shower (3-4 minutes) the water cools slightly and I adjust for more hot water. I still have plenty of hot water but I now have to make a slight adjustment. Its this way in 3 showers so its not a change in a showers mixing valve. We did go on vacation for 2 weeks and it was when we returned is when I first noticed the change.

Usually, Breaker tripping indicates a hard short ... probably the heating element is shorted and needs replacement. Possible bad C/B but don't think so.
Good Luck! handy Henry.

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5 years is about the lifespan for tankless.
You might have shorted element. That would explain cooler water and tripping reset.
Test element: power is off, disconnect wires from element; ohms = voltage squared divided by watt rating of element
http://waterheatertimer.org/How-to-test-water-heater-element.html

Call service technician./ Call Bosch
http://waterheatertimer.org/pdf/Bosch-AE115-AE125_manual.pdf

Gene

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The first thing that you should do is get a water sample and have it tested for TUBIDITY. Then see what the contaminates are. This will tell you what the problem is. I seriously doubt what Rinnai is telling you is the truth. Then you need to look into a whole house water filter that has an NSF recommendation. That will stop all the issues that you are having.

The rinnai is a computer that heats water. It is not ordinary water heater most people are familiar with, and repairs are not do-it-yourself, requiring a trained technician qualified in that brand who can diagnose both computer and water heater problems and plumbing problems.
Call rinnai for recommended local technician. Click on support tab. Note their webpage is very short on repair details, unlike companies that sell tank-type heaters... because tankless units are not do-it-yourself.
http://www.rinnai.us/tankless-water-heater
Problem might be simple blown glass fuse on circuit board or complete failure of computer and sensors.
Here is page written for Rheem tankless, that explains general complexity of tankless computers, and showing required maintenance, such as deliming each year:
http://waterheatertimer.org/Troubleshoot-Rheem-Tankless-water-heater.html
To save problems with tankless computers plus anticipated expense of maintenance and repairs, I recommend homeowner install tank-type gas water heater:
http://waterheatertimer.org/How-to-install-gas-water-heater.html
Also install one-time-use whole house surge protector, and install grounding rods to prevent further damage... lightning frequently likes the same places over and again:
http://waterheatertimer.org/How-to-wire-Whole-house-surge-protector.html
Gene
h

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Single-handle faucet cartridge that leaks can cause tankless not to function. Check for crossover or cross connection:
http://waterheatertimer.org/Crossover.html
Cut losses from owning tankless computer that heats water. Go to Lowes and buy new tank-type gas water heater for $390 and have hot water right quick with less maintenance and own a product that homeowner can repair themselves.
http://waterheatertimer.org/How-to-install-gas-water-heater.html
Otherwise call service technician who is qualified to work on tankless computer.
Gene
n

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Do not spend 500 on plumber to determine that condensate drain was not installed properly on vent line, and that acidic water from combustion by-product has been dripping down into tankless and burning hole through the special-alloy heat exchanger instead of going down into floor drain and corroding your plumbing pipes.
Do not spend hours fulfilling the required monthly and yearly maintenance, or spend money on service call each year to get the burner cleaned and gas pressure checked on your tankless computer that heats water...
Instead, fix any tankless problem, any brand tankless, same day do-it-yourself for $385, by replacing 1 part: the whole thing:http://waterheatertimer.org/How-to-install-gas-water-heater.html
Do not install tankless.
Gene

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Clean water filter.
Delime heat exchanger.
http://waterheatertimer.org/How-to-delime-tankless-water-heater.html
Add tempering tank to preheat cold incoming water.
http://waterheatertimer.org/Tempering-tank.html
Check for bad single-handled faucet cartridges.
http://waterheatertimer.org/Crossover.html
Don't install multi-sensor computer-chip water heater to water lines and gas line and vent pipe since tankless will develop unsolvable problems.
Contact Rinnai and hear them tell you to call service technician, only to discover there are no qualified service technicians in your area who might be able to solve problem. 1-800-621-9419
Ask why the Rainnai site has much promotional information and no service manuals. Nor does Rinnai offer forum where tankless owners can freely exchange their endless unsolvable problems for all to see.

If you need further help, I’m available over the phone at https://www.6ya.com/expert/gene_9f0ef4df2f9897e7

Your 150 amp service is undersized, and/or possibly your home shares the power company transformer with another home.
http://waterheatertimer.org/How-to-wire-Tankless-electric-water-heater.html

Do a main breaker stress test:
http://waterheatertimer.org/Troubleshoot-household-electricity.html#stress

If you need further help, I’m available over the phone at https://www.6ya.com/expert/gene_9f0ef4df2f9897e7

1) Rheem offers on-line chat, which you rarely see for typical tank-type water heater
http://www.rheem.com/products/tankless_water_heaters/customer_support/
http://www.rheem.com/hrc.aspx

2) Call local service technician, and put that person in your rolodex-wallet for future help.
Ask technician for tips and tricks for keeping unit working and needed maintenance.
They will gladly offer a yearly service.

3) Read manual several times to get accustom to yearly maintenance required for tankless water heaters. Tankless water heaters are maintenance heavy. All costs are energy.

4) Read manual several times and notice word troubleshoot appears many times.
Since tankless are complicated machines, many internal and external factors can affect operation, especially as unit ages.

5) Look at label on unit to identify exact product number. Identify same/similar product at Rheem, and become familiar with Rheem website
http://www.rheem.com/products/tankless_water_heaters/

6) Read other question/answers at fixya about Rheem tankless to become familiar with the tankless era
http://www.fixya.com/tags/rtg_74pvn

7) Install DIY tank-type gas water heater and avoid the whole headache
http://waterheatertimer.org/How-to-install-gas-water-heater.html

8) Add a comment for more free help.
Also take advantage of fixya expert assistance live.
For a price, expert works with you while you work on water heater or any do-it-yourself project.
Fixya is always less expensive than a service call.

step one... remove from wall
step two... throw in dumpster
step three... purchase and install a quality tankless water heater.

Rinnai, takagi, Noritz are the only brands I install now. Aquastar (Bosch) always had problems, then purchased by Bosch but never fixed the problems, just put their name on it and sold them at Lowes or home depot.
Garbage!