Gently let them know that if they continue to use the HVAC in this manner and it causes the unit to fail, they will be responsible for replacing it due to negligence. 

It is fine to take a gentle approach when talking to them, but those provisions are in your lease for a reason.

Bringing the $750 Tenant to the Real WorldAs for your tenant enjoying their $750-a-month sweet deal, it's time for a gentle reality check.

And they are being very kind and gentle with their thoughts etc..

I found myself missing out on more fun occasions than I'd like to admit because I needed to be at my properties handling some issue or showing it when vacant.I found that I had become conditioned by the sound of my phone (even though I helped alleviate it by having quiet hours on my phone from 7:30pm - 08:00am). 95% of the time when a tenant contacted me, it meant something was broken, there was an argument, or there was an excuse about late rent.My husband has been gently pushing me to get a PM for a while.

Super rookie here so be gentle...I am exploring the option for a possible rent-to-own situation.

Or she could be doing something totally legitimate like massage (minus the happy ending).

i know someone who does Gap lender for fix and flip and other deal, massage me if you want me to put you in contact with them

If I am being greedy, you can tell me, but be gentle, I only own two properties lol

I think @Jeff S. nailed it.If you were determined to buy this I'd give some thought to a small office cluster for therapists/ practioners like counselors, massage therapists etc.