moisture on fireplace window

[Peter Louis]

It is only appearing when fire is on. vent problem?

[Dom D'Agostino]

When you turn on a direct vent fireplace, the glass fogs due to the water vapor which is a byproduct of combustion.

It will burn off shortly, but it can etch the glass over time as it's acidic.

[John Kogel]

People should read up on glass cleaning. Google it. Some window cleaning products, like Windex, will leave a film. The glass must be dry before lighting. The film from normal use gets baked on and ruins the glass over time.
Gas Fireplace ignorance is rampant around here.
People never clean them.

[Eric Barker]

People never clean them.

Kinda the same for some kitchens I see. But that's ok as long as the plasma TV works.

[Bob Knauff]

The temporary "fog" is actually a bit oily and as such requires a special cleaner available at fireplace sales outlets or stores.

Although it clears up as the glass is heated it does leave the milky residue behind that builds up over time. If the glass gets too milky it becomes etched and cannot be cleaned and must be replaced. It is expensive fire proof glass.

The fogging can be a sign of poor air intake problems or a combustion problem. Always write it up in no uncertain terms as needing service.

[Stephen Tilleux]

Yes needs cleaning. That said any electronic ignition or IPI system will fog up when lit until the glass warms up.

Also when the units are new, some fake logs and bricks cure and out gas crud that needs to be removed or it bakes on forever.

Areas that have extra sulfur in the fuel gas will see white hard to clean deposits. The good hearth glass cleaner leaves a fine wax coating behind that helps keep the glass clean and easier to clean later.

Windex, Never use a ammonia based cleaner on wood or gas stoves or fireplaces with high temp non-tempered glass (ceramic glass) as the ammonia will react at temperature and will craze the inside of the glass. Rare but happens.
My2c

[Matt Fellman]

It will burn off shortly, but it can etch the glass over time as it's acidic.

Is it etching the glass or just mineral deposits like in thermal pane windows?