How to split/control heating cost on old triplex boiler

[Paul Kuhn]

I am in the process of buying a double brick triplex built in early 1900's (southern Virginia/historic district), 1650 square feet 2 bed/1 bath units stacked on top of each other. The whole house is heated by a boiler with radiators throughout the home, with the thermostat currently in the basement. No air conditioning installed, 1 window AC unit in each of the units.

Currently the heating(gas) bills can go as high as $800 in the coldest months and as low as $30 (tankless water heater) in the summer months with single pane lead glass. Tenants will have the windows open in December because it is so hot in the building! The current owner pays the gas/water bills, and is losing money on the property. 

I plan on living in the building for 2-5 years. My question is, is how can I lower the heating cost(make the tenants pay) and let the tenants individually control the heat between units? Part of my VA renovation loan/purchase is to have storm windows installed to maintain the lead glass and appease the historic district.

Thank you all in advance for the advice.