Question about Bosch PowerStar AE115 Tankless Water Heater...

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I have a Powerstar Electric tankless water heater, 45-551301, installed in a new home less than 3 years ago. We have always had minor problems with the hot water staying hot, fluctuations in hot and tepid and some issues with needing to turn the shower on then off then turning a nearby sink on until hot water arrives then turning the shower on to get the hot water. All-in-all a little frustating but ok given the limitless hot water available. However just recently the water heater has been fluctuating from hot to ice cold and then back to warm then staying ice cold. We are on a well system set to kick the wellpump on at approx. 40psi, the pump will bring the pressure up to approx 55 psi and then kick off. The water heater seems to stop working when the pump kicks on; then starts back up when it kicks off, only sometimes, sometiems it will not turn back on. This seems to be more of an issue with the tub and shower than the sinks. We live in Ohio.

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Tankless heater are a little frustrating sometimes but this is generally an issue of waterflow I believe. The fluctuations are do probably to the amount of water and rate of water going through the heater. If you have a shut off valve on the cold side of the heater, shut it back about half way. This will slow the water going through the heater and allow the heater to heat it up more efficiently. Then go to the shower, turn the hot on all the way (no cold) and allow it to heat up. Once it has heated up, test it by hand for a good temperature that you like. If it is too hot, turn the cold on just a bit until it comes to the temp you like. If it is too cold turn the hot down just a little (still no cold) and it will warm up. Ideally you should be able to turn the hot on full blast and then just turn on a little cold to get the right temp. Once you learn how these work then it makes it easier. You really cant run the water like you do with conventional hot water systems and expect to get the same results. Also, wanted to give you another little tip. Every 6 mos to a year, you should flush you heater with vinegar for about 30-45 minutes using a small cheap submerible pump like for a water feature in a garde. Just put in a bucket of vinegar and and run a hose to the "in" side of the heater and another from the "out' side of the heater and then back into the bucket. This will circulate the vinegar inside the heater and clean all the deposits that will DOA your heater. Funny how the manufacturer wont tell you this but these deposits will ruin the connectiosn on the heating elements. You can make your heater last for a long time if you do this.

Posted on Mar 27, 2011

Testimonial: "Thanks, this info helped me to remember what Bosch Tech support sort of told me 2 yesrs ago. I'll try the vinegar as well. Thanks, again."

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  • rrreddd Mar 27, 2011

    oh yeah, I would do the vinegar thing too as soon as possible. These depostis could already be affecting its performance and your flow switch may have been affecting and adding to these problems you are having.

  • rrreddd Mar 27, 2011

    sorry, just thought of another thing.. Do you have a sediment filter on the "in" side of the heater?? you need a cheap sediment filter like you can get at home depot or lowes. It should filter the water before it goes into the heater. Sand particles will wreak havoc and totally disable your flowswitch. Most well systems already have that but I wanted to make sure.

  • danrmiller Mar 27, 2011

    I've been testing the whole water system for the last 2 hours as well as reviewing other information on the web about troubleshooting this model. The issue with losing the hot water altogether ended up being our large bath tub faucet; the flow on this fixture is much more than on any other fixture in the house and the water gets extremely hot while the other fixtures seem to be less hot. Once the water is at it's hottest while the faucet is turned on full hot water flow, the water then turns ice cold and the water heater turns off completely. I think the flow rate from this faucet is more than the 4.0 GPM the water heater handles and one of the heaters/elements overheats. Or at least this is my best guess. I then need to turn on a sink faucet and the water heater will work just fine after a minute. I'll turn that off once I get hot water, then turn on the shower that is directly off of the same water line as the large tub; I'll get hot water there, turn it off, and turn on the large tub fixture and it will have hot water...then hotter water.. then extemely hot water...then ice cold all within a minute. I went through this three times to ensure the sequence and theory behind it, then I decided to turn the large tub fixture hot water on only a small amount to see if reducing the flow would help. The water was hot but not scalding, then I turned it up a bit more....then a bit more....turned the cold on a tiny bit...and got it to a good temperature, left it that way for a bit more...turned the hot up a bit more..ok..turned the cold off...still ok....and this after 5 minutes of running this test...then turned the hot completely up at full flow and within 5 seconds the water was ice cold. These are fickel pieces of equipment, but it seems well worth it once figured out. We do not usethe big tub much, my wife took a bath last night and had the problem of ice cold hot water, then after being gone all day today and using the hot water only very minimally I gave my son a bath and found the water heater not working on any fixture in the house for very long. Then after keeping a kitchen sink on for afew minutes, the water heater kicked back on completely instead of the light just flickering and never coming on in full. I tested all the fixtures this way and, as noted above all but the large tub would work well...still a bit of fluctuation from hot to warm to tepid then hot again, but better than ice cold. I'll try the vinegar and check the sediment filter, we have a pre-filter on the softener system as well. We do have hard water and the softner uses alot of salt given the hardness. The water pressure from the pressure tank/water pump kicking on and off did not seem to be a problem once the water heater was working better, except for maybe the slight flucutation in warm to hot, but not in turning the water heater completely off. Sorry for the length of this, just wanted to see if this may help you out at all with any further questions from others. Thanks for the help it's been really helpful.

  • rrreddd Mar 27, 2011

    yes sir, it does seem it is a water flow problem. I think it will make things easier if you install a shut off valve before the water enters the heater (just a cheap pvc valve is fine). Then you can regulate the waterflow coming into the heater by closing the shutoff valve maybe a quarter to half way to restrict the flow a little. This should alleviate all the stuff you are having to do at each individual fixture. And this wont greatly affect how much water gets to the faucets. If you dont have a shutoff there, I would put one there. If you do, try shuting it down 1/4 to 1/2 and see if it works better. I believe your water flow is too fast for the unit and the desired temp you are trying to achieve. .

  • redbirds09 Apr 01, 2014

    You can install a regulating valve in the line to help with this problem. You may have to play with the valve a little to find the right pressure to avoid those highs and lows. I installed a Bosch A125 and had this problem from the start. A little research un-earthed the regulating valve (I think around $25 at Lowe's or Home Depot) and we have had even hot water ever since.


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Having had this problem on two different installation locations, I would suspect that your flow rate sensor is dirty/sticking from deposits.
This can be resolved by removing the sensor and cleaning/replacing it - or flushing it with vinegar and allowing the mixture to soak for at least 30 minutes. Both techniques have produced excellent results without restricting your maximum water flow by partially closing down your intake valve.

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There is power to the system but when you turn water on elements do not come one and heat water. all phases have 238 volts to them. why will system not turn on and heat water

Do you have 3-phase water heater? Is this balanced or unbalanced 3-phase unit. Add comment and describe product in detail, including brand since i have some service resources for both types.
Or do you have ordinary residential split-phase 240Volt Bosch tankless computer that heats water, and has 2 or 3 separate elements, and a separate circuit breaker supplying power to each element? And testing across both hot leads coming from each breaker yields 238Volt?
Copy following link for Bosch tankless manuals, with brief troubleshoot section that does not provide detailed service procedures for repairing tankless computer:
If this is new water heater, then call installer. If you installed yourself, then remove tankless and return to seller for refund.
Tankless computers require certain amount of water flow to activate computer sensors. Turn on Hot tap and listen to pipe to hear if water if flowing through hot pipe. Check for crossover:
Clean water filter usually located just above where cold water connects to unit.
Delime tankless ... and use water softener. Tankless cannot be exposed to more than 11 grains hardness in water, or parts will fail within 2-3 years. Simply deliming tankless heat exchanger each year will not maintain full efficiency... although for typical family of 4, tankless comper water heater costs $100 more to operate than tank-type heater according to manufacturer's promo literature and independent studies.
Call local technician who specializes in fixing tankless computers. For plumbing problems call local plumber. These are two different things.
If tankless computer is older, then cut losses, and buy tank-type electric water heater at Lowes for $285 and make quick changeover and have hot water same day. Sell old tankless on ebay and cheat next guy, or sell at scrapyard for $3 per 100.

Aug 05, 2013 | Bosch PowerStar AE125 AE125 Whole House...

1 Answer

I have a PowerStar tankless electric water heater, model #44-557710 and it was replaced about 3 months ago. This morning I had hot water and this afternoon I do not. When I call for hot water a red light...

Call Bosch. And exercise your 10 year parts warranty rights.
Copy following link for Bosch power star manuals:
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.

Aug 03, 2013 | Powerstar Water Heaters

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AE-9.5 Bosch Tankless Water heater - no manuel or installation instructions

This is a "point of service" tankless water heater that was manufactured by BOSCH. BOSCH is now Technical support is available at 866-330-2729. P.S. check this site:

Feb 02, 2013 | Bosch AE-9.5 Electric PowerStar PowerStar...

1 Answer

Failed Bosch AE125

Contact Bosch 800-283-3787, or service technician.

Product has 10 year limited warranty against defects in workmanship, failed parts, failed heat exchanger for original homeowner with receipt. But does not cover neglect, failure to follow maintenance, installation etc. Pg 23 example manual
You generally have to pay for service call to verify installation, then send tankless into factory, or have service technician repair unit, at your expense minus cost of parts

To double check wiring:

Sep 29, 2012 | Bosch PowerStar AE125 AE125 Whole House...

1 Answer

I have a Power Star electric hot water heater, model AE 125, serial # 39-203967, model 45-55130. purchased 5 years ago at Home Depot for $850.00. I love the heater when it works. This is the second time I...

Bosch has 10-year limited warranty.
Probably the seller honors warranty first 90 days and Bosch afterward.

I think warranty is similar to car warranty where parts are covered, and they pay for parts to patch up the blown transmission but not give you a new car. But I hear of cases where the company just send a whole new Tankless unit and you install it yourself.

Contact Home Depot and present them with receipt.
Contact Bosch with the same information given in Fixya question.

Add a comment and say why unit broke down first time, and add what resolution was reached to help others with same question.

Upvote the help.
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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.

Jun 02, 2011 | Bosch PowerStar AE125...

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I have an AE125 - had it for a couple of years at the cottage. We got the water hooked up this spring and all was fine. Last weekend, the unit stopped working. I flipped the breakers a couple of times...

Can tankless electric water heater be repaired? Sure.
Is it worth the expense if problem is going to happen again? I can't answer, but can offer resources.

New tank-type electric water heater costs $207 and new tank-type gas costs $320 at local box store.
You can install new tank-type electric yourself in a couple hours. DIY repair on tankless is not easy since parts are not at local box store, and what part do you need, and tankless parts can easily cost more than new tank-type water heater.

Open following link for Bosch troubleshooting resources.

GENERALLY, Tankless require .4 gpm (gallons per minute) water flow to activate heating cycle, and then require .25 gmp flow to keep heating cycle going.
For example .4 gallon per minute means faucet can fill 1 gallon container in 2-1/2 minutes
Some tankless units require more water flow, for example .75 gpm

Tankless water heaters have flow sensor that tells computer chip when enough water is flowing to activate heating cycle.
If the sensor is clogged, or if water filter inside tankless unit become clogged with dirt or sediment from hard water, then the unit does not signal enough water flow and will not activate.
Home filtration system and water softener can also slow the flow of water.
Low water pressure and crossover problem on single-handled faucet and crossover on recirculation system check valve and high thermostat setting can also cause heat cycle to not activate.

Tankless cannot be exposed to water with more than 11 grains hardness or parts inside tankless will coat with sediment and not work. Within 2 years the unit will not work, with diminished efficiency from the beginning.

If water is hard, and you cannot soften the water, or if water is dirty and cannot be cleaned, then the best recommendation is to use tank-type heater instead of tankless.

All costs are energy. For average home: Tankless electric is NOT cheaper to operate than tank-type electric. Tankless gas saves $1-8 per month. However, cost of unit and installation, cost of potential repairs and cost of parts, difficulty of DIY repairs, required yearly-monthly maintenance, etc outweigh expected savings.

Upvote the help.
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Fixya is always less expensive than a service call.

May 31, 2011 | Bosch AE125 PowerStar 4 GPM Indoor...

1 Answer

Have an ae125 electric water htr. all power is at unit; elements ohm good and water is luke warm ; only pulling current on one circuit on each tank. power is present at board but does not seem to leave it ...

I don't know exactly how to solve your Bosch tankless problem, but there are resources for troubleshooting including contacting Bosch and reading manual.
You can buy 40 gallon tank-type electric water heater at Lowes for $207. Offer 10% less, and look for scratch and dent models with more markdown. Tank-type electric are easy to install and cheap to fix, cheap to maintain.

Add a comment with your experience so others with same-similar problem can benefit.

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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.

Apr 11, 2011 | Bosch PowerStar AE125 AE125 Whole House...

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Only using cold water and light turned of

You posted under Bosch electric tankless.
I don't know specifically how to fix your tankless from information given.
Tankless problems can be simple or complex, and include internal and external factors best gauged by experienced service technician who is there and can put hands on the appliance.
You can also glean information from the service technician how to best maintain your tankless unit, since they require yearly maintenance, such as de-liming.

Open following link for Bosch tankless troubleshooting resources, including Bosch contact information, typical manuals, error codes, tech support, parts, fuel-saver calculator, service bulletins, water heater forum, fixya links, disclaimers, and links for finding service technician.

Add a comment for more free help.
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Fixya is always less expensive than a service call.

Mar 21, 2011 | Bosch PowerStar AE115 Tankless Water...

1 Answer

My water heater wont refil with pressure

Open following link for Bosch tankless troubleshooting resources, including Bosch contact information, typical manuals, error codes, tech support, parts, fuel-saver calculator, water heater forum, fixya links, disclaimers, and for finding service technician.

Add a comment with your experience so others with same-similar problem can benefit.

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Fixya is always less expensive than a service call.

Mar 19, 2011 | Bosch AE-9.5 Electric PowerStar PowerStar...

1 Answer

Where do I find model number or serial number?I turned off main water valve to replace a leaking pipe and now I can't turn on the water heater. How do I do it?

1) You posted under Bosch tankless water heater.
There should be a nameplate on front of unit or located where you can see it.

Bosch problems at Fixya:

Bosch has online tech support form:

2) Replacing water valve probably loosened build-up inside pipe that lodged in water heater or inside elbow just beneath tankless unit.
If water flow is stopped or slowed, tankless will not operate.

3) Remove pipes and see if clog can be flushed.

4) Also U-tupe search "delime tankless water heater" and technique used for deliming will be good way to clear out tankless clog and sediment.
Your tankless should be delimed each year anyway.

5) Read your manual cover to cover, and see words 'troubleshoot' and 'maintenance' are used throughout manual. Get acquainted with your water heater and manual, because tankless are maintenance heavy, especially if conditions are not optimal and as unit ages.

6) All costs are energy. Install tank type water heater for ez do-it-yourself repairs and maintenance.
Tank type work under all types of adverse conditions and water pressures and water conditions, and even withstand sediment build-up for several years. Of course tank-type pipes can clog too.

Add a comment for more free help.
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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.

Mar 08, 2011 | Bosch PowerStar AE125 AE125 Whole House...

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