odd fireplace pilot problem
One of my customers has intermitant gas shut downs on a direct vent Valor gas insert fireplace after it's been on for a while. He's replaced the thermocouple a few times and the pilot flame is strong. This unit is very simple with an igniter, thermocouple, gas valve and no limit controls. He's about to try a gas valve change (expensive) but when he asked me to look at it while I was servicing his furnace, I noticed the very blue main flames became softer and more more yellow the longer it burned. This direct vent system is one vent pipe within another but these both terminate at the same length at the immediate outside surface of the house. Any direct vent terminations I've seen have had staggered lengths or some other system for preventing the recirculating of combustion fumes, so I am wondering if it's the real pilot problem?
Anyone else seen recycled combustion fumes from a sketchy termination.