Bosch 2400E-NG output temp fluctuates

If your Bosch Highflow 21e hot water system is experiencing fluctuations in water temperature while showering, it could be caused by a few different factors. Here are some possible reasons for this issue:

1. Flow rate too high: The hot water system may not be able to keep up with the demand if the showerhead or tap is allowing too much water to flow through. This can cause the water temperature to drop suddenly. Try reducing the flow rate by turning down the tap or using a low-flow showerhead.
2. Water pressure fluctuations: If the water pressure in your home fluctuates, it can cause the hot water temperature to vary. This can be caused by various factors, such as other appliances using water at the same time, or changes in municipal water supply pressure. Installing a pressure regulator or a flow restrictor may help to stabilize water pressure.
3. Blocked or dirty water filter: A clogged or dirty water filter in the hot water system can restrict water flow, causing the water temperature to fluctuate. Check the filter and clean or replace it if necessary.
4. Malfunctioning thermostatic mixing valve: The thermostatic mixing valve (TMV) regulates the water temperature in the hot water system. If it's faulty or dirty, it can cause temperature fluctuations. A licensed plumber or technician can inspect the valve and replace it if necessary.
5. Faulty temperature sensor or control board: If the temperature sensor or control board in the hot water system is faulty, it may not be able to maintain a consistent water temperature. A technician can diagnose the issue and repair or replace the faulty component.

If you're unsure about how to diagnose or fix the issue with your hot water system, it's best to contact a licensed plumber or technician for assistance.

A lot depends on the size of the tank. And when it heats some systems only heat at night , this is ok if the system is large , on most heaters there is a thermostat on the side that can be used to set to a higher temp. the thermostat on your heater may be faulty.

Open following link for Bosch troubleshooting resources:
http://waterheatertimer.org/Troubleshoot-Bosch-Tankless-water-heater.html

Bosch has a service bulletin that might apply to problem:
http://waterheatertimer.org/pdf/Bosch_Overcoming_fluctuating_temps.pdf
Links about crossover and low water pressure:
http://waterheatertimer.org/Crossover.html
http://waterheatertimer.org/Low-hot-water-pressure.html

Add a comment and say what you did to fix problem.

Bosch has a 'fluctuating temperatures' pdf
It may apply or not
http://waterheatertimer.org/pdf/Bosch_Overcoming_fluctuating_temps.pdf

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Ok i intall tankless water heaters and sell them and repair them . The trouble your are hving is more than likely ,the mixing valve on your shower has a ajustment on it to prevent you from burning your self . that is the number one reason in showers when water never get to temp . What it is doing is addng cold water to your shower for you bummer . Take handle off and adjust water temp to your hottest setting . Some times you have to remove the low flow washer in shower head to get the right flow . also every time you start up your hot water it shouldbe set all the way ot other wise withe the battery 2 D models the unit will not sence flow and shut off the unit ,with some units it is a fine balancing act to get them adjusted right. If your buying a tankless water heater always buy a blower type unit that has a computer that runs it . You will be happy . If the Battery operated unit is for a cottage or a smallcabin then your just going to have to fix the pumbing to make it work right many of the safety features of the newer showers are not made to work well with tankless water heaters .

Possible flow sensor issue. Check to see if your water pressure fluctuates a lot or is fairly steady. These units are sensitive to the pressure. take care.

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

Iam guessing your using it faster than it makes hot water. May need a larger unit or a second one,in the system

Sounds like the water flow sensor and servo, may just need cleaning, get it checked out by a Bosch agent

This sounds like a common problem with tankless type water heaters. The modulating burner has a set Btu range it can operate within, a maximum and minimum. With very low flow rates and very low temperature rises, the burner has trouble maintaining a steady output temperature because the btu load drops below the heater's minimum input. Typically the machine will overheat the water beyond it's setpoint, the burner will cut out, and then re-ignite once the output temperature has dropped low enough.
The solution to stopping this behavior is to simply increase the water flow through the device (i.e. higher flowing fixture or larger pump). This type of problem is exaserbated with recirculation systems that bring a very high water temperature to the unit.
Higher end tankless brands have a lower minimum input btu and thus fewer problems with this.