Having a problem only in the winter the hot water stops we have been solving the problem by running the bathroom hot water faucet while taking a shower. When running the dishwasher or washing machine, or sometimes doing dishes, we have to manually turn the unit off and on with the switch. We have developed this problem sometimes in the other seasons as well.

The hot side of the faucet is most likely plugged with something that came up thru the water system. The average thankless waterheater requires at least 1/2 to 3/4 gpm flow rate for the unit to sense flow and turn on. If there is not enough flow it will not fire and heat the water. Not sure about this particular unit but a few require at least one full gallon per minute flow. So chances are that if you fix the faucet, you fix the hot water issue at the same time

have a plumber check the installation as it appears that there is a temp control mixer in the kitchen line that should be in the bathroom line
Many states and countries now have made it mandatory the full uncontrolled hot water cannot be used in a bath, shower, wash basin but has to limited to a max temp of 70 C to prevent burns to the skin

1) Guesses: Sounds like a bad single-handled faucet cartridge, or low water flow at faucet, or clogged water filter.
Replace faucet cartridge, clean the aerator, and/or clean sediment out of pipes below sink, and clean water filter at tankless unit each month.

2) For exact measure of problem:
Typical faucet can have 2 gallon per minute flow rate.
2 gpm flow rate means faucet fills 1 gallon container in 30 seconds. Test faucet using 1 gallon container and a cooking timer to get flow rate.
Clean aerator to improve flow rate. Compare kitchen flow rate with other faucets in home.
http://waterheatertimer.org/Low-hot-water-pressure.html

Most homes have 1/2" diameter pipe running to kitchen faucet.
100 feet of 1/2" diameter pipe holds 1.2 gallons of water. This assumes no sediment build-up inside pipes.
http://waterheatertimer.org/Water-heater-recirculation-system.html

Assuming only hot is turned on: If faucet is 100 feet away from water heater, and pipe diameter is unobstructed 1/2" diameter, and faucet has 2 gallon per minute water flow, then hot water should arrive in 36 seconds.
50 feet away, and hot water should arrive in 18 seconds.

If kitchen has single handled faucet, and the handle is raised in the middle, then equal amounts of hot and cold water are drawn at same time. This doubles amount of time for hot water to arrive.

If single handle faucet has a bad cartridge, then faucet can have crossover.
Crossover means that cold water can enter the faucet even if faucet is turned to hot.
Bad single-handled faucet cartridge can delay or stop the arrival of hot water.
http://waterheatertimer.org/Crossover.html

Tankless require maintenance including cleaning combustion parts yearly, de-lime heat exchanger frequently, and clean water filter each month.
http://waterheatertimer.org/pdf/Tankless-maintenance.pdf
http://waterheatertimer.org/pdf/Bosch_Overcoming_fluctuating_temps.pdf
http://waterheatertimer.org/Troubleshoot-Rheem-Tankless-water-heater.html

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Since you have hot water at all the other fixtures, the problem lies in the sink, not the water heater. It could be that the sink is farther away from the water heater than the other fixtures. It would take longer to get hot water.

If some taps have really hot and others luke warm, then you might have defective mixing valve, check valve, or crossover on single handled faucet.
http://waterheatertimer.org/Crossover.html

Crossover can also cause stacking event. Stacking could be part of puzzle.
http://waterheatertimer.org/Stacking.html

Once crossover or check valve or mixing valve problem is solved, if you have really hot water to some taps, then turn down thermostat to avoid scalding.
http://waterheatertimer.org/How-to-adjust-temperature-on-water-heater.html

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bobo mayuk

No, the problem is not in the water heater. It could be the plumbing pipes were installed incorrectly.

A common problem with hot water is also found in the settings of anti-scald and pressure balance tub and/shower valves. You may only need to reset the temperature control on each faucet. Every faucet brand has a slightly different adjusting system. Hopefully you can find the faucet's brand name, and then visit their website to find the owner's instruction manual for your model.

A cross connection in the water piping system is rare, but it does happen from time to time. It would allow cold water to mix with the hot. You would need a plumber or leak detection company to locate every water pipe connection in and/or under your home to confirm and correct the issue.

There is a minimum water flow required to operate your Bosch tank-less water heater. It is around .75 gallons a minute. There are three things you can do to help the flow.
1.Turning the temperature down will help increase the flow.
2. Installing higher flow aerators on your faucets.
3. Clean the Bosch tank less water heater.
You are not alone in having this issue. The most common problems are low flow faucet aerators and low flow shower heads.

this is a problem they didn;t tell any one about, there is a min of 1 gpm that you have to run for it to sence for the unit to come on, if not the unit will not detect the water is even runnig there for will not come on,, as for the kit faucet, it might have a temp control and it may need to have a cartridge change, the water comming from the tankless is nothing like the old tank heater, the water only losses aprox 1 or 2 degree where a tank heater losses almost 5 or more because of the way the cold water is fed into the unit,, thats whay the sprayer works and the faucet doesn't ,, you have control on the sprayer unlike turning the faucet on,,

Since the cold water works, the problem is not the pump. Look for a blockage or possibly a major leak in the hot water system. if you have a kitchen sink and bathroom sink, are both hot water outlets not working? Does the water heater have shut off valve , or is its drain still open from when you winterized it (and the water running out to the ground instead of to the faucet?