How to Replace a Natural-Gas Furnace

Handyman working on internal furnace
What You'll Need
Duct materials
New furnace

Having a natural-gas furnace replaced or installing a new one can be pricey. However, you can install one of these furnaces yourself, as long as you exercise caution. Dealing with gas can be dangerous even for trained, experienced professionals. But, with the right tools, some wiring knowledge, and the steps below, you can begin safely installing a new natural-gas furnace in your home.

Warning: Working with natural gas can be dangerous. If you're unfamiliar with working with natural gas, call a professional in your area to complete the job.

Step 1 – Assess Your Home

Before beginning, assess your old furnace to ensure it needs to be replaced. Then, choose an appropriate-sized new furnace for your home. In addition, check to make sure the ducts will fit, as the newer furnaces are often smaller than older ones.

Step 2 – Turn off Your Furnace

Shut off the power and gas supply to your furnace with the shut-off valve and breaker. Then, secure your ducts with cleats and screws. Check that your ductwork doesn’t need to be replaced, and then remove your old furnace to make space to install the new one. When removing your old furnace, check to see how it's wired. This will help you wire your new furnace.

Step 3 – Align the Furnace

Next, align the new furnace with the existing ductwork and gas pipes. Install any doors, latches, wires, and switches to the internal furnace.

Step 4 – Connect Your Furnace

Connect the plenum, or the ductwork leaving the old furnace, to the new furnace. Use a clamp to ensure the duct is securely attached to the natural-gas furnace. Then, attach the boot to the new furnace.

Step 5 – Install your Natural-Gas Furnace

Now, you're ready to install you furnace to begin warming your home.

Connecting the Gas Line

Connect the gas line to the furnace and check it for leaks. You may want to use a gas-leak detector for this step. Remove any released gas by running the flue pipes.

Connecting the Thermostat and Power Supply

Next, connect the thermostat and power supply by wiring them. Your new furnace's wiring should look similar to the old wiring, so use the old wiring as a guide. If you are unfamiliar with wiring a thermostat and power supply, you should call a professional to help you complete this step.

Checking for Leaks

Before lighting the furnace, wait at least 5-10 minutes. Leave the room for a little bit, and then come back in to see if you can detect the smell of natural gas. If there is no smell, there shouldn’t be any leaks. You should be safe to light the pilot of the furnace by using the given manufacturer’s instructions and safety precautions.

Turning It On

After you have successfully installed your new natural-gas furnace, turn it on. Remember to follow the manufacturer’s instructions for letting the new furnace go through its initial run. Allow for about 1 hour for the house to get warm.