Had it with the Bosch water heater!

We have had a love/hate relationship with our water heater for 2 years now. It’s a Bosch 635ES (aka 250SX) tankless, which for the most part, we love. It’s energy efficient and is great when we have guests and need to do 4 or 5 showers in a row (or even run 2 showers at once). So what’s the problem?

Noise. Since day one, this heater has made one of two different noises:

  • A loud, combustion-engine-sounding, house-shaking groan/grunt upon startup, and/or
  • A loud whistle during operation that can be heard a block away

The noises are related to the gas valve in the unit. It is apparently very finicky and tough to get adjusted properly. If the gas delivery is too little, it’ll groan; too high, and it’ll whistle. We are often lucky enough to get both. Our next door neighbor has asked questions such as “did you know your house farts?” and “why did you have a 1kHz tone coming out of your house?” (He works for a radio station, so I guess he listens to sine waves in his spare time). While they’re laid back enough not to let it bother them too much, it bothers me that we keep projecting noise out into the neighborhood. Since we already have 2 dogs and a 4 year old, it’s best not to add to the list of noise makers! The part that drives me insane is that we paid a ton of money for this thing, so I don’t think it’s unreasonable to expect it to be quiet.

I have called Bosch tech support a couple of times, and they’ll walk me through the adjustment procedure. One time I even paid an “expert” listed on the Bosch website to make the adjustments. He turned out to be an ordinary plumber who also had to call Bosch to learn how to adjust the valve. Big waste of $$. Each time it’ll work great for as long as 24 hours, then it’ll start making one of the noises. I’ll then adjust it so it only grunts, since it only happens when firing up vs. a 10 minute whistle during a shower. I have probably tried adjusting it at least 20 times, but I can never find the sweet spot.

Earlier on, there was a possibility that the unit was not getting enough gas, since our heater is on the opposite side of the house as the gas meter. To rule out the possibility, I changed out the 3/4″ line with 1″. No change. A year later, I changed over half the 1″ line to 1 1/4″. Everywhere I tried to find it, I was asked “you want it for what??” Nobody had ever heard of someone putting 1 1/4″ in a residential application, but I pressed on until I found it at a local plumbing supply shop. There was a slight lingering doubt due to our being about 1′ over the Bosch-recommended length for 1″ line, once all the elbows were factored in. I was pretty sure it wouldn’t even make a difference, but I had to find out for sure. And besides, even though I was often mad enough to rip the heater off the wall, it was maybe $60 to change out the pipe again vs. the $1000 that the heater cost. Guess what…even the mammoth line didn’t fix the problem. Even with the furnace and gas dryer running full blast, the heater is still getting well above the minimum pressure it needs. I know, because I found a way to make a homemade manometer on the net and tested it out.

I’m not sure how we ended up 2 years down the road, not really any closer to solving our problem. The only thing that has changed is my willingness to do something drastic, if necessary. I just sent Bosch a long email today, basically as a plea to hopefully have them send someone out to replace (or somehow adjust) the gas valve. To really adjust it properly, a CO2 meter is required, but neither I nor the “expert” had one. In case worse came to worst, I found the valve online, and it’s a $350 part! For that money, we could almost buy a new (smaller) heater once the new Federal Tax credit is factored in. We’ll see if Bosch responds favorably to my email and how long my drive to replace the heater lasts. It usually dies out just as soon as I figure out how much we’d have to pay, at which point I start thinking “well, it’s just a little noise…”

Picture of the water heater right after being installed (with the original 3/4″ gas pipe) [note: the install was not quite finished, so you don’t need to comment about the partially-finished temp-pressure release or the lack of pipe insulation]



Picture of the insane 1 1/4″ gas line (the black line running to the right) as it transitions to the 1″ line. It doesn’t look too big in the picture, but keep in mind that our joists are true-dimension 2×10’s!



Feb 2, 2007 update:

This story did end happily. See the new post here.

The short version of the story is that Bosch offered to replace our two year old unit with a brand new one in an effort to take care of the problem. The new one works perfectly.

Aug 19, 2008 update:

We’ve now gone through two summers since installing the new heater. Around August, we’ll find that the heater will turn off at least once during a shower. I’m assuming it has something to do with the incoming water temp being higher than the rest of the year, but it still doesn’t make sense as the flow rate for a shower is well above the heater’s minimum. Since it has happened two summers in a row, I am further convinced that there is a design flaw in this heater.

September 2012 update: I realized that I never wrote anything about the fan on our water heater. Instead of turning on the fan in conjunction with the burner, it seems to be coupled to water pressure instead. So any time any water is run in the house, hot or cold, the fan comes on for 30 seconds. It’s a little annoying when we turn on the house outside, and then noise will start coming out of the exhaust vent, which is right next to the hose reel. I once asked a Bosch rep about this, and they said that the solution would be to install a check valve to ensure that fluctuations in the cold line wouldn’t trigger the fan. That just didn’t seem worth the bother. It’s a relatively minor issue, but I really question why they designed it this way. ..


102 thoughts on “Had it with the Bosch water heater!

  1. I think the main problem is lack of knowledge. What I mean by that is; you have to be familiar how tankless water heater works. It is different from tank type. First and major thing is to dimension the system properly… not enough gas, not enough power and hot water is not hot. Other thing is minimum water flow rate.

    Every tankless water heaters has the minimum activation rate and it can be as low as 0.4 gph… so you have to open the hot water tap little bit more than what you did with the tank heater.

    Tankless units are also more sophisticate and therefore, way too sensitive than tank type.


  2. Zee,

    While I agree that tankless heaters are more complex and definitely are more exacting in their requirements, there is no question in my mind that these Bosch heaters have a problem. In our case, we did multiple upgrades of the gas line, to no avail. As soon as the unit was replaced, it worked better, proving the first was defective. Even today, ours will (not too often) cycle itself in the middle of a shower (which is definitely over the min. flow threshold), causing a blast of cold water.

    I have not hear nearly as many complaints with the other brands.


  3. I also have a Bosch 635 ES and have a problem with it going cold (sometimes daily) after getting the EC error code. Then I have to go into the basement and reset it. I have never had the whistling but it does cycle repeatedly (flame goes out and tries to restart a few times)and make a noise when it is cycling. Sometimes after three tries or so to restart it will get an EC code. I have a long gas run so I ran 1-1/2 black all the way to just before the unit where it has to switch to 3/4. I also installed the second condensate drain as the service bulletin suggests but see no water coming from either one. I have removed the igniter a couple of times and found it had a slight reddish residue on it which cleand off with steel wool. I also found bugs on the top of the heat exchanger that flew into the vent so I cleaned out the bugs and put aluminum window screen over the vent cap. My problem seems to occur mostly in hot humid weather. I believe it is related to low flow rates since it mostly happens when you use the dishwasher or kitchen sink and occasionally the shower. I have it set to 120 degrees which is fine for showers but marginal for the dishwasher. We had a run of cool weather recently (first week of Sept) and it went for a whole week without requiring a single reset from the EC code. Then it went back to needing its daily reset. Maybe the incoming water temp is the issue, I’ll have to start measuring it and see if it correlates. My wife and I are sick of the problem which has been going on for six months and about ready to yank it off the wall. Any suggestions would be appreciated.


  4. We have a Bosch tankless heater and it quits producing hot water. The water flow is fine as I drilled a bigger hole into the shower head and have removed all the water restrictors from the taps. We have removed the cover to allow more airflow. I feel it ia a gas delivery problem. The gas lines in the area are 14 years old so small and low pressure is not an issue. Other people I have talked to have not had a problem wiht there heaters and love it. We hate it, cold showers are not fun. We have the spark operated by batteries. I have just contacted Bosch about changing it to ac. Make sure you don’t have batteries when getting one!! Any suggestions on the problem, we want to get rid of it and get a tank again. Our gas bill increased when this one was installed!!


  5. 3 years into a 10 year warranty I’ve just had a catastrophic failure of the heat exchanger.
    Bosch 250SX.

    Dripping water all over the floor, I’m waiting 2 hours on “hold”, I finally got Bosch service to answer the phone. At that point, I got Scott on the line who spent the next 30 minutes on a mission to figure out how to cancel my warranty. He wanted to know if I had a “momometer” of something like that, what altitude I was at, if I had a water softener, how long my exhaust pipe was… (he wanted me to get up in the ceiling and measure it)… Finally he figured out a was to exclaim that the water heater warranty was VOID because the exchanger had compromised the gave jet by dripping into it and that possible moisture in the 21 foot exhaust pipe had caused the heat exhanger to fail prematurely and that it must be my fault. Now I have a boat anchor…
    I installed an electric hot water heater yesterday.

    If it fails.. Throw it away and save yourself a lot of aggravation!
    Don’t call Bosch… They just want you to go away.

    Don’t buy Bosch anything!!


    1. Julio,

      I not sure if you still have knowledge of the Bosch AS 250 but my son bought a new house and he has this unit which we are having issues with. It worked fine for the first two months then this week it stopped working coming up with a error code of EA. He took the blower cover off and found it looked like it was melted from the electrodes next to it. He found replacement parts on line and bought them when they arrived he replaced the old ones and put the unit back together. He tried to turn it on and the unit fires up for about a second and goes out then comes up with the same EA code again. We reset the unit numerous times to no avail. The thing is there is a gurgling sound coming from the unit when it tries to start. It sounds like water inside of the unit. I wondering if you have ever heard of this problem and what would you suggest. I have been reading numerous columns and it seems like your the one who might have an answer for us.


  6. we have a Bosch and have all the same problems as others listed here hot, cold, hot, cold, have had it 3 years. checked with local plumbers and they are not hot on these either. could be scaling problem or water restriciton problem. just a pain. we have to let the water run forever just to get it hot and then don’t dare shut if off while doing dishes or else it is immediately cold. showering is a nightmare 5 minute shower usualy takes at least 20 with temp adjustments constantly. we are taking ours out this spring and going back to basic gas heater. this thing was really expenseive around 1500 total for all the special pipes etc.


  7. Just have to add my horror story to the bunch. Our Bosch tankless heater is a worthless appliance that frustrates us on a daily basis. Between the hot, cold, hot, scalding, cold and the shut-offs, we easily waste 20-30 minutes a day dealing with it, especially when trying to do something basic – like wash dishes or hands. Major pain. I would NEVER recommend this to anyone or purchase one again. Biggest nightmare ever. We are thinking of switching back to a tank.


  8. i have a bosch tankless and can appreciate all of the comments. i can deal with the loud bass noise at startup, which is my only problem. i will not invest any more money in this heater. i am disappointed with the bosch customer service when i asked to speak to a tech about this issue. i will not buy all the fancy equipment to test this or hire someone to do so. i live in an older neighborhood and now understand that it must be lower gas pressure. i do have a 1″ gas line from meter to heater and a water softener installed before the unit. this technology just isn’t there yet.

    i guess i am pretty disappointed in Bosch customer service. seems like all big business leave me hanging like this anymore (e.g. GE, Clearwire, Milwaukee, etc.) they build crap and don’t want to help solve problems. My recommendation is to start a black list and don’t buy these products anymore.


  9. I have the electric version, an AE125. The first one lasted two years and Bosch replaced it no charge. The second one lasted four years after that and the controller went out. After a few back and forth’s Bosch is going to replace it too. Can’t say enough about how well they’ve treated me. I love my tankless water heater.


  10. Can you post a picture of where to adjust the gas valve and some more detail on how to do it? I’d love to give it a go and see if I can get rid of this whistling. A few months back, I tried resticting the air flow in myself same as above by creating holes in a plastic cup and placing it over the intake. Works for a short period of time, but is not fool proof.


  11. I bought “someone elses nightmare” on craigslist.org for $400. I have two others and LOVE them. This one wheezed and farted on startup and gave inconsistent temps for about a week until I took matters into my own hands. First, LOOK AT THE MANUAL. It seems pretty clear to me that most of the people on this forum are the victim of either a) a lack of knowledge, or b) hiring a “plumber” that had no experience with one of these and just ripped you off. The “nightmare” I bought now runs like a champ … and all I had to do was experiment with the CO2 setting. No more noise and no more fluctuation in temp. At ALL. Fixed with 2 turns of the screw. There IS a sweet spot for CO2 adjustment. If you don’t have it right it’ll never work properly regardless of anything else you do. And every gas supply is just a little different so they’re just NOT going to work “right out of the box” without customizing it. On mine (LP gas) I noticed a strong smell when it was running. So I adjusted the CO2 screw IN a quarter turn at a time and before I knew it … BINGO. No more jet engine noise and no more errors and resets needed. It worked for me in about 10 minutes. I had to go two full turns IN (after experimenting with it in each direction a little at a time), but it was obvious when I had it set right. I love my Bosch heaters … all 3 of them !


    1. Marty – going to give this a try as we are about at our wits end with this thing. The manual mentions both a P1 and a P2 C02 adjustment Can you clarify which you did? Both?


  12. I have two Bosch 250SX. One is Natural gas and the other is Propane. I will never buy another Bosch unit. After 5 years of upgrades, testing and replacing I will get another brand that has better reviews. I’m sure Bosch has learned something from the design failures, but I’m not risking trying to get support from Bosch again. The heaters are great when they work.

    I have gotten down to only the EC error code on the natural gas unit that shows up after I got rid of the heat sensor errors that were fixed with the upgrade kit. The error occurs when the the heater is cycling on and off at low water flow. The unit will run all day at higher water flow rates, but doesn’t like the low flow cycling which is compounded when you have a well pump that varies the water pressure. I will be playing with the CO2 adjustment next to see if that will work. The propane unit seems to be working all right after the upgrades. Time will tell. Way too much work for a $1,000 piece of machinery.


  13. We have also experienced problems with our 250SX since it was installed at high cost by a local Bosch certified installer. We have experienced all the problems that can be seen in this blog. Jet engine noise when it starts, the unit have been shutting off while taking a shower quite often during the 5-6 years we have had it and now the unit shut off after only a few seconds after reset. We are getting an A2 error message. I was seeking for a certified service help on the Bosch web site and was informed that there are NONE within a 100 miles radius of New Orleans???? How is that for service?? Bosch technical hot line is not much of help either and when calling them today I was informed that they are closed during weekends and holidays???? I was earlier this week advised to contact local plumbing and HVAC companies for help and several was contacted and all refused to work on Bosch tankless water heathers.


  14. Tom, I think I spoke to the same guy, Scott…. What a jerk. All he wanted to so was blame me for everything. I told him it was put in October of 2010 and he said well it was made almost 4 years ago… I said, great.. I installed it on October 2010. He was very disrespectful and when I told him I gave up and replaced it with a new one and just wanted to fix this one for a backup or possibly sell it he wanted nothing at all to do with me then and said well, you have to put it back in, we wont warranty it like this and wont transfer the warranty to anyone else. My model is a GWH 425 HN LP and let me tell you this is the last company I will ever do business with again! My brother also bought a higher end model a couple of years ago and has had nothing but problems. Do yourself a favor and get one from Amazon. I got an Aviditi and love it. I think EcoSmart also make good ones from the reviews. Godspeed and good luck friends!


  15. I am blown away by how many people keep commenting on this years old post. There is clearly a problem with these heaters, yet Bosch has never admitted it. As I said in a follow-up post, our replacement unit worked MUCH better than the original, but it’s still far from perfect. It momentarily shuts off all the time during moderate flows (greater than the 3/4 gallon per minute threshold), but most of the time we don’t notice it due to the thermal “averaging” effect that the 30’+ of copper pipe and insulation. There is no excuse for it to not be able to keep running during a shower.

    I believe that the sensors and control circuitry allow it to overshoot the set temp, so that it has to turn the heat off in order to avoid scalding someone (which we do appreciate, of course.) This clearly could work better, but we are happy that it’s not making the loud noises that our first one did.


    1. I found your post about Bosch Water heaters. Mine is doing the groan also and the “Tool” Dwinel Bedard, at the Bosch tech support, said that it was my installation that caused the problem. Funny its been installed for 4 years with only a few problems and now its doing the grown and shake the wall thing. Someone explain how how the air intake and exhaust affects the gas valve turning on.
      It looks like I might be going back to the old fashioned water. Where is your house in Portland? My mom grew up at North East 49th and Sandy Blvd. in the hollywood area of Portland. I have many fond memories of going to visit Gramma there.

      Jeffrey Luther


      1. These heaters have a lot of sensors in them. This is because maladjusted heaters are a safety hazard. The computer always errs on the side of safety. This means that improper CO2 levels, heat detection, or failure to ignite will cause the unit to shutoff. The fan runs often in order to clear out any lingering gas. That is why it starts as soon as a water pressure change is detected. My unit failed when a slow leak in the heat exchanger dropped enough water to interfere with the ignition detector. This gave the EC error (and the strange whumping noise as the gas shutoff). This lead to the fun of reseting the unit, letting it start again, heat a little, boil off the water in the bottom, and then run so my wife could shower.

        I was told that the top of the heat exchanger has to be cleaned every year. This was news to me since there is no mention of it in the owners manual. bosch replaced the unit with a new 830
        ES unit, which had to be re-plumbed because of different spacing to the gas and water. The new unit hasn’t had any issues (other than the complicated install because of the pipes being close, but not exact, and the larger unit was difficult to install in the smaller space originally planed for the first unit. The first unit lasted about 6 years, the second is 2 years old. the old warranty period covers the new unit (warranty on replacement is not extended…).

        The old unit had soot on top of the heat exchanger. I was surprised by this. It looks like the majority of the flame was shooting through the center of the unit and the sides had much less air/gas flowing though them. My unit is mounted on an outside wall so there is no back pressure against the outflow. I placed a thin air filter on the oversize air intake to prevent any bugs or floating seeds from entering (note that bosch said NOT to do this) and I want to be sure that the soot wasn’t caused by anything entering through the air intake.

        BTW I use a 5 gallon electric water heater to keep a reserve and mix the incoming hot water. The reserve is below (in the basement) the bosch unit so I have a check valve installed that prevents the hot water from rising into the bosch exchanger and cooling off. As long as the bosch continues to ignite and run there are no problems and unlimited hot water is very nice to have, especially when I’m the last one to take a shower in the morning.


    2. Julio,

      We live in SE Portland as well. We have the same model Bosch tankless water heater ( GWH 635 ESN). Our makes all the same noises and shuts off multiple times a day. We have to hit the reset button each time we turn the hot water on and the water runs for several minutes before it gets hot, causing higher water useage with our astronomical water rates in this city, it costs us a fortune each quarter.


  16. What about a class action suite??? If they continue to blame installers, homeowners, location of heaters, etc… I bought two heaters for two different homes 4 years ago and after replacing the first one a year after install and being told the units waranty was void because I had a toilet in the same room as water heater???? Like that has something to do with it…And today my heat and air guy was told that the Heat exchanger was leaking and the so called 12 year waranty was void because there is a door on my utility room??? Are you kidding me. Someone needs to hold BOSCH accountable for their so called waranty and the voice of one obviously doesn’t have any power!! any ideas??


    1. Wow no kidding , I have one and have replaced 4 heater cores as well as 3 gas valves! I am going to buy a different unit tomorrow! We should get together and make Bosch do something! these units are crap! I am a contractor and have done all of the work my self so I didnt have to pay what some have.


      1. I bought this bosch in 2004 and had 3 cores replaced and have had several problems–in fact every problem mentioned above–i called last month because i had so many error codes–bosch said it was the gas line and they had no record of my heater–no it was not the gas line–now it is leaking water and making a loud growing sound– i guess i will call monday morning! has anyone bought the japanese brand heaters? Help


  17. I Have a 250SX as well… It worked ok for 8 years or so…recently the heat exchanger began leaking.
    This is suppose to have a 12 year warranty. I called Bosch to inquire about it and they said the following.
    Completely disassemble the unit and send us 8 specific pictures and we will let you know (he listed the types of pictures to take).

    Yikes.. but what the heck i’ll play I thought…
    Pictures were sent and of course they said “there is soot present and that indicates your air fuel mix is wrong so this is not covered”

    The unit was never adjusted. It never grunted or whistled. why would I pay someone to adjust this ?

    It never said in the manual that this was a requirement for warranty coverage!@
    Additionally I have spoke with at least 5 plummer that laugh and say that’s their way of “saving money”
    I found only 1 plummer out of 12 called that even had the analyzer needed to check the mix…
    He was VERY suspicious of why it was needed.
    Long story short I bought a new heat exchanger and the unit now works but acts funny and does not work right…. I guess I’ll need to buy a gas valve as the old one was compromised from the leaking core….
    To anyone considering bosch…. Run far away….. they have a VERY poor customer support team and product support…. The 250SX was a train wreck and they know it… They even discontinued it and changed the name after they updated the mess they created…


      1. Scott, I did all the plumbing, so I am sure that there is no crossover. It seems to be so sensitive that even a small change in pressure (from opening a cold water tap anywhere in the house) is enough to trigger the fan.


  18. I first read this post ~4 yrs ago when we moved into a house with one of these heaters (250sx). Since then, we have mostly ignored or learned to live with the same long list of problems reported on here (loud steam whistle noise; taking forever to heat up, then water running immediately cold again as soon as the tap is turned off; regular error codes requiring a reset, etc). I have called Bosch as well and they immediately rid themselves of me by tricking me into voiding the warranty. That’s all they seemed to care about. They wouldn’t even bother giving me any information on someone to service the unit, though I’m willing to pay for it and told them so.

    I returned to this post on a whim today as we are now resetting our unit 3-4 times a day and it is becoming too much of a pain to ignore anymore. Amazed to see it is still getting hits / updates all these years later! I will try adjusting C02 manually as in M. Spaulding’s post above from March 2012. If anyone else has had success with this approach, please let me know. I will report back. Wish me luck …


    1. Currently on day 3 of turning screw P1 in (cw) one full turn as per M. Spaulding post below from March 2012 (and as shown on p. 32-33 of 250sx manual). Left P2 alone (need special tool). Whistling seems to have stopped entirely and although I could just be imagining it, hot water delivery seems faster than it was. No error codes and resets yet either. Will try for one full week and then dial it back to see if that takes us back to the same problems we had before. Cautiously optimistic so far.


      1. One full week later … overall, much better than before I made the C02 adjustment (as noted above). No error codes reported / resets required for the entire week and only one episode of moderate whistling. While not perfect yet, quite an improvement. I’d say that it is worth playing around with this adjustment on your own if you are desperate.


    2. Just worked on mine. I had a AE error so I did some stuff and so far its working great. What I did was open the case. Then pop off the air intake case with two screws. Just so thats out of the way.next You need to remove the electronic sparker unit. It has 3 wire to it. Unplug and unbolt the two side nuts holding it up. There is a sponge gasget that I removed. Clean all three electrodes with a file then use high temp gasget silicone to make a new gasget. And seal it. Let dry for 24 hrs. I fired it up next day and works great so far. My thoughts are that gasget from factory over time now lets in air. And throws off air/gas levels.


  19. Same problems for about 10 years with this unit. Lately getting A 2 error. So tired of this. I will call Bosch tomorrow to complain

    Resetting this unit is difficult as it is in the attic


    1. Right there with you. Ours is in the attic, too. As was clarified in Julio’s blog, there is nothing to fix one of these things. Either they work, or they are a lemon. I don’t know how many good ones are out there, but there are obviously a lot of lemons.


  20. Horrible! Horrible! Horrible! I cannot believe Bosch knew about these problems and has let us all suffer for years. I did have mine professionally installed by George Morlan, and subsequently had a call into Bosch support and it was diagnosed on a CO2 analyzer with Bosch on the other end of the phone, but I have still had nothing but 7 years of horrible experience with this thing. Just had a plumber out the other night (as I have every year since I bought it) and the burner, which is made of some kind of non-metallic material has about a 1″ burn crater in it. The stupid heater now will not stay on for even one shower, so we have to have someone babysit it and press the reset button whenever anyone is taking a shower. One thing I did from the very first week of getting the sudden cold splash in the shower, was install a 10 gallon electric tank right after the Bosch as a kind of buffer that will absorb the cold stalls, and also give enough time to realize and rinse if the Bosch failed.


  21. We have Bosch 830ES and it has been nothing, nothing but a problem. I was installed by a plumbing company called ProServe in our area. At the end of the first year of operation ProServe was supposed to come out and do a de-lime service on it. They never called us an finally I called them and they came out only to find that the original installer had installed the hot and cold feed values upside down. They fixed this at no charge and then did the de-lime procedure. After that the heater stopped working correctly giving us an E1 error cold and display flashing. I had ProServe back at least 3 more times and they were unable to fix it. The last time I called them they washed their hands of it and said they no longer work on Bosch tankless water heaters. They told me to call Bosch which I did. I was referred to Barker & Sons Plumbing who have also been out 3 times and today will mark #4. They gave the heater better venting by running a smokestack into the attic. They pulled the burners and cleaned them out. The heater will work for short periods of time, but ultimately we get the alternating hot/cold showers and the E1 warning. The water gets so hot at times you can easily get burned. I was able to get about 3-4 weeks of good production out of the heater by cleaning out all faucets, removing restrictors and flushing. I’ve just come to the end of working cycle and did all this again, but it has not solved the problem this time. I have printed all the Bosch service bulletins that apply to our problem and have completed all the necessary procedures still with no luck. I have spoken to Bosch Tech Support several times and while they were initially helpful, now I’m getting nowhere with them. Based on what I paid for this unit, I really fee like a sucker and I really think that I’m reaching the point of just pulling it out and throwing it away and starting over with something new. At one point Barker & Sons contacted Bosch and they told them the answer might be a pre-tank. The Barker & Sons plumber wasn’t convinced and I certainly wasn’t. I bought a tankless water heater because I wanted tankless. If I have to install a tank I may as well put in the old style tank water heater and be done with it. Barker & Sons will be here today and we will see what if anything they come up with. Open to suggestions!


    1. UPDATE: Barker & Sons arrived this morning as promised and I might add – on time. I had the same service tech working on the heater since the first call. I might add he’s been equally frustrated with Bosch and the guys on their service tech desk. In any case he replaced the heat sensor and the unit seems to be working properly. He said that he has had several customer with same complaint E1 error code and although Bosch did not suggest this he finally came upon the solution himself. There was no charge for the call and he said to contact him if there were any more problems. I will post an update in a week or so and let everyone know if this ultimately solves the problem.


  22. I’m happy to report that since the replacement of the heat sensor my heater is working just fine. I hate to even say this out loud for fear I’ll jinx it, but so far so good.


  23. Well….. all of your stories make me feel a little better…. I thought I was the only one! I bought a Bosch GWH 635 es propane 5 years ago, My building supply store said it was the best. I was a single dad at the time building a new log home so $1700 was expensive but I wanted the best for my new home. I am a contractor and built and installed everything in the home. The entire plumbing system was built around this unit.
    I live in the Adirondack mts and it gets cold so I added Boshs freeze proof for the core as well.

    Yesterday morning the unit has frozen its 4th core! yes I have replaced it that many times! Most times it also ruins its gas valve as well and that needs to be replaced .
    It runs about $600 in parts.
    I have it mounted in a warm basement, the exhaust stack is protected from cold air as well.
    I have spent countless hours on the phone with Bosch….. wow! that is the worst customer service I have ever dealt with. I cant tell you all how much fun it is to go down the basement stairs to hear the unit spraying water all over! . I had adjustment problems but if you follow some of the ideas above you can fix it. I found it needed to be set different in the summer then re adjusted in the winter.
    We need to do something about these units and Bosch! Thanks!


    1. I have had the same problem with a brand new Bosch 520 HN LP. This water heater froze four times before bursting and dumping water all over my floor. I wrote a certified letter to Bosch explaining that I had an Aquastar VP125 for years and it never once froze. Bosch told me that the warranty was void because it froze. They also aggressively defended their product design, which I said was defective.

      I called Bosch prior to this incident to see if they had a backdraft preventer (which quite a few other tankless water heater companies make available). They did not have anything to say other than that I should get an HVAC person to check on the house. What a joke – given that these backdrafts occur so sporadically that it is impossible to predict (seems to happen during rapid shifts in barometric pressure or especially windy days).

      I filed a complaint with the US Consumer Product Safety Commission and plan to file a complaint with our state attorney general, plumbing board, and the Better Business Bureau. I do not have any expectations that Bosch will do right by its customers since they have an (F) rating with the BBB affiliate where their US operations are located (New Hampshire).

      My advice is to do everything in your power to raise “hell” with this company. They deserve it for selling such a lousy product without any support or desire to satisfy their customers. They must figure it is cheaper in the long run to alienate customers. I may just take them to small claims court too.

      Good luck with your water heater.


  24. Thanks to Steve in NM and M. Spaulding, my Bosch 250SX appears to have stopped whistling. I adjusted the CO2 screw (the plastic #40 torx behind the steel #40 torx) just a quarter turn CCW in my case. I started going CW, but the whistling got even worse (went from a whistle to a screech). Hearing protection made the adjustment possible.

    The system has been in place for about five years, but has gotten very little use. I talked to Bosch customer service when I put the unit in, and they lived up to their reputation. I replaced some 1″ gas line with 1-1/2″, with no effect.

    One problem it had early on was the inability to stay on, with an E1 (if I remember right) error. I use the heater as a peak-load supplement and as an emergency backup, hence it’s very little usage. During one of it’s needed moments as an emergency backup, while I was again trying to figure out why it wouldn’t stay lit, I noticed a lack of condensate from the drain tube. My system has a soft plastic tube coming out of the side of the flue just inches above the case. This has a loop to act as a trap. At 9 feet above the floor, it wasn’t obvious that the tube had been bumped and the top of the loop was about even with where the tube comes out of the flue. Something like surface tension was keeping the condensate from moving down a too-slight slope. I adjusted the zip tie forming the loop so that the top of the loop was 2 inches below the start of the tube, and the problem of turning off a minute after turning on has gone away.

    One other problem that still plagues me is the refusal of the unit to turn on if the inlet water is above about 70F. I didn’t see this in the specs. This isn’t a big deal for hot water consumption, but it’s a big deal for hydronic heat. Here the returning water is always warmer than room/house. What I have learned is that my unit, once on, will stay on if the incoming water rises above 70F, up until it hits something like 95 or 100F. So I can get some hydronic heat after someone takes a shower (as long as the hydronic heat isn’t on at the start of the shower), or, as I’ve now learned, after I switch two valves in my circuit and run hot water anywhere for two minutes. This is no good if you’re leaving the place for more than a day and it’s your only source of heat, but I have another heat source so I will live with it.


  25. I used to be a Bosch Customer Service provider. I have seen some terrible installations. The unit in the picture is less than correct. The gas piping is okay, but lack I no a dirt leg (not important due to the improvement in natural gas quality).
    The lack of servicing valve kit will be a problem when your unit needs descaling (read the manual). Any Bosch approved technician should have arrived with, and prepared to use, a combustion analyzer to bring the unit into performance standards. There is a need to measure manifold pressure, pressure during operation of all gas appliances before making any adjustments to the unit. Gas pressure too low? CORRECT THAT FIRST! Many factors go into a proper installation, gas volume, water piping size, intake and exhaust piping size and lengths (that corrugated intake pipe is lame). Blame the installer, not the product.


    1. Thanks for the comment, Frank. I do however respectfully disagree with some of your comments. While installation quality certainly can be a factor in any installation, we had two completely difference experiences with two heaters installed exactly the same way. Unit #1 was a complete piece of crap. It gave us constant problems. Unit #2 worked great. Based on all of the comments we’ve received about these heaters, I believe that there are many of them out there with a defective component(s), and Bosch has never acknowledged that.

      Regarding your specific notes about our install, everything about it meets Bosch’s specs. The gas pressure was not too low (I measured it at the time), but I went ahead an upgraded the gas supply line because technically I was slightly over Bosch’s recommendations for length. And I upgraded it a second time, at their request, even though I could tell that the customer support person was grasping for solutions. I’ve considered switching it all back to 3/4″ someday, just to prove that it never was a problem, although I’m too lazy to actually do that. And regarding the drip leg, my inspector called me out on that, so I did add one in just after that picture was taken. I don’t profess to be an expert in any of this, but I’m capable of following instructions and minimum specs. And I don’t believe that everyone that has commented on these Bosch threads had poor installs, although some may have.


      1. Bad components is a possibility. I cannot refute that. Having installed at least 50 in California, and another handful here in Virginia, I’ve had no problems. After my installs, I measure and adjust the incoming gas pressure (manometer required)(admittedly I mess with the regulator at the meter, shhh), followed by P 1 & P 2 adjustments with my combustion analyzer. Everything is recorded and reported to Bosch, establishing a record.
        Granted, I was trained by Bosch/Buderus, and built one of their classrooms in Livermore, CA.
        I’ve only ever had problems with bad installs done by others. Often times, venting causes problems.
        I’ve seen Takagi, Rheem, Rinnai, et. al. installed good and bad. There are many factors to consider.
        I hope your newest unit lasts forever.
        Don’t forget to descaling annually.


  26. I have a gwh-635es-n also and have always had periodic problems with EA and A9 errors. All it takes to clear it up for a period of time is to polish the 3-prong electrodes. The get a dark red haze on them which seems to affect them. The unit will begin getting errors on start up occasionally and then get progressively worse/more frequent. A reset will get you back in business pretty quickly and if you let it go too long, it will take 2 or 3 resets to actually get it going. So, would a CO2 adjustment help this problem?


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