Have no idea what is going on here. Getting the 4000 ready to go out on first trip, but is still quite chilly up here. To make it comfortable to work in the Cyclone, I have had to run the heater and also was running the fireplace to make sure it was working. In the middle of working I happened to look up from the bedroom and out into the living area and there was a haze. At first I thought it was the lighting or just my eyes (freaking getting old is not the most fun in the world, so don't believe what anyone tells you... is better than the alternative though!) But I asked my daughter (9 years old... good eyes) and she said she thought it was her eyes! There was not any smell or it did not make it difficult to breath. I shut off the fireplace and opened the windows. The heater was still running... slowly the haze cleared out. The heater quit and I verified we were out of propane on both tanks. The next day I ran the electric fireplace and did not have any haze. So by deduction this must have been the heater. I am wondering if it is the heater itself or something odd with the propane. Have a call into a propane professional, but was wondering if anyone else has had this happen and what anyone's take on this might be.