IMO the cleaning of the recirculation line won't affect the exchanger. I would do like you did on the Rinnai and connect your lines to hot and cold. Now, it may help to open the recirculation pipe to allow it to recirculate the vinegar, then close it back off. When you are through, circulate clean water through the recirculator to clean it up and remove the vinegar.

 

I tried the “Rinnai” way, but it didn’t work. Pumped through the cold service port, but nothing came out the hot. I tested the pump and it was working properly. I kept the recirc valve closed because I didn’t want vinegar going into the recirculation line. If I opened it, I would have to fill the entire line throughout the house with vinegar and then flush it, which ultimately would work but doesn’t make sense. Ultimately, I think I need a valve and service port on the recirc line.

 

Wouldn't putting a service port on the recirculation line negate what you are attempting? Just spit ballin' . You are isolating the hot and cold water valves and then once connected opening them to the pump, but keeping them isolated?

 

You would think, but the hot/cold ports thing didn’t work. I believe it has something to do with the internal recirculation pump used to descale and the internal plumbing rather than an external sump. It has a specific descale setting. I believe it recirculates through the heat exchange. There are several comments and a video on the interwebs (which if course is always correct) regarding the recirc port. I much prefer the old Rennai vs. this unit.

So, this is a new house build and my warranty expires in one month. Had I known I’d have so much trouble just trying to clean the thing, I would have objected. I’m going to add this to my list and address it with my builder. TBH, I wasn’t impressed with their plumber. Nice guy, but there were several mistakes I had to point out during the build.