DIY Network

How to Replace a Flue

A backdraft in this homeowner's flue is sending deadly gases into the home. Learn how to remove and replace a faulty flue and secure the top to the roof.

More in Remodeling

replacing a faulty flue
  • Time

    Two Days

  • Price Range

    $100 - $250

  • Difficulty

    Moderate to Hard


Step 1: Expose the Pipes

Unplug the furnace and turn the water heater down to the "vacation" setting to contain the CO gas.

To remove the flues, unscrew every connection at each of the joints and dismantle (Image 1).

Cut through the drywall with a drywall or electric saw, adjusting the blade to ensure it cuts only through the depth of the drywall; this will avoid the risk of cutting through any electrical wiring or the flue itself. Be sure to wear protective gear (Image 2).

Step 2: Remove the Old Flue and Pipes

As soon as the dysfunctional pipes are exposed, they can be removed. A flat bar is used to help dismantle the connected pieces (Image 1).

Remove the flue and any additional frame boards that will obstruct the necessary clearance needed to avoid a fire hazard. A minimum clearance of 1" is required around the flue (Image 2).

Step 3: Place the New Flue

With the old flue out of the way, the new flue, which has been cut to size, is put in place. The first piece slides in place, going up through the roof (Image 1).

Note: Make sure the directional arrow is pointing up.

As one person holds the first section up, slide the second section of the flue down into the floor and connect the two pieces together, again making sure the directional arrows are pointing up.

To keep the flue securely in place and ensure clearance from the wall, install
tightened metal straps. Cinch the screws in at an angle to pull the strap taut to remove any slack (Image 2).

Step 4: Seal the Flue to the Roof

Once the flue is in place, head to the roof. Apply a layer of roofing mastic around the flue piece to seal the joint between the new flue and the shingles (Image 1).

Add a storm collar, again sealing with roofing mastic (Image 2). Also, to keep moisture out, a rain cap is added, completing the exterior part of the fix.