The heating and cooling system is often the feature with which most people are dissatisfied after they’ve lived in the home for a while. The important features of an HVAC system are:
Design and type of duct work
If the ducts aren’t sized and balanced properly, the home will never be comfortable. Externally insulated round ducts are the most efficient; long runs of flexible duct are the worst.
Air filters should be located where they are easy to reach since the filters should be replaced about every three to four months.
Any air entering a room must also leave it when the doors are closed, or no conditioned air will enter the room. The ideal condition here is for each room to have an entering and a return-air diffuser.
As of January 2006, the minimum allowable efficiency for new air conditioners was 13-SEER. Think of it as miles per gallon: the higher the SEER, the lower the utility bill. Air conditioners rated close to 20-SEER can cost half as much to run as those with the current minimum limits.
The condenser (outside) coil type
Coils that are made of a single metal (such as aluminum coil and aluminum fins or a copper coil and copper fins) last longer and hold their efficiency better.
Attic-mounted furnaces will eventually leak and cause ceiling damage, unless there are special design precautions. Furnaces mounted in closets at floor level are less prone to cause leaks and water damage, and are much easier to service. The worst places to locate outdoor units are where they can be seen and heard, where they can be easily damaged; or under the edge of an un-guttered roof.