Some things to consider when looking at the high efficiency furnaces:

1. The exhaust piping must have a accessible line to the exterior from the furnace. 
2. All horizontal runs of exhaust pipe must slope back toward unit for proper drainage.
3. Many “slab” homes in the area have the furnace installed in a closet on the ground level, making the installation of the vent piping very difficult – sometimes impossible without running through living space. 
4. It is recommended that the exhaust outlet not be located within 6 feet (1.8m) of a condensing unit because the condensate can damage the painted coating.
5. Clearance to an electric or gas meter, or other similar items - 4 feet (required).
6. Clearance to window or door that may be opened - 12 inches.
7. Exhaust pipe must be at least 12 inches above grade - we prefer 18" or more to prevent blockages due to snow.
8. Terminations cannot point into recessed areas such as window wells, stairwells or alcoves.
9. Terminations cannot be directly above a walkway.
10. If your existing furnace shares a chimeny with your water heater, you will likely need a chimney liner to redcue the size of the chimney to handle only the one appliance.

These furnaces can be set up to be either Direct Vent or Non-Direct Vent.  In Direct Vent configurations it is required to run both exhaust and intake piping to the outside.  In Non-Direct Vent installations, combustion air is taken from indoors and flue gases are discharged outdoors (this would remove the need to run 2 pipes), but:

WARNING
Insufficient combustion air can cause headaches, nausea, dizziness or asphyxiation. It will also cause excess water in the heat exchanger resulting in rusting and premature heat exchanger failure. Excessive exposure to contaminated combustion air will result in safety and performance related problems. Avoid exposure to the following substances in the combustion air supply:

Permanent wave solutions, Chlorinated waxes and cleaners, Chlorine base swimming pool
chemicals, Water softening chemicals, De−icing salts or chemicals, Carbon tetrachloride,
Halogen type refrigerants, Cleaning solvents (such as perchloroethylene), Printing inks, paint
removers, varnishes, etc., Hydrochloric acid, Cements and glues, Antistatic fabric softeners for
clothes dryers, Masonry acid washing materials

In the past, there was no problem in bringing in sufficient outdoor air for combustion. Infiltration
provided all the air that was needed. In today’s homes, tight construction practices make it
necessary to bring in air from outside for combustion. Take into account that exhaust fans,
appliance vents, chimneys, and fireplaces force additional air that could be used for combustion
out of the house. Unless outside air is brought into the house for combustion, negative pressure
(outside pressure is greater than inside pressure) will build to the point that a downdraft can occur
in the furnace vent pipe or chimney. As a result, combustion gases enter the living space creating
a potentially dangerous situation.