We have a Power Star Bosch water heater it find for two years but now it take longer to get the warm water and it turn back to cold very fast. Model #45-557701

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

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

Call Bosch. And exercise your 10 year parts warranty rights.
Copy following link for Bosch power star manuals:
http://waterheatertimer.org/Troubleshoot-Bosch-Tankless-water-heater.html#Ariston
Check circuit breakers, fully off and then fully back on. Test if voltage is at least 80% rating plate on unit. Check that temperature dial is set correctly. Call for service again.
Don't install tankless computer that heats water. You cannot fix this computer yourself, and they are problematic for owners.
Go to Lowes right now, spend $285 and buy tank-type electric water heater, roll it into your house, and save money, since the electric tankless computer heater costs average $100 more per year to operate than tank-type heater for average family of 4.
Tank type heater can be repaired by homeowner same day for $25 using off-the-shelf parts from hardware store. And that empowers the regular guy, and gets you off the junk tankless treadmill.
Gene

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

Clean water filter yearly or monthly. Page 16 example manual.
http://waterheatertimer.org/pdf/Bosch-125BO-NG-LP-outdoor.pdf
Shut off the installer supplied cold water supply valve.
Disconnect water pipe going to the back of the water
valve. Remove filter from back of water valve. Clean the
filter and inspect for damage. Replace if the filter is at all
damaged. Inspect the top of the water valve for signs of
a water leak or corrosion. If present, rebuild the water
valve.
Bosch service bulletins:
http://www.bosch-climate.us/bosch-hot-water/aquastar-service-bulletins/38b-125-series-bulletins.html
Descale heat exchanger yearly.
http://waterheatertimer.org/pdf/Bosch_Descaling-heat-exchanger.pdf

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

Before calling service technician, open following links for Bosch troubleshooting resources and information that might help narrow down the problem(s):
No error code indicates that first suspect is a plumbing problem.
http://waterheatertimer.org/Troubleshoot-Bosch-Tankless-water-heater.html
http://waterheatertimer.org/pdf/Bosch_Overcoming_fluctuating_temps.pdf
http://waterheatertimer.org/Crossover.html
http://waterheatertimer.org/pdf/No-Error-Code.pdf

This should run two showers at once with no problem. If you have a shut off valve on the cold side of the heater, try closing about halfway. This will slow the water flow and the water will have more time to heat up as it pases through the tank. If you dont have a shut off valve, just cut back the hot water valve on your shower and do not use any cold water. If it gets too hot then turn on a little cold water. The object is to slow the speed of the water going through the tank. Tankless heater take a little getting use to but they work great when you know how to treat them. On my shower I turn the hot all the way on until it gets hot and then add a little cold water to bring the temp down. You may have too much water going through too fast to be able to heat it up to what you need. Try this and see how it works.

I had the EXACT same problem (installed my tankless last winter and worked fine, now this summer it fluctuates, have to turn the shower completely off and then on again to get any hot water and then it only lasts for a couple of minutes!) Argh - it is frustrating!

Here's what I found: the demand for hot water is lower during the summer (hot) months because the "cold" water coming in is already quite warm. You don't need as much hot water to make the shower temp comfortable. SO, my tankless was shutting off due to low flow demand. I've removed the flow restrictor from my shower head and it helped alot but didn't completely solve the prob. Next step is what has been recommended to me by several people but is much more expensive - change the pressure valve on your shower to a temperature valve. Or just wait til winter and it should work ok again :)

Bosch will give free tech support to figure out your problem. 800-798-8161. Check your paperwork if it is still in warranty they will send free replacement parts. If out of warranty go to www.houseneeds.com for replacement parts. Check the "Flow switch" a long two tubes with two little micro switches and a little round hat or sombrerro that MOVES when the H20 is turned on. IF YOU HAVE ONE OF THESE, it should turn on one or two micro switches. If you have this part it should be described in your "Owners manual". If you no longer have the owners manual you can get one free at www.bosch.com It will give you trouble shooting guide and or you can call them even out of warranty then purchase replacement parts.

water from heater is no longer hot

Slower the flow hotter the water is how it works . Sounds like the water section is blocked and could well need cleaning out . Draw back with this system is you need a good inlet flow and flow will drop when you open other taps . they are good for a small house a family house not that good . Not much you can do put in a bigger water inlet to heater ?

This's been a cold year!
Is this an instantaneous water heater?
If it's outside or on an un-insulated wall in an unheated area this could very well be all that's wrong with it!
Is your heater well insulated?
Are your pipes well insulated?
Is it in a heated area?
If not I would insulated it real good and try moving it to an area that's heated!
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