Why would you want to hold and supplement a tenants rent.

You will likely end up supplementing your tenants rent.You are however in CA where appreciation is prioritised over cash flow.

Now pardon me while I go take my vitamin D supplements and antidepressants and wait a few more months to see the sun again...

Most deal analyzers (not talking about one particular person) are super basic and are sold to supplement the income of the person selling them.To be honest, most deal analyzers will not have material around IO payments, bridge financing and other slightly more complex issues.

Look around and you will see the average joe is NOT getting six pack;meanwhile six pack book writers, gyms ,pod-casters,personal trainers, supplement sellers, workout equipment sellers,nutritionist,plastic surgeons etc are getting rich. 

I am new to real estate investing & am looking to generate an additional stream of income to supplement my full-time career in Chicago.

The biggest risk I would like to safeguard against (as much as possible anyway) is tenant eviction etc. as I have a full time job and real estate is just a hobby/supplemental pursuit. 

And if you're in the middle of a lease term, you won't be able to increase rent to account for your new heating bill.Cheapest way to get around this, IMHO, is to leave the existing heating gear in the hallway, and whichever heating system it's connected to, pay that tenant's heating bill(and adjust the rent upwards accordingly as heat will be included.)Installing an electrical resistance heater in the hallway to save money would be like running on a treadmill and doubling the speed because you're not getting anywhere.One other possibility- if the building is very well constructed/insulated, the heat from the apartments might be enough to keep it sufficiently warm in the hallway without supplemental heat(doubtful, but you never know.)

From our perspective Airbnb is a way to supplement some income in an unused portion of our primary residence where we have a walkout basement with 4th bedroom and full bath.

The prevailing attitude in your area is to rent with negative cash flow, having to supplement your tenants rent, and hold on till you die due to the unusually high appreciation rates.