Thanks everyone! @kevin9408: I ended up returned the Vesta and paid $89 more to get an Navien NPE-240A2, just for a little bit peace of mind.
By the way, I was thinking of installing a descaler and a water filter. However, after I called Navien local distributor, and that guy (Tim) was so kind and helpful and asked a lot of his guys, he told me that for San Francisco, the water quality is darn good (47 PPM and 2.7 GPG). He mentioned that Navien only recommend adding a descaler "PeakFlow S" (for residential) if the GPG is larger than 4.0. So I guess I won't be installing a descaler and a water filter, just to keep it simple. Will probably do the vinegar descaling like Cathi mentioned.
By the way, my biggest learning point is that (1) Vesta.DS seems to be a group of people originally from Navien and started their own company. Navien seems to have a proprietary recir pum, while Vesta.DS uses grundfos 1558 pump, which according to Vesta's tech support, is a slightly better pump. I also "tested" calling tech support for both company, and both were the same. A real human did pick up the phone. I heard that a regular customer can not directly order parts from Navien though, which I'm not sure if that's true. Anyway, just want to share all these info with everyone here. Hope this helps (man, I probably spent too much time on analyzing this)...
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