Zoning Heat in Old Multi-Unit Building with single boiler

[Dan Bosak]

I just bought an old (100+ yr old) 6-unit building with a single hot water boiler on a single thermostat.  The heat works fine and the annual utility costs are reasonable, but there is no zoning and no good way to zone from the basement since various apartments and floors are tied into the same pipes from the basement.  I'm sure we will run into the issue that it is cold in apartments on the 1st floor and hot on 2nd/3rd which will result in tenant complaints and/or upstairs tenants leaving windows open to regulate their heat (I had this issue in a similarly configured 5-unit building).  

I'm thinking a solenoid valve on a couple of the radiators tied into a local thermostat in each apartment would be ideal but I've never seen or heard of this. Does anyone know if this does exist or have any suggestions for any good ways to regulate heat in individual apartments or a way to automate the individual radiators in an apartment?  

The building is vacant so it'll never be easier than now to do it.