Question about Terminator Aquastar 125FX Tankless Water Heater: FX Hood Horizontal Vent

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Bosch Tankless HW Heater...Hot water flow becomes cold then hot again during shower. Have had only Bosch plumber in area out numberous times but never fixes the problem.

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You are using a low flow shower head. you probably own the 250/2400 they have activation rate of .8gpm. If your shower head is low like this, your unit will deactivate. Higher flow shower head is a fix, or pull the aerator out of the shower head you have now might help.

Posted on Jan 19, 2009

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Hot water varies


I do not know how old this unit is or what model but many water heaters are not sophisticated enough to measure the temperature, but heat based on water flow and temperature rise. To start, tankless water heaters turn off every time you turn the hot water off. Lets use an example. You use the facilities, wash your hands then turned off the water and take a shower. You will bring the already heated water (from washing your hands) from the pipe up to the shower. Then the cold water that comes into the pipe from the reactivation of the water heater comes up the hot water pipe with warm then hot water just behind.

If this is not your problem, keep reading. Every unit I have been called to see were installed wrong. Check your installation manual to see if the problem with the system started with the contractor.

First, check the temperature of the water from the heater. If it is too hot and you are mixing too much cold water in, the unit will turn off because there is not enough water flow to keep it running.

Second, turn off the valve at the water heater and turn on the hot water at a faucet. If the water does not stop running, there is a "plumbing cross over" usually caused by a failing pressure balance valve in a shower.

Third, if the unit has not been serviced, go to bosch-climate.us to find a tankless water heating professional in your area. Totally Tankless LLC

Mar 14, 2014 | Bosch & Tankless LP Hot Water Heater

1 Answer

On demand hot water heater gets cold for a short while during showers.


Check for crossover.
Bad single-handle faucet cartridge causes cold water to enter hot pipe, so hot never arrives and tankless does not run consistAntly
http://waterheatertimer.org/Crossover.html

Apr 13, 2013 | Bosch AquaStar 2400ES-NG

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I installed a dhc tankless heater, but when I open only the hot water everything is ok, but when I open the cold water at the same time the heater turns off.


Hi and welcome to FixYa, I am Kelly.

What is happening is that your cold water flow is greater in volume (gals per min) than your hot water. So when you open the cold water it is mixing with the hot water inside the faucett or shower plumbing and actually reverse flowing back into the hot water heater plumbing. The reverse flow slows the water flow sensing turbine inside the heater and it is thinking that there is not any water flow. (There isn't actually due to the equalization of water pressure when you open the cold water faucett) Since this is a safety feature of the tankless heater to keep it from overheating, it shuts off.
Somewhere in the hot water plumbing there is a major restriction. It may very well be the internal plumbing of the tankless hot water heater or the flow detector design.

There are 2 ways to correct this. The easy way would be to restrict the available cold water flow. You can do this by installing another shut off valve inline with the cold water plumbing and just not open the valve all the way. You would have to adjust the setting so that the hot and cold water can be on at the same time and the hot water heater stays on. The hard way is to locate the hot water flow restriction and eliminate it.
It is possible.... if you have a flow restrictor in a shower head or a screen in a faucett areator that they are partially clogged. Clean any flow restrictors and screens then see how the hot water heater performs before you start modifying plumbing.

Thanks for choosing FixYa,
Kelly

Apr 10, 2011 | Stiebel Eltron DHC-E 8 Tankless Electric...

1 Answer

Hi there, I have a bosch aquastar 125b waterless tank. My problem is that i am not getting hot water at all. The water runs during the shower and it stays warm. HELP!!


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.

Mar 26, 2011 | Bosch AquaStar 2400ES-NG

1 Answer

Hot water tank does not work when shower is on, it does however work when the faucet in bathroom is on. It quit working in the shower about a month ago.


  1. COLD MIX, HEATER DEACTIVATES
    If inlet water temperature is over 70°F due to geographic location, water maybe very hot out of the tap. This requires a lot of cold water to be added with it in order to attain a useable hot water temperature. The addition of too much cold water will overpower hot water demand from the tankless water heater. This slows the flow within the tankless water heater, decreasing it below activation point, which shuts off the heater. The end result is nothing but cold water coming out of the outlet. See: TWH E2-04 Overcoming fluctuating temperatures [PDF].
  2. FLUCTUATING WATER PRESSURE
    If the water pressure in the home is erratic and the water flow is not consistent while a tap is opened, then the temperature of hot water will fluctuate. The minimum water pressure for the home should be 30psi or greater. For people on well systems the minimum pressure range is 30-50psi. The use of a pressure reducing/regulating valve is an effective way to maintain constant water pressure to the water heater. Watts brand 25AUB- ¾" or N35B-¾" pressure reducing/regulating valves or equivalent is suggested.
  3. CHANGING FLOW RATE
    Major changes in flow rate can adversely affect the output water temperature of the heater. Increases from one major fixture running to multiple fixtures running at one time can cause the temperature to fluctuate. Stay within the heater's specifications. See: TWH E2-03 Outlet temperature vs. flow rate [PDF].
From the Bosch website.

Good Luck,
Gary

Dec 16, 2010 | Bosch PowerStar AE125 AE125 Whole House...

3 Answers

Water turns cold very quickly


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 :)

Jul 06, 2010 | Bosch & Tankless LP Hot Water Heater

1 Answer

Heats up the rapidly decreases within in minutes.


Problem new or old? How long you have unit? Sounds like not enough flow to keep burner on. Tankless need at least .6 gpm to keep hot water flowing or else the burner fires then shuts fires then shuts and you get a sandwich effect...hot cold hot cold. If you have new diverter valve in shower -vs- old two handle type try turning hot water ALL the way ON and see if hot stays hot. If it does stay hot then slowly adjust to cool and see if you maintain a good MIX for proper temp. If it goes cold again try this......LOWER control pad setting to a comfortable 105 degrees (ladies) 100 degrees(men) and turn shower valve all the way to HOT. See if your temp remains consistant now and if it does then problem solved. Tankless mindset...set temp on control pad and stop trying to MIX at shower valve. Thats how these things work. Thats why I install a control pad in each bath and one in the kitchen for adjusting ash temp during dishwasher on time. ditto for wash machine. You should never need to turn on cold water if your unit is set to proper temp at controller. Hope this helps

May 17, 2010 | Rheem Tankless Hot Water Heater...

1 Answer

Water becomes ice cold in the middle of a shower


Sounds like you have a problem with water flow. Since your ground water is warmer in the summer, you mix less hot and more cold in order to get the same warm temperature. If your hot water flow gets low enough, your tankless will start to "short cycle" in order to prevent scalding. Use one of these three methods to increase your water flow at the shower:

1) Clean your shower head and descale
2) Widen the hole in the showerhead's flow restrictor
3) Remove your flow restrictor entirely

Good luck!

Jan 24, 2010 | Noritz NORTIZ Outdoor Propane Tankless...

2 Answers

Water runs hot then cold then hot during showers


YOU MAY HAVE TO DESCALE YOUR UNIT I have this problem along with a bunch of error codes. Please don"t call the Bosch 888 number. They want to speak to a tech. In addition, their off during the weekend. Tankless water heaters are great when thier working. Mine is not.

Oct 17, 2009 | Bosch 2700ES-NG Natural Gas Tankless Water...

2 Answers

Bosch AquaStar 125b ng suddenly goes cold during shower


Does it happen to any other fixture? If not, it's most likely your shower valve and not heater.

Sep 01, 2009 | Bosch AquaStar 2400ES-NG

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