House Calls with the RV Doctor
By Gary Bunzer
Dear RV Doctor:
In one of your articles on holding tank probes you offered the idea of using “well nuts” by drilling a 3/8-inch hole. Great article, by the way. What size well nuts would you recommend buying, and does this move the actual probe far enough off of the tank wall to ensure it will take awhile for the buildup, etc., to really be a factor?
RV Doctor: Thanks, Bill, for the compliment. I use 3/8-inch well nuts; that is, the hole size is 3/8 inches in diameter. Be very careful drilling into polyethylene, ABS, or any plastic holding tank. The drill bit has a tendency to grab the soft plastic and create an off-round hole. I recommend using a Forstner-style drill bit. You’ll want the holes for each probe to be perfectly round to avoid any chance of a leak. Simply push the well nut through the hole all the way up to its flange. There is no need for any sealant behind the rubber flange of the well nut itself; however, it will be necessary to apply sealant to the threads of the machine screws. The screw passes through the well nut and is threaded into and through the embedded nut and into the tank. Thus the machine screw becomes the actual probe. If no sealant is used on the screw, liquid will “follow the threads” and leak down the outside of the tank. I also recommend using a washer with an outside diameter a little larger than 3/8-inch so solid pressure remains against the outside wall of the tank.
Tightening the screw bulges the nut section inside the tank and creates a good seal (see photo). Do not overtighten the well nut. If the monitor panel wires have ring connectors attached, be sure to insert the machine screw through the ring connector first before threading it into the well nut. Be sure also to use stainless-steel machine screws and washers. Unfortunately, paper and waste can still hang onto these probes, because they stick into the tank a little farther. But if you clean the inside of the holding tanks once a year with very high water pressure and fill the tank before evacuating, such happenings will be minimized.
Dear RV Doctor:
I have an Atwood water heater in my 1989 motorhome, and I am getting large amounts of soot on the side of the coach 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 out 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?
RV Doctor: 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 gas is what draws in that primary air. It must be set to 11 inches of water column with the regulator at about a 50 percent load. A manometer is used to measure LP-gas pressure, so if you don’t have one, make an appointment at your local service center. You cannot adjust the LP-gas pressure correctly by simply monitoring the size and color of the burner flame like you can with the air adjustment. Eleven inches of water column is only 0.4 of one psi “” a very minute amount. Once the pressure is set correctly, readjust 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, soak it in acetone, and let it air dry. Replace it if you see 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-gas pressure, and air adjustment.