GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

Can you increase the efficiency of a gas fireplace with a long exposed flue pipe?

mangler66 | Posted in Mechanicals on

With the death of the Mantis by Empire, and with Nu-Air IFP-45 not yet certified, high efficiency direct vent gas fireplaces seem hard to find.

Add in specifics for you design (linear, 40″+), and it’s basically a lost cause. You are stuck with 65-70% efficiency. Which is a significant penalty for the “ambiance”, when you can’t get a gas furnace that is less than 90% efficient.

My question is, since 30-35% of the heat seems to be going out the flue pipe, could you artificially increase the efficiency by using a long flue pipe and boxing it in with air intake and exhaust vents, to recuperate most of the heat that was going to get lost? I was thinking of having the fireplace on the first floor, and the exhaust on the second floor so to speak. Forced ventilation in the chase would be the last step, but I realize you could go too far and create a “condensing” fireplace with no provisions to deal with the acidic condensate.

Has anyone done this type of install before?

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  1. user-2310254 | | #1

    Mai Tai,

    Why not use an electric fireplace?

  2. GBA Editor
    Martin Holladay | | #2

    Mai Tai,
    Flue design is specific to an appliance. In general, the hotter the flue, the better the appliance performs -- at least for atmospherically vented appliances.

    Cool flues perform worse because they encourage condensation of liquids from the flue gases. Some of these liquid condensates are corrosive. Ideally, the flue will stay as hot as possible, from the appliance to the top of the chimney, to minimize condensation problems.

    I'm not saying you'll have any problems with your jury-rigged invention, but you might. The small amount of heat you intend to harvest isn't worth the trouble. It's worth neither the expense of your fan and enclosure, nor the risk of condensate formation.

  3. mangler66 | | #3

    Most gas fireplaces allow a vertical flue pipe of 24' or more. Some of this cooling would naturally occur there, and that's an approved install. I suspect they would not allow an install that risked major condensation/corrosion, given that most fireplaces have a lifetime warranty on the heat exchanger. It is actually quite difficult to force a gas appliance into condensing mode. Just ask the hundreds of people who have bought a condensing water heater and never get the rated efficiency in radiant heat mode.

    I believe that the initial efficiency is related to the heat of the flame (more to optimal air/fuel ratio), but I don't think what happens past about a foot away would affect that. I'll let you know how it goes :)


    Although electric fireplaces are way easier to install and quite a bit cheaper, they mostly lack in realism until you get into the 1500/2000 dollar range, and once your there you may as well get the real flame and emergency heat source in case of a power outage a direct vent natural gas fireplace can provide.

  4. calum_wilde | | #4

    I doubt you would get more cooling of the flue gasses with conditioned air from the building than you would with winter outside air. Even with mechanical ventilation to move the conditioned air I suspect that wouldn't come anywhere near the air movement of a mild wind outside. If anything, I would guess that a long internal flue pipe would increase the temperature of the gasses leaving at the top.

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |