How inefficient are gas fireplaces?

I've measured the temps and I got 380F at the base of the unit and the outside flue was registering the temp of 380F. That is a lot of heat just flowing outdoors and heating our atmosphere and a lot of gas being burned. Inside, the fireplace puts out some heat to the room but just enough to heat the immediate area. It maybe bumped the room temp by about 6 degrees.

The efficiency is mixed. First, I am unaware of any condensing gas-fired fireplace so the best would be 86% but most sealed combustion units come to about 80%. But fireplaces are generally not the main source of heat for a house or even a room but rather, an incidental heat source to supplement the occasional use of a room. As such the efficiency is of little significance in any regard but comfort. In the shoulder months and to supplement on a cold morning they can't be beat. We also specify many sealed combustion gas fireplaces in cabins or remote homes as they do not require electricity to function.

Then of course there is the undeniable luxury of having an intense local heat source when you get the chill.

I'd be very surprised if ANY gas fireplaces did better than ~80% for raw combustion efficiency. To hit the mid-80s requires well controlled excess combustion air volumes & mixing which is nearly impossible to achieve without forced draft. By contrast hitting 80% with an atmospheric draft is pretty easy. If vented into a masonry chimney it can be no better than 83% without running into flue condensation issues in most climates.

A stack temp is 380F with the incoming combustion air at 30-70F (net-stack delta of 310-350F) would also indicate something between 78-81% combustion efficiency, dependent upon the amount of the excess combustion air:

http://www.nwfpa.org/nwfpa.info/images/stories/articles/improve%20your%20boilers%20combustion%20efficiency.jpg

If the thing is built into an uninsulated (or barely-insulated) exterior wall chase, the as-used "system efficiency" will be much lower than the raw combustion efficiency.

Exterior wall chimney? Only in hillbilly land...like where I grew up. Still surviving up there without the benefit of advanced degrees or condensing boilers but still there...hehhehe Fireplace efficiency is improving and we should not confuse the folks with AFUE vs steady state combustion efficiency, which is why we take as much care in sizing a fireplace as we do with a full-blown radiant floor design. http://www.heatnglo.com/Products/Escape-Gas-Insert-I30FB-I35FB.aspx?page=Specifications

I see these framed-in and vinyl or fiber-cement sided "chimney: chases all over the McMansion (and lesser) new housing developments in New England, going for a "Colonial" (read "inefficient") look or something. eg:

http://www.reporthost.com/AtHomeMonte/Sample2/2C37ACA4-B127-4315-8C7D-CC2BCFBCAEB8.jpg

They are typically utter crap- rarely air-sealed well from the conditioned space, and often uninsulated (at best insulated with leaking R13 batts), blowing more than half the heat from the fireplace/insert out the back.

Their ubiquitous masonry predecessors come in many vintages too (including circa 2012), yet when I recommend insulating them I usually get push-back on aesthetic grounds, with some noise about how it isn't really necessary since the fireplace is warming up the room. As if 50-100 square feet of ~R2 brick is LESS lossy when you raise it's average temp by 50F with a gas or wood burner... Yes the cold hearth syndrome is gone, but it blows 3x the amount of fuel to heat the room than it might otherwise.

Now that you mention it. But you will be happy to know that had my foam guy shoot the old brick chimney so I have a hot roof and chimney. The fireplace is like the vaulted ceiling; an absolutely worthless essential. They are just being human so we try to accommodate their personal style while pushing making clear the challenges we face in efficiency and comfort.

Posted By BadgerBoilerMN on 15 Jan 2013 05:35 PM
Now that you mention it. But you will be happy to know that had my foam guy shoot the old brick chimney so I have a hot roof and chimney. The fireplace is like the vaulted ceiling; an absolutely worthless essential. They are just being human so we try to accommodate their personal style while pushing making clear the challenges we face in efficiency and comfort.

You have to be careful with that on retrofits, since code requires 12" of masonry between flue liners or fire-boxes (usually the thin-spot) and combustibles such as when the combustible material (in your case foam) is in contact with the masonry. 

Rock wool batt insulation is nowhere near as air tight as foam, but the rigid high density rock wool panels are measurably better, and can make code compliance a bit easier.  You can put foam on the outside of the rock wool if the thickness of the fiber gets you to a magic 12".  There's not a whole lot of science (and some real contradictions) to the code surrounding this, but one persnickety inspector can ruin your whole project.  (Rock wool + steel studs + fiber cement, anyone?)

http://publicecodes.cyberregs.com/i...par020.htm

http://publicecodes.cyberregs.com/i...par061.htm

Mine is a well constructed and lined chimney foamed in the attic between ceiling and roof line. But if I need any help I will be sure to call the local inspector. I am always so impressed with their knowledge of the code counterbalanced by the ignorance of the most basic physics and aversion to exercise their statutory prerogative for anything but obstruction of progress. But your words are to the wise.