Where would you place the thermostat in a single zone, 3 story building. Tt's a multi-family residential, 3 units, one unit per floor. The boiler is a high-efficiency condensing boiler in the basement, with hot-water cast-iron rads in all units. I know the ideal is to have multi-zones, one per unit, but that isn't currently a possibility. So is it better to put the thermostat in the first floor, since heat rises, or in the top floor, since the top floor is farther away from the boiler?
This is a common problem in NYC. There's no perfect solution.
What I've done is put in an electronic thermostat with remote sensors on each floor.
Then, using the data I've collected, I tell the thermostat which devices to obey at each time interval.
In general, each floor is a degree or two off from the other ones, but the top floor can be much hotter/colder than the other ones, and, of course, you get tenants who leave windows open in winter...
I lived in the third floor of a building set up like this.
If you are the landlord and paying for the heat, you want the thermostat in the top floor apartment, at a high-ish temperature (maybe 72*F) and let the tenants on the 1st and 2nd floors deal with it.
If you are a tenant, and the landlord is paying for the heat, you want the thermostat on the first floor set at the minimal that tenant can tolerate, and let the 2nd and 3rd floor tenants open windows if they need to.