I have two zones in my first floor forced hot water heating home. Each has a separate thermostat. There are no doors between zones and the air flow is unobstructed.
One of the zones (LR, DR) is hardly used. Hence, I want to keep the thermostat low.
The other zone (Kitchen, family Room) is inhabited constantly and set at 70 degrees during the day and 65 at night.
My question is: What thermostat setting should I use in the non-used zone? I am tempted to set it at 55 degrees but would that cause the other zone to work too hard to maintain temperature?
"working too hard" isn't really the way to put it. A properly installed system could easily handle 24/7 operation with no problem. That zone would work longer to be sure. But that's not the real issue.
My guess is that energy savings won't be that impressive. The walls between the two zones are not insulated. So as you've alluded to, the operational zone will inevitably heat at least a part of the other zone. And if you do successfully manage to get that unused zone cold enough then you may end up having to deal with condensation and mold. Warm moist air from the occupied zone will seep to the cold zone and could condense onto cold surfaces, like the outside windows and walls of the unused zone. Thereís probably other disadvantages that Iím not even thinking of.
mold would be a problem for sure