No windows that open (only frosted blocks for light) and the only heat source is from the ceiling light/vent/heat combo.

Then call again until they put some heat on the property.

I don't think anyone can be 100%, especially in this heat.

When the owner lived there the PG&E I believe was somewhere between $100 to $180 a month for one person (who ran the heat constantly when it was cold). 

Maybe do a sewer cam inspection if it hasn't been done, and ask renters what problems they have (ie Hot water lasts 5 minutes, no heat in Unit B bedroom, etc...)Job A Problems$$$/How long to fix: inward movement on the foundation with masonry (tree roots?

They even turned off the heat!!

I'm about to close the contract (3 weeks to closing date), but all of sudden my lender refused to process the loan (underwriter refused to proceed) after they found there was a heating oil leakage found (still looking for mitigation but cleaned and controlled until now).

One of the units found out have bed bugs, called an exterminator and their doing a Heat treatment!

This is the simplest method, but it also means your tenants are more likely to abuse the utilities by leaving windows open with the heat or A/C running, leaving lights on, ignoring the toilet that constantly flushes on its own, etc.3.

Due to the recent summer heat, our HOA is trying to get a couple units together to bulk buy and upgrade the individual AC units for each condo unit owners.