I recently added an expansion to my home. I got a used Enviro gas EG33 insert fireplace to go in the bedroom. I had it framed according to the instructions, venting comes out the back of the unit 45's and about 20 inches to the termination cap, about 3/4" rise from unit to term cap. My problem is this, I light the unit and it will burn for about 10-15 seconds and just shuts down, pilot goes out and all. Now if I lift the burps plate(its what I call it, its a plate across the top so if you get an large combustion it opens a touch to release pressure) the unit will remain lit and burn just fine, which lead me to believe it was a venting issue. I pulled all the venting out and double checked, it looks great.. At a major loss of what to try next. Helppp

P.S. also check the thermopile, coupler, and valve. All are good.