flue heat signiture 12 hours after fire

To our amazement one morning there was a "heat signature" coming out the roof flue pipe 12 hours or more after the fire was out! When the heater burns clean the exhaust is clear but you can see the heat waves just above the flue. That is what it looked like. Do I need a flue dampener to keep from loosing heat from the mass to the outside long after the fire goes out? Or should I just close the stove air inlets?

Your thing being soo weird, it's hard to tell. Normally closing the feed of a J tube should be enough. You will always have a convection movement in your chimney. Is it worth blocking it, at the risk of killing yourself or the loved ones? I can't remember if your last tank is realy a bell. Plus your latest modifications i haven't followed. The question is, is it realy worth it?