Originally Posted by tscarps
Having the same issue in our brand new 40D and scheduled a visit for the dealer early next week.
We have had ours since June of last year.
We experienced absolutely no problems all year. As we don't as yet full time
and live in NW-PA I winterized it for over the winter.
This spring before starting the season I took it to the dealer for several minor recalls one of which was related to the Bypass valve for the Truma.
On the 1st trip after the service we experienced the same conditions as you describe.
There is always a slight noise when using the unit as the closed plumbing loop has a circulating pump that has to run so the Truma sees flow and will turn on producing the practically instant hot water. This time the pump sounded noisier but didn't put much thought into it then my DW yelled from the shower that the hot water had stopped. I checked the unit and found the Truma to be in error mode. I reset the unit and hot water came back but only for a very short time. As I do work on pumps in the course of my business I thought the pum sounded like it was pushing against a closed valve (or was going bad). The heating circuit would light for a short time and I felt the main control was OK (I hoped)
I finally checked all the valves and found that the Technician had turned the bypass valve and it was mainly in the BYPASS mode. While it was slightly open it would allow a little water to flow at first but couldn't keep up with the needed flow which would shut the Truma down and eventually it would trip out on fault and power would need cycled.
After turning the switch to the FULL Correct position the pump ran quiet again and the Truma has worked flawlessly as before.
On mine the Bypass Valve is located below the 3-Way valve that is located directly behind the unit.
The Bypass Valve in mine can't be seen but I followed the piping down until I found it buried in the middle of Furnace runs