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Building Science

Is a Ventless Fireplace More Efficient Than a Condensing Furnace?

An energy conversion conundrum for building science geeks

Is an unvented gas fireplace more efficient than a condensing furnace?
Image Credit: Image #1: Virginia State Parks via Flickr.com

One of the primary benefits of a ventless gas fireplace is that you don’t lose any heat up the flue. That’s because there isn’t a flue, of course. (The potential problems with indoor air quality, however, outweigh any benefits, so don’t run out and buy one just yet. Or ever.) That ought to make it a winner for heating efficiency in comparison to any vented heating appliance, such as furnace or boiler. Even the highest efficiency condensing furnaces still lose some heat in the exhaust gases that go up the flue.

So does that mean ventless fireplaces are more efficient than condensing furnaces?

Heating efficiency numbers

Let’s take a look at the efficiency values for these two heating appliances. Before we get to them, though, let’s talk about the standard furnace: the 80 AFUE induced draft furnace. AFUE stands for annual fuel utilization efficiency, and it is the percent efficiency averaged over a year. We see a lot of that type of furnace here in the Southeast. When the gas gets burned in one of these furnaces, 80% of the energy in the gas gets transferred to the air moving through the furnace, which then goes into the conditioned space, while 20% of the heat goes up the flue.

A high-efficiency condensing furnace does much better. They start at about 92%, although most condensing furnaces are in the 95%+ range. We did a quick search on the AHRI Directory this morning and found the highest efficiency model at 97.7%.

When you look for the efficiency of unvented gas log fireplaces, you see claims for 99% to 99.9%. (For purposes of this article, let’s just say it’s 99.9%.) That number is so high because, as I said at the beginning, none of the heat is escaping up the flue.

Percentage of what?

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4 Comments

  1. User avater
    Reid Baldwin | | #1

    Factoring in humidifiers
    The analysis might come out differently if you assume that the house needs supplemental humidification. In such houses, the water vapor added by a ventless fireplace is a benefit. What is the efficiency of a condensing furnace attached to a humidifier when you discount the heat that is used to vaporize water rather than heat the space?

    All things considered, I would not advocate for releasing combustion products into the house as a good way to increase humidity. However, it is an efficient way to accomplish that when you only consider fuel use.

  2. User avater
    Stephen Sheehy | | #2

    CO2?
    Don't these ventless contraptions add enough CO2 to the interior air that the occupants lose the mental ability to assess the energy issues discussed in this article?
    Seriously, why even discuss this?

  3. Charlie Sullivan | | #3

    Academic only, I hope
    Stephen, I trust this is only a discussion for the purpose of sharpening our understanding, not promoting actually installing those.

    But for Allison, I have this comment: If the water vapor it produces condenses on your windows, does it count as a condensing appliance? If the envelope is sufficiently tight that it all the water vapor condenses on the windows rather than leaking out, maybe we could get up to 99% HHV efficiency. Then the only question is whether the residents can mop up the puddles on the floor under the windows before they are overcome by the combustion gasses.

  4. User avater GBA Editor
    Martin Holladay | | #4

    A new TV game show
    Charlie,
    We could create a TV game show for GBA readers. Two teams would compete for fuel efficiency in side-by-side homes. Team A would sit in their chairs, reading Fine Homebuilding magazine, kept comfortably warm by a condensing furnace.

    Team B would be in the house with the ventless gas appliance. They would run back and forth from window to window, wielding towels -- rushing to the kitchen sink to wring them out. Look! One of the team members is staggering due to the fumes! He's down!

    Looks like Team A wins again this week. Tune in next week for another exciting episode.

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