The house we just bought came with one of these in the master bath, and our shower is basically unusable because of it. The general process goes like this:
Turn hot water all they way on. Wait several minutes until water slowly starts getting warmer. Barely turn on the cold water. Wait a couple more minutes to ensure the temp stabilized. Get in shower. Water temp suddenly goes either ice-cold or scalding-hot, randomly. Adjust hot and/or cold taps to prevent bodily harm. Wait a couple minutes to ensure temp has stabilized. Repeat.
I've tried adjusting the temp control on the unit, and while it changed the max temp it had no affect on the symptoms. I discovered that any more than ~2-degrees of rotation with the cold water tap caused the heater to switch off. That seems odd to me?
Is there any hope, or is this a POS than needs replaced/upgraded?
I have read that the regulator in those units needs to rebuilt occasionally.
But I thought that had more to do with demand than temp regulation....
Rant - Mine will shut off if I am not using a significant amount of hot, like when shaving.
In reply to bentwrench:
Interesting. This one is a few years old, so that could be a possibility. After messing with it a while, I think a temporary workaround is setting the temp of the heater = our desired shower temp, then just leaving the cold water off completely. However we noticed the shower water pressure seems low, but we're not sure if it's the pressure, or the shower head. I'm going to install our old shower head from the last place & see if that makes a difference.
I also noticed the unit is plumbed with 3/4" copper, so maybe we need a reducer(or something) to increase line pressure?
They need to be back-flushed annually. A lot of people don't and the performance will suffer.
Sounds like the problem can be in the shower valve. Is it a single handle pressure balance anti scald unit like a Moen or Symmons? The Symmons has a floating ball to balance the pressure that gets stuck. If its one of those old school shower valves with separate hot and cold handles, then its the water heater. The water heater will only make hot water if the flow sensor sees at least .4 gallon a minute. Less than that, like for shaving, it ignores the signal to make hot water. After the hot water comes out of the unit, there should be a tempering valve that mixes cold water back in to prevent scalding water. Those go bad as well and that might need a rebuild. So many possibilities, and not to toot my own horn, but thats why us licensed plumbers never want for work.
In reply to tr8todd:
It has separate hot & cold valves, so that confirms its in the unit. 
Calling a plumber is our next step, I just didn't want it to be something simple I could easily DIY.
What's your opinion of these Bosch units? I've never been a fan of any of the tankless units, but if this brand/modes is a POS and some other brand/model isn't, that may be the easiest upgrade path to a happy SWMBO.
jstand
HalfDork
1/23/16 8:22 p.m.
As mentioned above, it sounds like a flow rate issue, and you are at the edge of the minimum required to turn the unit on so any fluctuation causes it to cycle off and back on.
Before calling anyone, I would check to see if you can remove the flow restrictor from the shower head to see if that helps.
You can also turn down the temp on the water heater. While seeming counterintuitive it may help, since it will require more flow from the hot to reach the desired temp, moving you further away from the switch point and reducing the sensitivity to fluctuations in flow.
Also, if the faucets are older ones with washers, the washers can swell with the hot water causing the flow to be reduced until the reach the same temp as the water.
In reply to jstand:
I switched out the shower head with the one from our old place & the flow seems normal, compared to using the sand shower head at our old place. However, with the temp on the unit set exactly the same as it was this morning, the water temp was barely above ambient. I increased the temp gradually until it got a bit warmer, but I noticed that even though the LED remained lit, it sounded like a relay buzzing occasionally.
I've never had to deal with the Bosche on demand units other than walk right by them in Home Depot. Can you adjust the temperature output on your unit or is it controlled externally by a tempering valve? If you are getting fluctuations in temp and it isn't controlled externally, then its one of three things.... flow sensor, outgoing temp sensor, or an issue with the heat exchanger. The way its suppose to work goes like this. You turn on water, flow thru the unit gets detected, unit fires up full tilt to make heat, water starts to leave hot, temp sensor senses temp of hot water and computer modulates the flame to keep water temp leaving at the set temperature. If its an external tempering valve, its probably regulating temp with a spring inside the flow of water that expands contracts just like a t stat on your car.... until it gets stuck and just lets everything flow unregulated. Sensors are cheap enough to buy and replace. Some units will have a screen on the incoming water that gets clogged. Its easily removed and cleaned. Looks like a shower head airator. If the heat exchanger is a flat plate style, you need to back flush them every year to 18 months with a vinegar solution to remove scale build up, but most people don't bother. They would rather just complain that they aren't getting enough hot water any more. Whenever I have to fix any of these high tech plumbing gadgets for the first time, I google up the manual and read about it. Start with the cascade chart which outlines how everything is suppose to happen and in what order. Once you figure out what isn't happening that is suppose to be happening, thats your problem. Most of these units now have an error code screen that narrows down the search for you, but those usually only flash once a shut down occurs. In your case, the unit doesn't think its doing anything wrong.
It seems like there must be a problem with flow rate issue. I am having a [40 foot canoe] and is ideal for household needs.
