1 boiler vs 3 boilers
I have a 3 unit property in which there is only 1 thermostat across all 3 units. It is an old hot water / cast iron radiator system. I currently set the thermostat at 72 in the "coldest room" and pay about $3000/year in heat and hot water. Inevitably, someone always complains about being cold.
I have 2 options to update my heating system.
1) Install 1 modcon boiler + indirect tank. Plumb the supply and return (home runs) 1/2" pex tubing for the cast iron radiators. I would install BTU meters and charge each unit a fraction of their gas usage. Each unit would have their own zone and thermostat.
Pros: only 1 system to maintain. About 1/3 the cost ($30,000). The total bill per tenant is probably cheaper due to modcon.
Cons: I have to deal with billing each tenant for their heat usage.
2) Install 3 hot water boilers. Tenant would directly get billed for their gas/hot water usage. Each unit would have their own thermostat.