Cold flow is much less volume than hot

Refer to the manufacturer's recommendations for back-flushing/descaling to flush out mineral deposits and build-up in the heat exchanger. You can check to see if this is a problem by comparing the full cold water flow with full hot water flow at the faucet. If the hot water flow is significantly slower than the cold then the heat exchanger is likely clogged with deposits and will not transfer heat sufficiently. If the flow is good then the heating element(s) maybe defective.

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It is your mixer value. Replace your value which it is not a very expensive piece and you are on the road again.

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

water sounds like it is off at the mains which could be in the flat below , need access to there to check, or it been cut off for non payment , heater would not stop main flow , it is off at the main

best I can tell you ,

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

If you have water pressure to the cold inlet valve and the valve's filter is clear, you probably have a failed inlet valve...Replacing the cold valve should fix the problem..Not a big or expensive job..Good luck....Nomess

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.

disconnect your hoses and check for pressure without connected o machine, then check with int he water inlet valce for a clogged screeen slowing the pressure

the water inlet valve has to be replaced,its the part where you seen the screen ,where the hose hooks up to,theres a cylinoid that opens and closes and lets the water come in,your cylinoid is no good,but you change it as one unit,and its called water inlet valve,easy fix you can do your self,if this helps please vote me a fix ya

its in your cold water line valve to the washer if it works good with hot water the valve are incoming line being blocked when you turn it on the pressure comes in fast then when pressure is gone water slows down so when its off the pressure builds back up it runs fast then slow