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Community and Q&A

Why the extra flue for this fireplace?

leoj100 | Posted in Energy Efficiency and Durability on

I have a question about direct vent fireplaces venting and sealing behind them.  I have two direct vent fireplaces in my home.  I have one in the basement and one on the main floor.  Both vent out the side of the house.  Picture attached.  I also have a flue out through the roof. Why do I have a flue out the roof if I have direct vent fireplaces? Also I know there isn’t insulation behind the fireplace because when I look through the giant hole for the gas line I can see OSB. Few questions, is there a way to know if I can remove the Vertical flue?  Maybe the initial plans called for a normal fireplace and then they upgraded the fireplaces? What are my options for insulating behind the fireplace?  I can feel air rushing in the hole where the gas line enters.  

Check the attachment for a picture of the back of my house built in 1996.  

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  1. Expert Member
    Peter Engle | | #1

    The flue out through the roof may serve the furnace. Is there a furnace in the basement below your fireplaces? That's a very common design in some parts of the country. If you can verify that there are no combustion appliances using that flue, then yes, you could remove it and seal up the hole. A chimney sweep will be able to help you determine whether the flue is in use and how to best terminate the top of the chimney.

    As far as air sealing and insulating around the fireplaces, you need to be careful. All manufactured fireplaces have limits on clearances to combustible materials, and most insulation materials are considered combustible - certain types of rock wool are the most common exception. But the fireplaces also require some air space around them to allow the units to cool, and insulation is prohibited in contact with most of them. As with any manufactured product, take a look at the installation data for complete information.

    You should be able to seal the air leaks if done properly. For the gas line, canned spray foam would work fine. There are probably air leaks around the flues too, and those are tougher because of required clearances. Some of them will allow rockwool stuffed around the flue. In many cases, it can also be OK to air seal the surface of the rock wool plug using fire-rated caulk, but again, the manufacturer's installation instructions govern this. Talk to a local certified chimney sweep. They will be the best sources of information and techniques.

  2. leoj100 | | #2

    The flue out the roof is definitely not from the furnace. The furnace has a separate flue on the other side of the house. I like the idea of the foam to seal the holes. I will ask a chimney sweep about the flue. I am hoping it is not used so I can close the hole up! Thanks for the quick response.

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