I have just installed a new gas hot water heater. All faucets are running except the kitchen. Do I have a vapor lock? If so how do I fix it. Thanks

Do you have one handle, if you do replace the faucet with dual handle faucet. A single handle faucet has built in valves which mix the hot and cold water as you change position of the handle.

Does it run cold? Or not run at all? If cold and bathroom hot, it's piping problem. If doesn't run, valve closed. If no hot water anywhere but runs cold, water heater problem. If gas, fire out.

Turn Off Power or gas to heater, shut off incoming water supply, open the hot water side of a nearby faucet, attach hose to bottom drain valve, open drain valve, drain tank. Opening faucet should stop it from becoming vapor locked.

Open a hot water faucet and let it run until steady water flow without air comes out.

Should be valves under your sink for the faucet. #1
Make sure they are opened up enough.
If either device is new, they should support this issue.
Old plumbing can also cause this. Build up in the lines.
They can only be replaced to fix. Some water heaters have adjustable valves, make sure if you have them they are opened as needed. Remember, too low flow can cause over heated water.

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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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.

your problem can be fixed by running a loop in the plumbing. If you hired a professional in the first place it would already be done. basically the water pressure isn't enough, i bet your faucet is on the second or third floor, or even just THAT far from the water heater.

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.

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?