Place the furnace in a centralized location. This will allow the ducts and pipes to each room to be generally the same length. This homogenization of ductwork in turn ensures that each room receives roughly the same amount of hot or cold air as any other room in the house.
Ensure that there is enough room left around the base of the furnace to allow cold air to return from ducts and pipes around the house. This will facilitate the cold air rejoining the furnace as rapidly as possible. The bottom of a cold air duct is known as the boot and the boots need to be as close to the floor as possible.
Locate the furnace in a spot where there is enough room for the installation of the hot water tank and heating coil. Keep in mind that you will occasionally need to perform maintenance on the furnace, so be sure that you leave yourself enough room to get this maintenance done. Otherwise, you'll need to call in a professional for repairs that you could have done yourself.
Order pipes, ducts, registers and grilles from a supplier if they do not come with your furnace, and most of the time this will be the case. Calculate exactly how many registers and grilles are needed to supply adequate heating and cooling to every room in the house. Estimate how much ductwork will be required, and always give yourself a little extra padding in case your estimates are off or if you damage the ductwork during installation.
Make sure that the capacity of the cold air return pipes matches that of the warm air pipes. Ensure that the horizontal lengths of warm air pipes have an upward pitch of at least one inch per linear foot. This pitch is necessary to bring the warm air up efficiently to the register box from which the warm air blows.