I recently had repairs made to a cracked mains water pipe. The plumbers cut out the faulty pipe and replaced. In the same area there was a new outside tap installed one month prior, tapped into the mains. The new tap and the Rinnai gas continuous hot water system are all in the same area. At first I thought it was just the one tap that caused the hot water to ignite, but now it seems that both outside taps do this. I had the plumbers back, they had no idea, and said the problem was mine, (it was the gas hot water) and not theirs. They would not listen to me that the problem only occurred following the mains repair work. I'm suspecting something to do with pressure, and maybe the sensor in the hot water box. Please, any ideas would be most welcome.

What are the conditions that make it ignite? What has to happen first?

turning on the outside faucet makes the rinai fire?? that makes no sense..

I'm not a pro, but I believe that is caused by "bounce". The water is flowing slightly outward from the heater, and activating the flow switch.

Even if that were the cause, it would turn off immediately when it did not sense water flowing continuously.

Is the unit firing, then shutting right down?