How to Replace a the Exhaust Stack of a Gas Furnace

gas furnace
What You'll Need
Replacement Exhaust Stack
Utility knife

If you have an old furnace that doesn’t work very well, then you may need to consider replacing the exhaust stack. Also known as the gas flue, the exhaust stack is the pipework on the furnace that removes toxic gases from the furnace into the outside air.

Replacing this kind of flue is rather complicated. It may not be suitable for an amateur to do, as doing it incorrectly can lead to exhaust fumes building up in the home. If you feel like you are up to the task, then follow these steps to successfully replace the exhaust stack of your furnace.

Step 1 - Find a Replacement Flue

You may need to find a replacement flue that has the exact same model number as the one currently in your furnace. Your owner's manual should be able to tell you the number of each of the parts used in the making of the gas furnace, so you will be able to find the right model by reading it. You could also contact the manufacturer to ask for a replacement exhaust stack for your gas furnace.

Step 2 - Preparing the Flue

Once you have your replacement part, turn off the electricity and gas supply to your furnace. Leave it to cool for a few hours to allow it to become cool enough to touch. You should then remove the exhaust pipes from the back of the furnace. Doing so could involve removing a layer of caulk from the edge. Use a utility knife to cut through the caulk. Pull off the exhaust pipe. You should be able to see the flue.

Step 3 - Remove the Flue

Reach inside and unscrew the flue from its position. If you cannot get to the flue from this side, you will need to access it from inside the furnace, through the access hatch. However you do it, unscrew the exhaust stack from the furnace, and place it in the trash. You should then clean around the flue space with a damp cloth.

Step 4 - Replace the Flue

You can now install the replacement exhaust stack for your gas furnace. Make sure that it is facing the correct direction. Use your screwdriver to secure it into position. The screw holes should match up on each. When you have done this, you should then reattach the old exhaust pipes to the exhaust stack. Screw them into place using the wrench. They should be firmly joined together. You should then caulk around the connection between the flue and the pipes, and leave it to dry completely before starting up the furnace again.