Tenants having issue with ac/heat in side by side duplex

[Bob Billingtonz]

Hello BP community - we are new landlords and we are having some issues with our side-by-side duplex. We have installed a mini split in their unit which is located upstairs. The downstairs receives some heat/AC from the other side since the place was one house at one point and only had one furnace. The other side that they don't live in is kept between 68-73 degrees throughout the year depending on the season. The tenants know the mini split is their best bet for controlling temp on their side, but they do not want to not pay for the energy to run it and would prefer the gas gets used (gas is included in their lease.) We have installed the mini split for them and given them plug-in heaters. Below 71 degrees they claim is too cold and above 72 is too warm for them. We have offered they can terminate their lease at no penalty charge because frankly they complain a lot and are unhappy with the residence. They have claimed their thermostat changes temperature (ex. they set it at 72 before bed and wake up and it's 65 degrees in the morning.) They want the heating element to be inspected however we know it is working properly and the main heat/AC for their unit is the mini split, not the thermostat. We don't want to install another furnace as we just spent well over $1000 on the mini split. What should we do?