Remove the gravity furnace from the basement by disconnecting it from the chimney and air pipes. If the furnace is too large to remove in one piece, disassemble it into smaller pieces to be taken out one at a time. A fridge cart helps to move the larger sections from the basement.
Remove the air pipes from the inside of the wall systems. This is done by following their path and cutting open the plaster walls with a hand circular saw. Remove the wood laths by prying them off the wall with a pry bar. If any of the furnace's components have asbestos wrapping, contact an environmental company for their removal.
Determine paths for the supply and return ducts to run through the wall system. This typically uses interior walls to house the duct work. The walls you cut to remove the old pipes can most likely be used for either the supply or return duct system.
Install the supply and return ducts and run them to the location in the basement you have chosen for the forced air furnace. Hiring a qualified HVAC company to determine the ducting requirements and install them is wise, because it requires some knowledge to calculate the proper airflow through the building.
Connect the supply and return ducts to the forced air furnace. Vent the furnace's exhaust products out the side of the building to a location that is at least 3 feet from any windows, hot water heater exhaust vents and air conditioner condensers.
Connect the thermostat and power supply to the furnace. Have a gas fitter attach a natural gas line to the furnace.
Contact your local building authority to inspect your installation before using the new furnace.