Currently have a floor to ceiling brick wall with a wood burning stove that connects to a round flue tile about 32" from the floor and then has a standard cinder block with tile liner up to the roof. Want to get rid of the stove and install a direct vent freestanding or wall mounted gas fireplace. Can I use the existing flue and install the coaxial type vent pipes required by the direct vent fireplace? Don't want to use a gas stove and can't use an insert. I want the finished product to look like a real fireplace.

Spoke with a fireplace pro over the weekend who told me for direct vent units the flue opening should be about 36 inches high to allow about 22" of vertical rise before entering the first right angle into the flue. Will use flexible coaxial pipe for venting all the way to the roof. Prefer this method rather than breaking through the outside brick chimney.

2 Answers 2


Hard to say without seeing it, but typically you can use the existing flue provided you add the proper direct vent chimney insert for your particular gas fireplace. Alternatively, you can possibly knock a hole in the back of your chimney and just go out the side if you'd prefer (more labor, but less flue inserts you'd need to purchase).


We actually did this recently. We ended up taking a direct vent unit and have the exhaust going through the existing flue (about 20'). For intake air, we're letting it take it from inside the house (since this isn't a primary heating unit, the loss in efficiency isn't a big concern for us). We've ran it a few times and seems to be working just fine.

  • I would generally agree with the shortest route from insert to the vent. Direct venting in your unit can be sensitive to the differential air pressure between the vent exit and the location of the heater unit. (See next comment).
    – Michael Karas
    Nov 13, 2013 at 13:09
  • 1
    I once installed a direct vent heater unit in a basement and had to arrange for the vent to rise up about 5 to 6 feet above the heater level so that the vent exit on the outside was above grade. This caused great difficulty in getting the heater to work properly till I purchased a whole different pressure safety switch assembly. The switch in the unit had been specified for an application where the vent went straight out the back of the unit and was not adjustable enough to deal with the differential pressure, during heater operation, caused by the elevation to the vent exit.
    – Michael Karas
    Nov 13, 2013 at 13:10

Get a copy of the installation guide for the prospective direct-vent fireplace, and read it carefully to verify that what you want to do is within specifications. Some may not handle that long a flue, especially if designed as "condensing" units, which strip nearly all the heat out of the exhaust gases. Without that heat, the blower inside the unit has to blow the exhaust all the way out the pipe without assistance from thermal buoyancy.

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