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?