After showering - pressure builds up and I have to turn other faucets in the house on to drip to reduce ths pressure. Outside water pressure is @ 60 when hot water turned on in shower (approx.10 minu

Well when it is heating up the pressure will rise
mine set at i bar but will go to 1.5 when hot

it could be a leak around an element if it is an electric heater.

Have you ever tried to pump water for a fountain... It is so tough and you need larger and larger pumps to get it to go 7 feet high to let's say 10 feet high. Having the on demand water heater in the attic dramatically drops it's pressure. It's best to feed it with a 1 inch or at least 3/4 inch pipe downstairs before it splits the hot and the cold to get the best pressure everywhere then the least pressure will be when it reaches the 2nd floor. but the heater won't cut out because the pressure it even greater downstairs.

Has nothing to do with the expansion tank, the expansion tank whole enough air so that when the heater heats the water it does not build up pressure. You know from school that you can compress ait. you CAN NOT compress water. When you haet water it expands until it turns to steam. So lets rule that out.

First look at all the plumbing in the house where there is a cross conection. This would be like a shower valve that has a personal shower head on it. and you turn of the water using the personal shower handle and not the valve at the wall. This then would let cold water to go into the hot water side piping.

Most people do that with out knowing it at the laundry sink. they put a hose on to was the dog and forget to turn off the faucet and then BANG cold water in the hot side.

Go around the house and turn off all the faucets and valves by using the handles. no add on things.

Now last could be the shower valve itself. . Do this happen in just one shower or all of them???

If its just the one and you spend the day doing the walk around, even check the bidet if you have one

its the valve. you need repair to that.

Best of luck in your hunt Tom

http://waterheatertimer.org/Troubleshoot-Bosch-Tankless-water-heater.html#flow
1) Tankless require minimum flow rate before burner will activate.
2) Sounds like the water pressure drops quickly when faucet turns on. This implies combination of problems including sediment build up inside lines, crossover at single-handled faucet, or sediment filter clogged or water softener clogged.
3) Low flow faucets can reduce needed flow rate.
4) Other problems can contribute: sensor failure, loose wires, PC board failure, high temperature setting, cold incoming water temperature during winter months.

If you spin the shower head off there should be a metal screen inside. If it is clear, Soak the shower head in CLR for about 4 hours. After 10 years even the best water supply will leave behind a build up that can plug the tiny holes.

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

Do you mean the other faucets in the house are hot and only the shower is warm? If so there is an anti-scald adjustment on the faucet that needs to be turned up a few notches. Havent adjusted a Grohe faucet but most faucets you rmove the handle and behind the handle is a nothced adjustment ring that prevents the faucet from getting too hot. This ring, or stop, is pulled out and twisted clockwise(or counter clockwise to set higher or lower max temp, then pushed back. maybe you can look up grohe to see which way ring or stop adjusts for hotter water.

it comes on when the pressure changes so when you open another faucet it changes the pressure and makes it come on,, check your pressure regulator valve on the main line comming into the house it has a screw in the center, if you have a pressue gage put it on either the water heater drain or out side hose bib , check the pressure when turning water on and off if it changes more then 5 lbs you need to change your prv valve. pressure regulator valve,

I found a way to get warm water in the shower, but I'm not sure it qualifies as a solution. After trying everything else I thought that maybe when I was feathering the faucet in the shower toward the cold side so I would get warm water that at a certain point it blocked the flow on the hot water side enough so that the sensor in the hot water heater sensed there was no demand in the line and it shut the water heater off. To test this, I went to the kitchen and turned the kitchen sink faucet all the way to the hot side and then turned it on so that I got a small flow of water. I waited for the water to get hot to be sure the hot water heater was working. Once it was hot, I left it running to keep demand in the line and then I went to the shower and turned the faucet on. When the water got hot I started feathering the control towards the cold side and soon I had warm water. So I think I'm right in that the sensor in the hot water heater was shutting the water heater off when the demand in the shower was lowered to a certain point when I was trying to adjust the water temp in the shower.

So now I can get warm water, but it means I have to leave the hot water running at another fixture in the house to create a false demand in the system so the hot water heater doesn't shut down. Is there a way to adjust the senor in the hot water heater so that it will stay on when I'm using the shower only, so I don't have to waste water by running another faucet when I want to take a shower?