I am looking for insight and expertise with the HVAC system, because I am newer to real estate investing.There is currently one natural gas furnace in the basement that pushes air to all three units and floors.  

Make sure the house is clean and spotless and I even put air fresheners in so they smell good.

The home does not have separate meters for gas/water/electric and all the ducting is connected to the same furnace and A/C... the thermostat for the air conditioning and heater is in our unit. 

Or should I say is there a cleaner way to get you guys and gals the most accurate repair cost? 

Forced air heat. 

It is a lot cleaner and could help your account when it comes to filing taxes. 

I've dealt with bad tenants and having these tenants is a breath of fresh air.

People have to remember to clean them and put water in them.I’d be concerned that warm moist air is being blown inside the unit.

We're trying to decide on what material to use to replace it.There's not great air flow to inside which is why we're considering moving away from carpet and something that may hold less moisture, easier to clean, look more modern etc.

It looks like the listing mentioned forced air.