Heating a Duplex on one furnace HELP

[Craig Rich]

Hello everyone, so i purchased a duplex back in 2018 (CONNECTICUT)  to use as rental property (my first home). As i was eager at the time and not as knowledged as i am now i didn't do the proper investigating to notice the homes heating source was a single unit oil based furnace (thermostat being located on the first floor with the tenant). I bought the house with a tenant who no longer resides at the property and myself moving into the upper unit(2nd floor). The tenants lease she signed did not include her having to pay for oil but just electricity, (1st floors heat and hot water are fueled by oil) Me not being a home owner before and or landlord thought nothing of it until winter came along and this oil bill became very expensive. I called in a Hvac contractor to get his opinion and he insisted that the duct work was built to heat a one family home and apparently putting another unit in (i had a gas line installed to the home 2019) would be very difficult to do assuming the lay out of current duct work so having it re routed would be difficult. He then insisted having a gas unit installed into the attic and having a new duct work done would cost nearly $9,000. Fast forward to 2020 i now have family renting out the first floor and i am getting prepared to buy a single family home for myself and i will have to rent out that 2nd floor. 

Note the second floor does not have a thermostat and it wouldn't be fair to my 1st floor tenants who have to pay to heat the entire home...

Any insight would be highly appreciated!!!!