See my response to this thread where it was posted on the tank heater forum:
If you are having troubles with your Navien system cycling on and off or going cold , I can help. I found the problem to be a vacuum issue. After talking to tech we determined our house being foam insulation type was too air tight. So I removed two bottom screws from the cover of the Navien system. I then placed to plastic spacers about 1/4- 1/2 inch in size and used longer screws to attach. The cover now stays open from the bottom. The unit works perfectly has never gone out again. That was over one year ago. You could just take the cover off but I feel better with it just open at the bottom. You must use a firm spacer because the vaccuum is so strong it will close with a soft one. Try it I think you will find it works!!