When in the shower, the heat fluctuates from hot to lukewarm over a 10 second cycle. The water will not stay at a constant temperature.

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.

It *may* not be a problem with the water heater itself necessarily. When you say "keep adjusting the water temperature when you're taking a shower" I presume you're talking about turning the shower hot/cold knob(s) and not the temp thermostat on the water heater itself.
If so, when it starts to run colder, turn the knob all the way to full hot. Is the water then very hot? If so, the water heater is working fine & doing its job, and the problem is likely to be in the shower faucet mixing valve itself. Sometimes a faulty one will expand the inner seals/cartridge as hot water flows thru it, and starts to (wrongly) restrict the hot water flow, thus causing the constant need to "turn the dial up a notch".

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 .

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

This electric demand water heaters has one heater element. The flow switch turns it on. The thermostat cycles it on and off according to temperature. There is no medium.
You need to run the sink on hot while you take a shower. This kind of cancels out the savings on electricity.
Or get a higher flow shower head. Your screen on the shower head may be partially fouled.
Here is the brochure. It shows the flow rates for the different models.
http://www.stiebel-eltron-usa.com/sites/default/files/pdf/brochure-dhc.pdf

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 :)

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

It might be a question of inadequate flow. All tankless water heaters have a minimum flow rate that must be met, typically .5-.9 gpm, before the heater will operate. In the summer time the incoming ground water temperature is higher, causing you to mix more cold water and less hot water to get a lukewarm temperature. This can reduce your flow rate near or below the minimum flow rate.
Here's a test for you, try running running a sink while taking your next shower. If the hot water remains mostly constant throughout, then your problem is with flow at the showerhead. The flow at a shower head can be reduced by buildup of scale and sediment. Also all showerheads have a flow restrictor that limits flow to 2.5gpm (some even less). Removing the restrictor will increase the showerhead flow rate as well.
Also look for any error code displayed on the digital read out of the heater. Some heaters will flash on their burner indicate light as well.

Sounds like there is not enough flow in order to keep the burner in the heater firing continuously