Jym, yes. It would respond to the temp in that upstairs room, but provide to the whole house. Same as your family home. Even if you had a remote sensor upstairs, the downstairs would continue to heat until the upstairs reached temp.
Add to this, steam pipes don’t always go in direct lines (alas.) So the pipes to the room (study) above the DR, which is above the boiler, apparently is not a just a matter of 20 feet up.
Now in my case, we balanced those vents today. Max setting is 8. MBR 6 (was 8,) BR2 6 (was 8,) study from 6.5 to 7,5 , first floor is set lower. That sends more heat upstairs before the first floor. (All these variables do bend my mind.)
This boiler is new, right sizing, the rest is old. With today’s fix, the study is heating. The two other bedrooms are nice. (4th room has its own elec.)
With vents, if I have dinner guests (some friends are older and temp sensitive,) I can crank that vent to a higher number, temporarily. Or if someone stays in D2’s BR and is chilly, that vent can be opened more. If the whole house needs more heat (say, guests with little kids,) then I’d up the thermostat.
All this spins my head. Today was a Eureka moment and I don’t want to lose that, lol. But it shows me that if Isell, the system is capable of heating the house to another’s specs.
Last concern is if, when the weather warms a little next week, this will still work as it has today.