If it's not the bypass, it may be when the hot water tank was installed they used to long of a hose and it became kinked. That happen to mine. CW "fixed" my hot water and swore that they ran the hot water for a few minutes. I had told them that I had hot water but within one minute it became cold. They just assumed that I didnt know how to work it. on the day we were leaving for a trip I had to take it back again and told them I wanted to watch them run the hot water, it turned cold. It took them 5 hours to figure out that when it was installed the hot water hose was kinked. They had to trim it down and now it works.