We have a 10 year old gas fireplace that has been operating flawlessly. There is no blower/fan unit. After annual cleaning, I've always replaced the rockwool and situated the logs as before using a before-photo.

Last fall, I didn't have any new rockwool, so omitted that. And I forgot to reapply the 2 bottom spring loaded clamps of the glass panel.

Over the winter, it hasn't seemed to put out as much heat. Finally got a hold of some more rock wool. Took off the front screen and discovered the unapplied bottom clamps. This time, I placed the rock wool over the only the front row of orifices, and moved the front log to on top of back log, leaving the rockwool exposed to view. And have left off the front screen.

Sure looks gorgeous with all the rockwool embers glowing. It is definitely putting more heat into the room, and might be more heat than it ever has. But it could be due to any of the three: 1) no screen mesh, 2) properly clamped glass front, 3) rearrangment of logs.

What do you think, and for maximum heat, is there an optimal strategy for placing rockwool and/or logs? Does rockwool aid heating?

  • 1
    4) Perception bias? I am out of my depth on thermal efficiency of gas fireplaces (wood burner myself), so honestly asking if this could be "felt colder because you knew you forgot" and "feels hotter because it is back to nominal" situation.
    – bishop
    Feb 19, 2020 at 13:01
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    Yep, that too could explain the perception post-fix. But over the winter we also felt it wasn't putting as much heat into the room, and were not aware of the semisealed glass panel which presumably was sucking floor level air into the fire chamber, creating room draft and cooling the glass. So the received reduced performance probably wasn't a psychological artifact.
    – Andy Fitz
    Feb 19, 2020 at 13:16
  • Is this a vented fireplace?
    – JPhi1618
    Feb 19, 2020 at 15:40
  • It vents to the outdoors at about the height of the mantle, which is about 4 feet above grade.
    – Andy Fitz
    Feb 19, 2020 at 15:42

2 Answers 2


The "optimal" placement of the logs is that shown in the manual for the unit.

Rearranging the logs doesn't change the amount of gas being let through the gas valve, which is the only variable that will change how much heat the fireplace produces.

It can however cause the unit to burn dirtier and create soot, if you place the logs in a way that causes impingement (flame touches them, essentially).

Chances are it feels warmer now because you've moved the front log which is allowing more "radiant" heat to come straight into the room, but the unit would not be creating any more heat than normal.

  • I understand the idea that gas consumption determines the amount of heat generated in the chamber. But I also think the Mfr log configuration is primarily determined by aesthetics when the fireplace is off. When fireplace is off, our now exposed swath of rockwool is pretty ugly (to us), and wouldn't have sales appeal in showroom. It is not clear to me that gas consumption also sets amount heat put into the room. My thinking is that one wants the fire to heat up objects in chamber, that then radiate into the room. For this, the amount of outward glowing material would seem to be a measure.
    – Andy Fitz
    Feb 19, 2020 at 14:23
  • It just occurred to me various log/rpockwool arrangements can be tested by measuring the temp of the exhaust air. Reasonable log arrangements that minimize the exhaust temp would maximize heat-into-the-room, since there is no other place for the heat to go. I suppose.
    – Andy Fitz
    Feb 19, 2020 at 14:30
  • Rock wool has absolutely zero effect on heat output or deflection. It's purely an a aesthetic thing. As someone who works on the things for a living I can pretty much guarantee that you won't see any difference no matter what arrangement you put the logs in. You will see an increase in soot within the appliance if you put them in wrong though.
    – Perdog
    Feb 19, 2020 at 15:10
  • Its hard to argue with a tradesman that specializes in this. Where should I pay attention regarding soot build up?
    – Andy Fitz
    Feb 19, 2020 at 15:41
  • If you do end up rearranging the logs, keep an eye on any spots of the log where the tip of the flame touches it, it'll build up there, typical black soot. You'll also notice it on the "ceiling" of the fireplace and along the top of the glass but that's more of a white/gray color.
    – Perdog
    Feb 19, 2020 at 15:53

Manufacturers are very specific on log placement to avoid soot buildup. If you don't have a manual it's best to search manufacturer and model# on-line for correct placement. I've learned that soot buildup also depends on flame height (blue vs yellow). You want a blue flame with just a tip of yellow for maximum efficiency. Also it's best to keep that glass enclosure in place for all sorts of reasons.

  • Glass panel will stay in place, sealing against the rest of the unit. The screen, which is a separate part altogether, might not. Besides cleanliness in the chamber, what issues are there with soot build up ? I look at the flame color next time it's running.
    – Andy Fitz
    Feb 19, 2020 at 18:35
  • OK, from a cold start, there are a lot of flames 95% blue, After 30 minutes running, about 80% of the flames are gone. What are left are blue with some other color at the tips. In the center back row of orifices (no rock wool), there is a 5" flame rising between two logs that comes about 3" from touching the height of the arc of the newly moved 'front' log, the bottom of which is glowing red orange. That flame is 70% blue with an yellow orange streak in the middle about 4 inches long. There is about 80 square inches of visibly glowing rockwool and log,.
    – Andy Fitz
    Feb 19, 2020 at 19:20
  • I'm not sure there is a single measure of efficiency. There is efficiency in the sense of % of gas combusted. And efficiency in the sense of % of energy transferred into the room, rather than out the vent. And I think a professional in any field has no choice but to install and setup according to MFR specs. And MFR design of log arrangement is weighted heavily towards sales appeal.
    – Andy Fitz
    Feb 19, 2020 at 19:24
  • I was referring to burn efficiency (low soot) not energy transferred into the room. Again I'm not an expert in this field but have plenty of experience and my understanding is that the cleaner the flame burns the greater the heat gain from more efficient combustion. That doesn't mean that that energy gets transferred into the room. Again - maximizing flame efficiency is one, but only one, important aspect of a properly functioning gas stove.
    – HoneyDo
    Feb 19, 2020 at 21:32

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