Question: I had a house built with a gas fireplace(marco). It worked normally last winter but now the gas shuts off after approx. 1 - 1 1/2 hours of operating. Is it a valve? do I need a new insert? Should I still be using it? Answer: You need a service call. There are a couple of things it could be: You may need to replace or tighten/clean the thermocouple on the unit. You may need to test/replace the overheat sensor (if it has one). The chimney may have a birds nest, etc. in it, causing heat to back up and turn the unit off (overheat sensor again). I doubt that you need a new insert, just a checkup and possible adjustment.