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.
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."
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.
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.
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.
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. .
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.
I’m happy to help further over the phone at https://www.6ya.com/expert/john_bf86cec48e61dfe0
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