My navien 240 recirc has had the same problem going warm after 5 minutes. I bought a check valve for 6.95 and installed in five minutes. The old one was pulled out in pieces and would crumble like clay in your hand. It will be a bi-yearly Maintainence item for me to replace. I had the same experience with navien service. I understand liability issues but the problem was not electrical or gas and should be listed as a regular Maintainence issue. I won’t waste any more time with Navien.