That's because the combustion byproducts are in a liquid form, so they need to be drained out of your home in a similar way the condensation from an air conditioner is drained outside.At least, that's my understanding of it.

So one room can heat and another can cool at the same time even if they are on the same condenser unit. 

There is condensation in the window of the bedroom and the stick that keeps the window from opening has mold growing on it. 

And you have to back the truck right up to the front door.. if you leave your truck exposed to the street you will get stuff stolen.. just like condenser units etc.. a vacant house in many of these areas will get stripped pretty quickly if just left alone. 

Typically, you find the condenser on the ground connected to an air handler in the attic above the third floor which supply's heat or cooling to the unit.Your contractor should know what to look for but we all know what happens sometimes when you just trust your contractor.

For AC systems I have a wonderful local AC company in my town do two services a year... one in spring and one in fall... they make sure the filters are replaced, blow out the condensation drains, clean the coils and make sure all is well.

All new fixtures. 5000 Replace improper basement ductwork 3225 A/C coil and condenser 3200 Stairway into basement - bring up to code 2250 Washer/Gas Dryer (stacked) 1800 Paint throughout 1500 Transfer taxes 1200 New carpet - stairs and both bedrooms 1000 Gas water heater 800 new fridge 800 Exterior paint 800 Demo/creation of fridge location 725 Replace wood rot along roof line and cap with metal 700 Roof coating 700 new gas stove 600 Trim repair, doors for both closets, door for 2nd bedroom 500 Window blinds throughout 300 Minor siding repair - rear 200 New screens 150 Total costs 34200 Offer ALL CASH 21799

Making sure vents are Not closed near the air handler which can lead to ice forming on condenser and unit running continuously.Most of my units are in Condos so I look for the price difference between Seer models, and air handler\outside A\C quietness also. 

I hope the insulation doesn't have a paper backing (vapor barrier) on one side otherwise this will cause a condensation problem. 

But when your tenants move as often as these folks do and walk away from their security deposits your left with turn over after turn over and very high maintenance costs along with getting your condensers stolen or your house stripped when its vacant a week or so.