I have a gas fireplace (natural gas) that shuts itself off, the pilot too after it operates for about an hour. When I remove the logs it operates longer, for about two hours before it shuts down. Any suggestions?
Typically this problem is caused by either a bad pilot generator or thermocouple or a weak pilot flame. When you light the pilot, the flame from the pilot should surround the thermocouple which is the copper or silver rod. If it is surrounded by a strong pilot flame and the pilot goes out, the thermocouple or pilot generator is bad. If the flame does not surround the thermocouple, the problem is the pilot flame itself. A weak pilot is usually caused by a dirty orifice on the pilot assembly. I usually disassemble the pilot, remove and clean orifice. Hope this helps. Thanks
Jim's answer is the most likely cause, but there is one other possibility that sounds like it might apply since the fireplace runs longer when you remove the logs. Thermocouples need what's called a cold junction in order to operate properly. This means that only the top ~1/3 of the thermocouple should be in the flame. If the bottom half of the thermocouple is heated up it will not function properly. Since the operation improves when you remove the "logs". The logs may be directing the flame towards the bottom of the thermocouple.
Refer to the two other answers given and pay careful attention to the descriptions of how the thermocouple's top 1/3 should be surrounded by the pilot flame if not the pilot assembly must be removed and the orifice cleaned.The only thing we haven't covered is overall gas pressure. Is it very cold in your area of the country? Are the other gas appliances in your house operating normally?