If you have significant DIY experience there is no reason you can’t handle all or most of the job yourself. If gas or electrical lines need to be run, that is one area even experienced do-it-yourselfers often call in the professionals, but you’ll have to make that call yourself.
Connecting a new line to the box can be dangerous at the moment, while incorrectly connecting gas lines can lead to problems down the road. Whether you handle the gas and power yourself, most handy guys and gals can install the basic furnace with relative ease.
First, disconnect the power and gas from the old unit, being sure to shut off the electrical circuit and the gas valve. If your electrical box is equipped with a lock out, insuring that the circuit cannot be turned back on inadvertently while someone is working in the line, this would be a good time to use it.
Next, remove the sheet metal that connects the furnace to the duct work. Having done this, remove the old furnace for recycling. Move the new unit into place and determine whether or not the existing sheet metal, sometimes called a plenum, is going to match. A new plenum may be needed, or the furnace may need to be set on shims or blocks, to connect it to the duct work properly.
From there, you’ll need to run the exhaust line out. Making sure the vent is level is crucial. Slanting upward can allow for rain to enter. Slanting downward may produce inefficient exhausting and a backup of dangerous gases such as carbon monoxide. Once done, reconnect the gas line, making any adjustments needed to the length or size of the fittings. Finally, reconnect power.
The manual you receive with your new gas furnace will provide more detail, but if that overview sounds like it is something you want to tackle yourself, there is no reason you shouldn’t give it a try.