It is a radiator hot water system that is now divided into 2 zones: the original house which had a gravity system and a new kitchen addition.

You do not do that unless you have back up electric power. The safety control system needs electric to work.

Makes sense to me, though I recall the original system working when we lost power during a winter storm about 15 years ago. Maybe it's just a faulty memory. Would it make a difference if it were just a gravity system with a mercury thermostat?

I am sure with a gravity system with a millivolt generator type control it could be done but your system has now been modified for sure.

The millivolt systems have been discontinued due to minimum government standards or maybe I should say government control.

I started my boiler during the '78 northeast blizzard using "D" batteries but I don't remember how many I had to string together but it was no more than 4.