Direct vent gas inserts-flames ghosting?

[ernie]

Has anyone seen a direct vent insert burn with what I call ghosting flames? What happens, and it is intermittent, when the unit is first turned on, the flames look very thin and blue. the unit will only run a minute or so and then shut down. I am guessing that the venting is reversing and going out the intake instead of up the exhaust. This is a system with two units installed back to back in a see thru fireplace, they have both intakes and exhaust run up and connected to a special common cap made for duel inserts. I have been told that some times it works better to only run the intakes up a short way instead of all the way to the top. Both units seem to do this a different times with no pattern to it. Some times they can go a week or two with no problem and then do it again.
All you installers out there, do you install the intake all the way? Have you seen this problem? Any solutions?

Ernie

 

[jtp10181]

We had problems with units running out of air with the stubby fresh air liners so we started running both to the top. Normally the fresh air comes in way down at the bottom through a small hole, it would be pretty hard for it to reverse. In my years of doing service I have only seen ONE gas unit reverse draft and it was an old stove with a huge opening under the burner for the fresh air.