Determine the type of furnace:
There are various types of furnaces like electric furnace, natural gas furnace etc, so you need to first decide what type of furnace you want to install. The type of furnace is determined depending upon what kind of fuel is available to you.
Familiarize yourself with the furnace
Once you have decided and bought the furnace, take some time to understand how it works. Most of the furnaces these days work on the same principle - drawing room air into the ductwork through small holes (air returns) before warming it and throwing it back out.
Power down and remove your old furnace (optional)
Once you have identified the functioning of your equipment, get ready to install it. Disconnect the power and gas from your old furnace (you can easily locate the shut off valve near the furnace), and carefully remove it to make space to install the new one. Also check the ductwork of your furnace for any wear and tear; if you find any, replace it as well.
Align the new furnace
Now align your new gas/electric furnace with the ductwork and gas pipes (electric wires for the latter). Also install wires and latches to the internal furnace. Connect the ductwork to the new furnace using a clamp.
Connect the gas line
After you have properly attached the plenum and boot to the new furnace, connect the gas pipe and check the furnace for leaks (use a gas leak detector to be sure there is no leakage). Now connect the power supply and thermostat to the furnace and wait for several minutes (5-10) before lighting it.
Light the furnace
Go out of the room for a while and then come back to detect gas leakage. Now light the pilot of your furnace following the manufacturer’s instructions.