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I have a 425 gas tankless water heater. In 3 years in the winter (and sometimes warmer weather) I have never gotten a hot shower. The water starts out warm, and in the middle of the shower changes to freezing cold. It is much worse in the winter, almost unbearable. Help, before I rip it off the wall and go back to a tank.

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  • duckyb1 Nov 30, 2008

    It is a Bosch #425, and I believe the shower restricter has been removed. My son has another manufacturer, and loves his too. I heard it is the "venting", then I heard that this was a "bad model, loaded with problems", then someone said "it works on pressure and not flow". Apparently flow works better. Someone said "turn the faucet water on first in the sink, and then turn the shower on before turning off the faucet at the sink". I'm tired trying. Not sure what model you have that works for you.

  • duckyb1 Nov 30, 2008

    I b elieve the letters might beGWH 425 HN. Not sure about the alltitude. I am in Pa. right outside of Philly. The other thing they said it might be is the "single" faucet control in the shower. I was told a double faucet (1 hot, 1 cold) works better then the "single" knob. But the walls are closed up now, and the tile is installed (very expensive tile) so it would mean major repair if we were to change handles now. My son thinks it is in the diverter at the single shower. It doesn't always do it at the other faucets in the house. But who needs that shock when the temperature is well below freezing. :You don't even get a warning....................

  • duckyb1 Nov 30, 2008

    Well your good....................The water was very hot. It stayed hot for a long period of time, even when I made an adjustment, to a temperature that you would be able to stand under if I wanted to. It did not go ice cold. I felt I kept it on for a good deal of time. I have 2 1/2 baths, the kitchen too so it is 6 hot faucets in all. They all stayed hot, and then I turned them all off, but the main shower. I thought it stayed hot pretty long, even after the adjustment I made. Its not freezing in Philly today, but it is probably around 40 degrees outside now. Hopefully I did this right. Does that give you a clue as to the problem????????????????

  • duckyb1 Nov 30, 2008

    Yes the shower in the main bathroom did remain hot for what seemed like a longer time then it has before (once the problem starts happening in the cool weather) After running it for a while I shut it off, but up to that point it remained hot. This time it did not go cold. That being said "what do I have to do when I turn the shower on to get that result without turning on all the hot water faucets in the house, and why do you think it happens to begin with. any solutions that would work??????????????????

  • duckyb1 Nov 30, 2008

    by the way I do have just 1 knob.

  • duckyb1 Dec 01, 2008

    thank you. I hope I get a good result tomorrow morning.

  • duckyb1 Dec 01, 2008

    Hi, just wanted to let you know. The shower worked this morning. YA, AND HALLLELUYAH............................I turned on the hot water in the sink, brushed my teeth, and at the same time turned on the water (all the way to its hottest point) in the shower. Then turned off the sink water, adjusted the temperature in the shower, stepped in and prayed. The water never went "cold". I was even able to adjust it to warmer without the hot water cutting out. If this works everyday I can't thank you enough. Also my son since I rent this gorgeous home from him, and he was ready to throw in the towel after many tries. Again thank you, and you have a wonderful holiday.

  • duckyb1 Dec 01, 2008

    Hi, just wanted to let you know. The shower worked this morning. YA, AND HALLLELUYAH............................I turned on the hot water in the sink, brushed my teeth, and at the same time turned on the water (all the way to its hottest point) in the shower. Then turned off the sink water, adjusted the temperature in the shower, stepped in and prayed. The water never went "cold". I was even able to adjust it to warmer without the hot water cutting out. If this works everyday I can't thank you enough. Also my son since I rent this gorgeous home from him, and he was ready to throw in the towel after many tries. Again thank you, and you have a wonderful holiday.

  • duckyb1 Dec 01, 2008

    THANK YOU, THANK YOU, THANK YOU. THE SHOWER WORKED THIS MORNING. I CAN'T THANK YOU ENOUGH. IT ISN'T SUPER COLD HERE THIS MORNING, BUT HOPEFULLY THINGS WILL STAY THE SAME. AGAIN THANK YOU, AND YOU HAVE A WONDERFUL HOLIDAY. GENEVIEVE

  • duckyb1 Dec 01, 2008

    Can't thank you enough..........................It worked......................This morning I was able to get a shower without the water turning cold. This is what I did. I turned on the hot water fully at the faucet in the sink, and brushed my teeth, not to waste water, at the same time I turned the shower on full force "all hot", then turned the sink off when i was done, left the shower on, and adjusted it to a warm temperature. Stepped in, held my breath, prayed a prayer, and guess what. NO COLD WATER CHANGE. I can't thank 'you enough. My son also thanks you since I rent this gorgeous home from him, and he was ready to throw in the towel after many tries. The next one would have been costly. As I said "he has a tankless and it works perfect". Not the same model or manufacturer. He owns his own construction company, and had many trades at his disposal to give him advice, but everyone kept saying. "those systems stink", "you can only use them in warm climates", "its the venting", "its the single control". He got it all, even getting in touch with Bosch. They were no help at all. Sorry for the long winded comment, but I am so happy. Thank you again, and you have a wonderful holiday. Fix, Ya, is great. I will put this website among my Favorites. By the way I am a widow, and live in Springfield, Pa. I am 73, and my name is Genevieve. The nicknam

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Is it a bosch.What is the exact model number.I have one and i love it once you get it set.It seems that there are two flame levels low water flow and high water flow.If your shower has a water saving restricter in it it may not flow enough to let the flame get too high.So i would remove it.If there is a knob on the front turn it all the way clockwise and try it.There is also a screw on the inside that you can adjust the low water flow flame , it the brass one with a locking screw on it.Your manual should show it.If you need to adjust it turn on your shower and go check the flame and use that screw to increase the flame.Hope this helps and let me know how that works

Posted on Nov 30, 2008

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  • good wrench Nov 30, 2008

    I have an aq 125 frow the orange home supply store.Do you have a better model # any letters.Not sure on what you have but it should have a sensor that monitors the venting and if it is insufficient it will shut it down.Hot water tanks usually have a 3"vent and the tankless requires a larger one for the larger btu output.Try leaving the hot water on in the sink and the shower and see what kind of temp you get.If it gets hot then shuts down it is a vent problem backing up.I also had to get a better gas meter as the one i had could not flow enough gas for the furnace and the water heater at the same time and when the heat kicked on the water temp would drop from the restricted gas flow.Is there a dial on the unit beside the flame adjust.I had to turn my dial down if i didnt i had warm water not hot it seemed the water was flowing to fast through the unit to get hot and the flame wouldn;t come on unless i had two faucets on.If you turn on your hot water and adjust that knob back and forth(assuming you have one) you will see the water flow rate change.Turn it to slow the flow all the way and try that.Hope you get it working as there is nothing like a long hot shower or fresh hot water to cook in,not some water that has been sitting in the tank.Also what alltitude are you at.and is it nat gas or propane.

  • good wrench Nov 30, 2008

    Do you have one or two nobs on the unit.If it has two turn the one on the right all the way clockwise.If it is one turn it all the way clockwise.then turn on as many hot water outlets you have,kitchen bath shower and tell me if the water gets hot.It should get very hot.If it gets hot then turn off everything except the shower and let it run and see if it stays hot.Let me know

  • good wrench Nov 30, 2008

    Did the shower stay hot?The knob on the right or the single knob decreases the flow through the unit and lowers the flame on per water flow.If the shower stays hot you can tweak it up if it is too hot or you need more flow for two showers.But if it went cold then there is some kind of restricter in the water supply that doesn't allow enough flow for a higher flame.Let me know if you need some more help

  • good wrench Nov 30, 2008

    Did the shower stay hot?The knob on the right or the single knob decreases the flow through the unit and lowers the flame on per water flow.If the shower stays hot you can tweak it up if it is too hot or you need more flow for two showers.But if it went cold then there is some kind of restricter in the water supply that doesn't allow enough flow for a higher flame.Let me know if you need some more help

  • good wrench Nov 30, 2008

    You should be set.When you turn on the shower you slowed the flow by turning thet knob and that lets the water get hotter and also lets the flame come on with less flow.

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Bosch gwh 425 hn


Fix problem immediately same day, yourself, for $385, by replacing whole thing:
http://waterheatertimer.org/How-to-install-gas-water-heater.html
Bosch posts some service manuals: or contact Bosch. Lack of error code indicates failed PC board? or bad single-handle faucet cartridge?
http://waterheatertimer.org/Troubleshoot-Bosch-Tankless-water-heater.html
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http://waterheatertimer.org/Crossover.html
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http://waterheatertimer.org/Tempering-tank.html

2) Reduce temperature setting on tankless unit so there is less differential between incoming cold water and thermostat set point.

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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 :)

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