IF THIS IS A NEW UNIT THE PROBLEM SOUNDS LIKE THE BYPASS WATER VALVE IS CLOSED NOT ALLOWING WATER INTO THE WATER HEATER. We had the same problem our first time out. The unit was new and winterized so the valve was still closed. The water heater would fire up and run for about 20 seconds and shut off. Check the pressure valve and see if you are getting water out when lifting the valve. If not there's no water in your water heater. The reason it quits after 20 seconds is because the high temp switch is getting a hot reading telling it the water is hot but it's only the air in the tank that is hot. There should be a valve under the bathroom cabinet to allow water into the heater. I learned this by experience and was a little embarrassed but hey it's a learning experience LO
Hope this helps.
