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Thursday, October 22, 2009

Sooty Sidewall

I have an Atwood water heater in my 1989 rig and I am getting large amounts of soot on the side of the rig above the exhaust vent. I have checked and cleaned the tube for obstructions and I adjusted the air gas ratio to a blue flame. I cannot figure why I am getting incomplete combustion. No changes or maintenance had been done to the heater just prior to the onset of this problem and the heater had been used several times a month prior to the problem. Any ideas?
Garth, (Saratoga, CA)

Garth, cleaning the mixing tube and adjusting the air shutter is a good start, but there are a couple of other factors that, in combination, ensure complete combustion. As you know, adjusting the air intake affects the burner flame size and intensity. The delivery pressure of the incoming LP is what draws in that primary air. It must be set to 11.0 inches of water column with the regulator at about a 50% load. A manometer is used to measure LP pressure, so if you don’t have one, make an appointment at your local service center. You cannot adjust the LP pressure correctly by simply monitoring the size and colorof the burner flame like you can with the air adjustment. Eleven inches of water column is only .4 of one PSI; a very minute amount. Once the pressure is set correctly, re-adjust the air shutter to the blue flame you have now. 

Another important aspect is the cleanliness of the main burner orifice. As with any appliance orifice, remove it and soak it in acetone and let it air dry. Replace it if there is any evidence of damage. 

The final thing to check is the alignment of the mixing tube around the fitting that holds the orifice. The orifice should be centered in the open end of the mixing tube and the mixing tube should be in-line with the direction of the angle of the orifice fitting. Any discrepancy here will result in turbulence or an imbalanced mixture inside the tube once the primary air enters. This could lead to the overly rich mixture you apparently have now. So, the key words are cleanliness, alignment, LP pressure and air adjustment.


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