glass on gas stove turning black

maybe you guys can help me out: my brother-in-law has a vermont castings gas (propane) fireplace insert. he's had it for five years without a problem. this year the glass is turning black on him. the outfit that installed it isn't being much help. they claim it's not getting enough air, although the conditions haven't changed any. then they said it's probably the propane. sounds to me like they're not much interested in helping him out. any ideas as to what would be causing this condition? thanks for your help in advance.......tonyg

Typical with propane fireplaces. It may be time to get in there, remove all the fake embers and rocks and do a good thorough vacuuming and cleaning. It's true it is likely not getting enough air probably because of dust or dirt on the burner assembly. Give that a shot, it's the cheapest thing to rule out. Besides they should be cleaned yearly.

Same as above, BUT.................. he may also be getting black siding too if he has a direct vent and it isn't getting enough air.

Would be best to get a pro in there if you can. Probably just need a thorough cleaning. Could have something blocking the air shutter underneath (leaves, dead bat... whatever). Also could be blockage in the orifice or burner port holes. Something could have shifted inside the firebox causing flame impingement. There are lots of reasons why a gas fireplace could soot and LP is more sensitive to things that NG is.

The reason there isn't enough air might be because there is too much gas. Pressure regulators are known to go bad raising the pressure at the jet. More pressure means more gas flow, affecting the air/fuel ratio.

Spiders are another source of problem. They tend to clog up the air intake at the jet.

Also, once you're in the fireplace, verify that the primary air shutter (tube that connects burner to orifice/gas supply) is set correctly to the specifications in the manual. In my opinion, an incorrect air shutter is the #1 cause of black residue inside the DV gas firebox... close second is incorrectly set logs.

I agree with Inside Guy, it's usually the air shutter or logs, but if it worked without issue for 5 years and this is a new problem, and you are sure no one has tampered with the unit, e.g.; got inside the unit and adjusted the primary air shutter, nothings in the flue/intake, the logs are in the same position etc. Then I wouldn't rule out the quality of the LP gas and I say this from experience. Sometimes when a tank has been used for a few years and has never gone "dry" it collects what I've understood is the heavier Butane. So when the tank gets low you end up burning butane gas which burns dirty in most LP appliances.

If your tank isn't low and you don't think its the quality of the gas, then as LLigetfa suggested, it could be your tank regulator has gone bad and is allowing too much gas pressure at the valve.

If you vacuum the unit out and it still soots then I'd call a dealer and have them take a look so they can confirm the pressures and BTU.