We have a bosch tankless water heater in a 1600 sq ft home...it works great except in one faucet (the spare bathroom sink)...sometimes the hot water comes out, sometimes it doesnt, sometimes it takes forever...if we turn the bathtub faucet on at the same time, the water will get hot in the sink...can you help us? I'm at our office, not at the house...not sure of the modelm just know it is a bosch tankless water heater

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

Problem might be crossover from failed single-handle faucet cartridge:
http://waterheatertimer.org/Crossover.html

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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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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.

try bleeding that section of pipe. or the whole system if not.

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,,

I had a customer a few weeks ago that had the same problem. When the water heater was installed they crossed the supply lines. They had the inlet as the outlet. Esentially, the water heater was plumbed in backwards. Also there could be a miss piping somewhere in the house but it would probably be near impossible to find it right now without tearing up foundations. Also check that the faucet is not plumbed backwards as well. Especially if this is a single handle faucet that you are referring to.

Do your sinks have a hot and cold handle or just one handle. how old are the faucets. If they are very old.the first thing i would do take one faucet out and replace cartridge.

you can put in a circulatating pump that is controller with a timer and a thermostat so the when yo need heat water the most its readilly available and when you don't it's off, the only thinsg thing that you would have to do it tye in the supply and the return and then insulate the pipes.

diaohragm valve or micro switch