I remember (well searched and found) a thread in 2016 that said two things 1) There does not seem to be a battery low warning for the CS92 and 2) The CS92 and BDR91 need rebinding after battery loss. Replies said that the rebinding was not necessary.

I lost hot water function today (controller showed hot water on, and system status showed 100% demand) but hot water temp was not increasing. (Heating fine).

So with no errros in the system log, I replace the CS92 batteries, and the controller continues to show communications and hot water demand, but 3 hours later no increase in temp.

Cleared binding on CS92 and BDR91, removed HW from controller, then rebound.

Within 2 mins the system was back up and running and temp increasing for HW.

I have changed batteries before, and this has not happened, so it is not consistent. Would love Honeywell to comment?

Just posting in case it helps someone.

...Jon.