MiniMax nt 400 heater carbonizing flame

Yep probably spiders or something in the burner tubes. Cleaning them on this heater isn't easy or fun. The worst part is trying to clean all of the soot off of the heat exchanger.
 
You guys nailed it and I thank you. I took out the eight burners and found the first two had mud daubber nests in the front of the tubes. I found that the brush designed for cleaning refrigerator coils was just right for cleaning the burners and it would go to the bottom of the tube. A little steel wool polished the outsides just right. Getting the burners back was not a simple task as it was difficult to get the tabs on the burner tubes back into the slots on the back of the cabinet. It is back together now and works well. As a footnote on this heater, the engineers on this design have a bit to learn - and I am a Mechanical Engineer. Why would they design a unit for outside installation that had a burner vent on top of the unit for all falling water to enter? A side vent or a snorkel tube on top would have been a better design. Also, why would they design a unit for outside installation that would allow a mud daubber to get to the burner tube? A little screen wire over the vents and other openings would have been better engineering. I see lots of engineering mistakes on the stuff this company sells.
 
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