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The boiler section is overheating.
Below are two pictures that indicate what is meant by the boiler section.

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The boiler section is the part of the rv refrigerator where heat, by either electric or propane, starts the refrigeration process. When a cooling unit is operated level, has the correct heat input, and has good ventilation, the unit runs at a safe temperature (with just a few exceptions) and can expect a long happy life. But when one of these components is missing, bad things happen to the cooling unit. 
When the cooling unit reaches a critical temperature, the refrigerant in the system starts breaking down. This may be noticeable in small increments by a fridge that doesn't get as cold as it used to and progressively gets worse, or the unit all of a sudden ruptures in the boiler tubing, causing a release of all the refrigerant. These ruptures are caused by fatigue cracks in the boiler tubing. These fatigue cracks form for two reasons: First, the heating of the boiler by its heat sources causes thermal expansion and contraction along the weld seams. Every time the unit cycles on and off, thermal expansion takes place. The tubing that the factories use isn't heavy enough to handle the stress and eventually forms stress cracks along the welds in the boiler. Second: overheating causes a breakdown of the refrigerant and its additive sodium chromate. The brown sludge is sodium chromate that reached a critical temperature and solidified. Along with the sludged chromate are also flakes of metal from the interior walls of the boiler. When the sodium chromate breaks down, the interior walls of the boiler tubing are destroyed which make the already too thin tubing even weaker until fatigue cracks form, causing a release of the refrigerant ammonia and hydrogen. There have been several rv fires in recent years linked to the boiler failure in the cooling unit. Even if a fridge fire isn't the end result, the cooling unit is still destroyed and in need of replacing.
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How can this type of failure be avoided in the future?
1. Use a "bulletproof" boiler built by RV Cool. We use heavier tubing in our boilers than any cooling unit builder in the country. Heavier than Dometic, heavier than Norcold and heavier than other cooling units in the industry. But just using heavier tubing isn't going to make the cooling unit last longer. The boiler must be designed properly so it runs at a safe temperature that won't damage the refrigerant in the system. We have done extensive studies on ammonia boilers and their design. We use that latest, most "state of the art" software to evaluate and graph the performance of our boilers. Our studies have given us a far better understanding of how ammonia boilers work and what they need to last longer. Our heavier boilers are safer than boilers built by any of our competitors. 

2. Control the heat in the boiler: Remember the three things an rv refrigerator needs....must be operated level, has the correct heat input, and has good ventilation. Well, today's recreational vehicles don't always meet these requirements. RV owners are often boon-docking in unlevel parking lots, pulling up and down long steep grades. Many towable campers don't set level on the tow vehicle. Each of these things will overheat the cooling unit boiler, damaging both the refrigerant and the boiler tubing, thus significantly shortening the life of the refrigerator. Unlevel operation can destroy a cooling unit faster than almost anything else and if extreme enough, can ruin the cooling unit in a matter of hours. Slighter instances will have a cumulative affect on the cooling unit but will shorten the life.  

Why don't today's rv refrigerators last as long as those built 20 years ago? 
  • The way they are quickly, cheaply built 
  • The way they are operated as described above  
  • Ventilation problems from the way they are installed by some rv manufacturers. Explanation: The biggest ventilation problems are from rv refrigerators installed in a slide-out room. An rv owner can do everything right and always level your rv/camper and the refrigerator can fail all of a sudden from overheating due to ventilation problems with the slide-out room. Because the slide-out room doesn't have a roof vent, the refrigerator relies on cooling fans to keep the unit from overheating. If the fans fail or the limit switch doesn't turn the fans on at the right time, the cooling unit can be destroyed very quickly. In extreme heat, even properly operating fans aren't always enough to keep the cooling unit from overheating, especially larger refrigerators like the Norcold 1200, N1200, 1200LRIM and the Dometic RM7732, 1292, NDR1292, RM1350. 

1. A strong ammonia smell inside the box. This would indicate a leak in the evaporator section. If the unit has leaked for an extended amount of time the ammonia smell may no longer be present but you will probably hear a gurgling sound coming from the back of your refrigerator a few minutes after you turn it on. 

2. If the controls are working and the cooling unit is heating up in the back, but there is no cooling taking place inside the box, this could be a sign of a blockage in the boiler section. 

3. If your unit is not cooling but you don't smell ammonia inside the box, look for a yellow powder around or just above the burner located at the back of the unit. This would indicate a leak in the boiler section. Since this leak is outside the box you would rarely smell it inside.  These boiler leaks appear more frequently in the newer Norcold and Dometic cooling units, found in most campers and RVs.