Unless you are going with chimney hookups that would probably cost more for the chimney/liner etc.Furthermore, If the furnaces all 'reside' in the same mechanical room you would be advised to include intake combustion venting to avoid O2 depletion.

You still have to factor in leaky roofs, broken hvac, crappy tenants, and the occasional "spontaneous combustion".

Trying to get the pipe dry enough to re sweat in a combustible wall is daunting.One snap of the crimp tool makes pex bomb proof. 

The C$ was at parity (or stronger) with the US$, real estate in Phoenix were discounted up to 70% off, every agent who was still working wanted to help me, and there was absolutely no competition from anyoneI applied that same lesson that I learned when buying Berkshire Hathaway stock during the dot com bust - buy when everybody wants to sell. 2009 was the third crisis in which I bought assets.

Debris near electrical panel, furnace, or other combustible areas7.

Very happy with the Geothermal system as it will last longer, be cheaper to operate, safer because there is no onsite combustion of fossil fuels, eliminated the use of propane, and reduces our carbon footprint.

Your basic smoke/CO detectors will not detect a NG leak you will need the explosive gas detector units 'First Alert' has one.The one I useUEi Test Instruments CD100A Combustible Gas Leak Detector.

Debris near electrical panel, furnace, or other combustible areas7.

Not nonmetallic/PVC, not MC, not rigid conduit/EMT, nothing.So there are some potential NEC code issues but let's take a look from a practical standpoint: with boiler vent stacks/flues running through there and with it being a fire enclosure, it just seems like one probably shouldn't put any combustible materials in there including wire and PVC. 

Also, there wouldn't be a combustible gas running through the building that the tenants are responsible for.