Hello, I am thinking about purchasing a property in North Carolina for the purpose of Air BNB and would like to know what percentage of vacancy loss should I expect since this will be a short term lease agreement. 

Air BB a room in your house at $100-125 a night when you visit your grand kids rent your whole house .

I never liked propane because when there is a propane leak the gas is odorless, heavier than air and you find out you have a leak when it is too late.

Currently own duplex there and looking to do more ( flip or possibly Air BnB)

I did make bel Air money in real estate without uncle Phil!

I've been researching and I see there are products out there called "mechanical vents" or "air admittance vents" that install close to the drain and act as check valves, allows air to be sucked in but not allowing the drain gases to come out.My question is, are these allowed to be used in Philaldelphia per the plumbing code?

House has central heat and air, dual pane windows. 

We can do that and still leave the machines in place.The only issue is that the direct route for the plumbing would go right through the air handler of the heat pump in the crawl space.

Look at rentals figure out the areas you like and what sort of amenities you want to offer or what will make you stand out (number of bedrooms, bathrooms, air conditioning, parking, etc.).  

One option would be to buy RM-20 unit A now, use it for your Air BnB purposes short term and sell it prior to the Jan 2022 moratorium to another investor so they can take advantage of it for their purposes and let them deal with the hang up of reselling it.