Fogged Glass/Gas Fireplace

New construction

73 degrees in house with air conditioner running.

Factory built fireplace vented to exterior through side wall.
Gas operated by a wall switch.

Glass fogged over when testing. Is this normal?


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It should clear after a few minutes of operation.

There is a lot of water vapor when you burn gas.


Yes, but that fog also has a small amout of sulfuric acid in it, so after many years the glass can be etched so badly that you have to replace the glass if you want to see the fire in the fireplace.

Flue gas emissions from ‘fossil’ fuel combustion

Saw the same thing yesterday on an inspection. It cleared in a minute. You can see the fog lift from the bottom when it heats up.

Yes, But I have seen many of these that do not clear up after several minutes even as long as 15-20. I also have noted and spoke to a fireplace technician that some of the double pane glass gas fireplaces have seals(?) between the panes and that if the seal is bad, you can get fogging and condensation between the two panes. Thought is made sense since I have seen what appears to be air movement and haze fogging around the corner after running the fireplace for a long time. Evidently the warmer air hitting the colder air of the outer pane.:frowning: Ever heard of this? Was not sure!

So, My question is what is the problem if the fireplace continues to show fog that does not clear. And is it possible to be in between the two panes of glass?

No problem really, as it does not affect the fireplace operation. It’s more of a cosmetic issue. I point it out to customers, and let them know the glass can be replaced if they want.