* Water Heater Flashback Repair & Protection *
Fire


A common problem with LP fired appliances in RVs is bugs ...bugs are attracted to the scent of LP, and like to build their nests in places that are not designed to function with such obstructions.

It is not uncommon to find appliances (refrigerator, water heater, furnace) that will not work on LP in RVs that have been stored, particularly if they have been stored in a barn or similar location. Many times the problem will be a mud dauber nest built in the burner tube, burner box, or flue. Or it may be a spider web/nest in the burner tube. In some cases, the appliance LP burner simply will not light. However, in other cases, the obstruction in the burner tube can cause a gas build up outside the burner box. When that gas is ignited by the pilot or electronic ignitor, the result is a "flashback," or fire where there is not supposed to be fire.

When the appliance is a water heater, the fire will normally be contained inside the metal door of the water heater. Atwood water heaters have a thermal cut-off device (a fusible link) in the circuitry that is designed to fail under intense heat. When the thermal-cut off fails, it shuts down the gas flow to the water heater. The thermal cut-off is located just above the LP RV firemain burner tube so that it will be in the likely path of the flames in event of a flashback fire. Some RVs have had problems with air circulation behind the water heater metal door, causing the thermal Thermal Cutoffcut-off to fail for no observable reason. Atwood has blamed the RV manufacturers for putting an exterior door over the ugly metal door of the water heater, and the RV manufacturers have attempted several fixes, none of which have been particularly effective. Many creative owners have worked with the problem for an extended period of time, concluded the problem is primarily bad design by Atwood, and gone on to work out their own solution to getting better airflow behind the metal door. A few owners have discovered that all they have to do to make their water heater work is to simply omit the continually failing thermal cut-off from the wiring. Yes, that will work, but in event of a burner tube obstruction and resultant flashback, those owners are literally playing with fire! Without the thermal cut-off in place, LP gas will continue to flow until the water heater wiring is completely melted and breaks the circuit to the burner, shutting down the gas flow. By that time, burned wiring may be the least of their worries!! The picture on the right is the result of an RV appliance fire during vehicle refueling. It not only shows you the relatively quick result of an RV LP appliance fire, but also serves as a strong reminder to ensure all LP appliances are TURNED OFF before entering a fuel station! And DON'T OMIT THE THERMAL CUT-OFF from your water heater circuitry! Atwood thermal cut-offs normally come in a package of two, (pictured to the left) and include a short length of wire with a spade connector on one end in the event that you need to replace the wire running to the thermal cut-off that failed due to a flashback. That is good, however, there are other wires that are also in the path of the flames!

Although my Atwood water heater is protected by a stainless steel screen over the exhaust to prevent mud daubers from getting inside, and although I have a ventilated bottle of mothballs inside the water heater compartment, and although my motorhome is used fulltime and never stored, I STILL experienced a water heater flashback due to a spider web in the burner tube. We rarely used our water heater on LP as most of the time we have 50a electrical service at the RV parks where we stay. One day as cold weather was approaching and as I had just finished the quarterly draining and flushing of the water heater, I decided to test it's LP function just to insure it was working well. I could see no obstructions, so closed the metal door and went inside and flipped the LP switch, then went back outside to observe. As I walked up to the water heater, I saw a whiff of smoke coming out around the edges of the metal door, and smelled rubber burning. As the smoke dissipated, I opened the metal door to see most of the wiring on my water heater had been creamated! Insulation was melted on several wires, with one wire left bare for several inches. The picture below shows you what I saw.

Burned wiring

The damaged thermal cut-off is the partially silver/partially melted item in the lower left. It functioned as designed, failed under intense heat, and shut down the LP gas flow in seconds. Unfortunately, the other wiring around it also experienced that same intense heat, and was badly damaged. Fortunately, the wiring insulation was all that was damaged.

It is not possible to see around a right angle bend in the burner tube on my water heater, so I removed the burner tube support brackets in order to get the burner tube out for inspection. At the far end of the tube, I found a complete blockage by a spider web, complete with spider. Well, I evicted the spider from it's poorly selected home site, and reassembled the water heater. Due to several previous failures of the thermal cut-off, I had a replacement on hand. With a new thermal-cut off in place, the water heater ignited and operated normally on LP. Now I had a water heater once again working on either electric or LP gas, but I had some badly burned wiring needing repair. In order to restore the wiring to it's original colors to avoid confusion in the future, I used the brown wire with connector included in two Atwood thermal cut-off kits, a piece of green wire I had on hand, and bought both red and blue wire, cut out the damaged wiring, and spliced in the new wiring with crimp-on butt splices. Then, in an effort to protect the repaired wiring in the event of future flashback, I covered all the repaired wiring ...WITH THE EXCEPTION OF THE THERMAL CUT-OFF!! ...with flame retardant wire loom. Below is a picture of what it looked like when got finished.

Burned wiring

Project Flashback Repair completed ...hopefully if there is a next time, I won't have as much damaged wiring to repair!

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