How to Charge for Steam Heat in a Duplex

[Chris Luger]

Hello BP!  I closed on my first duplex.  All my previous props have been Single Fams.  I'm wondering how to charge the renters for heat.  I'm including water/sewer as there isn't two meters. 

Each unit has a thermostat.  I don't know if I need to pay for the fuel, and then charge back the tenant? 

How typically have landlords done this.  Any advice would be greatly appreciated.