Loss of efficiency leads to rising temperatures in ammonia absorption refrigerators.
By Bill Hendrix, F761S
March 2015
The performance of ammonia absorption refrigerators, which are common in motorhomes, can be affected by several factors. Some, such as the weather or grandchildren who frequently raid the fridge, may not be within your control, but most are.
The American National Standards Institute (ANSI) sets minimum design criteria for refrigerators fueled by propane, and you should expect your fridge to perform accordingly. The standards state that refrigerators larger than 6 cubic feet must be capable of maintaining 43 degrees in the food storage compartment and 15 degrees in the freezer with ambient temperatures up to 110 degrees; for smaller refrigerators, the ambient temperature benchmark is 90 degrees.
The Food Safety and Inspection Service of the U.S. Department of Agriculture recommends that a refrigerator’s main compartment be set to maintain a temperature of 40 degrees. However, the primary concern is the temperature of the food, not the temperature of the air around the food. Each time the door is opened, the air temperature is adversely affected. But the food temperature might not change if the door is not open long.
To more accurately monitor the food temperature, use an aquarium thermometer placed in a small jar of water. When you lower the temperature in the main compartment, a corresponding lowering of the freezer temperature should occur. The freezer compartment temperature, by design, will be about 28 degrees lower than the temperature in the main compartment. Most automatic ice makers will not perform well with freezer temperatures above 15 degrees. Also, storing ice cream tends to be a topic of discussion among RVers. Quality ice cream has a higher fat content and thus requires a lower temperature to freeze solid.
When a cooling unit is performing at peak efficiency, it first evaporates ammonia in a chamber adjacent to the freezer compartment. By design, all of the available ammonia cannot evaporate in the primary evaporator; the remaining ammonia, still in a liquid state, is routed by gravity to a secondary evaporator behind the fins in the food-storage compartment. For gravity to work, you must park the motorhome so the fridge is level; operating it critically out of level can damage the refrigerant, hastening a premature failure. Once the remaining ammonia reaches the secondary evaporator, enough of it evaporates to accommodate food-storage temperature requirements.
When a cooling unit first starts to lose efficiency, there is not enough leftover ammonia for the main food-storage compartment. So, the initial symptom of an underperforming cooling unit is a slow rise in the temperature of the main food-storage compartment while the freezer temperature remains satisfactory. This could be an indication that the cooling unit is starting to fail, or it could mean that other factors are affecting refrigerator efficiency. If the temperature rises in both compartments, that can signal other problems.
Slow failures are usually a result of the boiler and pump tube being contaminated from overheating of the refrigerant; or, a minute leak of hydrogen gas can cause a slow reduction of the internal pressure. A weld fracture that allows ammonia to escape will cause a faster failure, and it usually is detected quickly by the presence of an ammonia odor.
When you camp in extremely high temperatures, care should be taken to minimize the heat load on the cooling unit. Open the refrigerator door as briefly and as infrequently as possible. Use the air conditioner to keep the motorhome interior cool. If possible, park with the refrigerator on the shady side of the coach and get as level as possible. Turn on the fridge a day or two before adding food and make sure to precool the food. Also, inspect the fridge door seals using the dollar bill test. Close the door on a dollar bill; as you pull the bill out, you should be able to feel the door seal lightly gripping it.
If the fridge is still struggling after that, the cooling unit must be evaluated. If it does not cool well on propane but cools okay on electric, a tune-up of the propane system is in order. Check the system pressure; it should be 11 inches of water column, which is the amount of pressure required to displace 11 water column inches in a manometer. Also, clean the jet, the burner, and the flue, and inspect the venting. If the fridge cools okay on propane but not on electric, the heating element (or elements) must be tested for the proper resistance. These tests can be performed at most dealers and any RV service center.
Pay particular attention to the venting of the back of the fridge. Varmint nests, sagging insulation, or a damaged vent cap all can affect the natural upflow of the hot air. Some refrigerators are equipped with one or more fans to assist the airflow, especially if the fridge is in a slideout room and/or has an upper sidewall vent. If you can’t hear the fan, it probably is not running. If you want to add a fan, the recommended place is above the absorber coil and below the condenser, about two-thirds of the way up the back of the fridge.
If the refrigerator is still not performing well after the above steps have been completed, a cooling unit test is in order. This must be done by an authorized service center. Technicians will start with a warm refrigerator. They will test the heating element (or elements) for proper resistance, bypassing the control and plugging the heating elements directly into a 120-volt outlet. This eliminates any possible malfunction of the controls, and supplies the correct amount of uninterrupted heat to the boiler. Some shops may use a slightly different method, but something similar typically is necessary, especially if a warranty issue exists.
If the refrigerator passes that test, then the controls and installation will be evaluated. If the refrigerator fails the test and is under warranty, a new cooling unit will be installed. If it is out of warranty, you must make a choice: buy a new refrigerator or purchase a new or rebuilt cooling unit. Obtain a price for each option and consider the warranty, especially on the rebuilt cooling unit, as some may require the unit to be returned to the installing dealer. That would be a real bummer if the failure happens when you are hundreds of miles away.
The useful life of a cooling unit is completely unpredictable. Some models may fail within the warranty period, but I have seen 30-year-old vintage units that were still working well. In my opinion, not using the refrigerator has little effect on its lifespan. Despite the presence of a corrosion inhibitor, the corrosive ammonia is still in contact with the ferrous tubing whether the unit is dormant or active. The best way to extend the life of your refrigerator is to read the owners manual and follow the recommended procedures.
RV Food Storage Tips
- Cool the refrigerator before placing food inside. A good way to do this is to turn on the unit the day before you pack to leave on your trip.
- Never put hot food or drinks into the refrigerator; always cool them first.
- Don’t overpack. When the unit is full, it takes longer for temperatures to lower. An overcrowded fridge will take longer to make ice, and a heavy load may cause defrosting.
- Arrange food with space between items to allow free air circulation. This will ensure heat is removed efficiently and quickly.
- Don’t block the shelves with large storage containers or paper items over the shelves. This blocks circulation and will reduce the unit’s efficiency.
- Wipe excess moisture off of items before placing them inside the fridge, which helps prevent condensation and frost buildup.
- To help reduce condensation/frost formation and increase efficiency, keep the door open only as long as necessary.
- Keep all food and drink containers tightly covered. They’ll be traveling, too!
- Cover vegetables and lettuce to help retain crispness.
- Cover highly “piquant” foods — onions, garlic, certain cheeses, etc. — in covered dishes, sealable bags, or foil. This will prevent odors from permeating other foods in the refrigerator and keep odors out of the motorhome.
Source: Dometic Group