If they're relatively small, open concept and 1 story then you could do a vented gas rinnai monitor heater.Or if it's more cut up and you've got high ceilings then you could run a forced air furnace with the duct work at the top of the ceiling w/ a drop ceiling below to hide the duct work.

However, for rentals, you do need to have someone monitor and add water to the system.   

Monitor the topic for updates.  

If so, that is exponentially more challenging than staying local and will require a great deal of dedicated time to mitigate the high risk: researching sub-markets and neighborhoods in your target area, visiting the area (probably more than once) before buying, interviewing PMs, assembling team in your target market, connecting with local investors (joining the local REAI is a good start), and building processes to monitor everything effectively from a long distance (which might require instructing your PM to customize the information they provide to you).You are really going to need a very trustworthy associate in your target market to make sure you don't get screwed over, especially on the rehab.

Great 👍 Hopefully Karla is still monitoring this post 2 years later …?

While it sounds amazing to have 20 doors rented out, you also have 20 different families to contend with, potentially several property managers, the tax returns and monitoring the cash flow and expenses, plus overhead of physically owning the real estate (mortgage, repairs etc).I wanted to have time and geographical freedom, which is something I feel most investors want - but it often gets covered up with "financial freedom".

It's not completely passive as you have some work to do in finding operators, learning about asset classes, etc - but once you make the investment you have nothing to do but monitor the ACH's into your account (Hopefully) and read reports.I have done both active and passive investing and had great results with both - you just need to decide which route you want to go and then commit to it.Good luck!

I acquired a property in Illinois that had higher levels of radon and it was as simple as putting down sensors to monitor. 

I am going to continue to monitor it as I think it will make a great case study of how greedy people were getting

I don't want to rely on guests to monitor the levels.