Hi Bannerman, was wondering if you knew if the new A2 series units like 210A2.. requires the branched cold line plumbing to the recirculation port to have it working when unit is set in external mode.. The drawings/manual say otherwise.. but my common sense and research says it should be done how you've noted here on the A series.. Navien is completely no help what a surprise.. but yeah I think they've got their drawings and manuals screwed up and it should be as the navicirc install manual shows.. with cold line branched to recirc port with check valve in place..Good to hear about the vibration, but always concerning when no cause has been identified. While the problem is gone for now, will the vibration recur again in the future?
Many homes are now plumbed with central hot and cold manifolds that feed each fixture individually using PEX piping. Unfortunately, plumbing a home this way leads to the situation you identified, that is, locating a recirculation return valve at the most distant fixture from the WH, may not necessarily assist hot water to arrive quickly to other fixtures even within the same room. If your home is equipped with central manifolds, the hot manifold should remain warm along with the individual hot feed to the sink where the NaviCirc unit is located, but all other feed lines exiting that manifold will remain at room temperature when no hot water has been drawn through them for some time.
Perhaps the current hot plumbing line that feeds the shower, maybe cutoff and re-plumbed to T directly into the same hot line supplying the sink where the NaviCirc is located. It would be best to T in directly below the sink and before the NaviCirc valve.
Alternately, perhaps a second NaviCirc valve may be installed within the feed line serving the shower? This way, hot water will be quickly available at both the sink and shower. The NaviCirc for the shower will perform best when located as close as possible to the shower itself.
Another possible option dependant on findng a suitable path for a continuous length of PEX piping, is to install a dedicated return line from the shower and sink back to the WH. The NaviCirc valve would then not be needed as each hot line at each fixture would flow through a check-valve to ensure the correct flow direction, and also through a partially closed valve that will balance the return flow between the two fixtures. A dedicated return line will also eliminate the tepid cold water situation since the cold water lines would no longer be providing the recirculation return path back to the WH.