I'm not quite as far along in the burn as you and I can only get 110* at the most reading in that same spot. Now I can get 130-140* reading around the front of the front duct connector, which is why I have that duct wide open and the back two throttled to .2 SP. Even with the top reading 130-140 around the connector, I still only have 110* internal duct air temps. If I throttle the front duct to .2, internal air temp goes up a bit, but CFM to the house drops and overall heat output drops.Here is a picture of the video. This is back side of burn no strong flames, no more nice secondary burn going on. Mostly logs are about to brake up in larger coal chunks. The second picture of the top ( see red dot in middle ) is to show where i took the temp reading. Its 130*-135*.
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Right now I am leaning toward making a whole new top for the furnace to put a plenum onto. I will bias the opening toward the front of the furnace since that is where the higher temps are.