By Bill Henderix F761S
March 2006
Recently confusion has arisen in the RVing community concerning temperature adjustments of Dometic refrigerators. The confusion relates to moving the temperature sensor (the thermistor) in order to increase or decrease the cabinet temperature in the absence of, or in addition to, the refrigerator’s thermostat. The thermistor is a simple solid-state semiconductor whose electrical resistance decreases with an increase in temperature. So, the thermistor reports a resistance value to the control board that is relative to the temperature at its location.
In the upper rear of the food storage compartment (not the freezing compartment) is a bank of vertical aluminum fins that are cooled by the evaporation of ammonia in the secondary evaporator of the cooling unit. The temperature sensor (thermistor) is located on the extreme right fin and normally about halfway, top to bottom. The thermistor is held in place by a plastic clip that may be moved up or down on that fin, but some models have a limited travel due to the length of the wire to the sensor.
The confusion related to the misleading information that seems to be circulating is that the side-by-side refrigerator has a much different cooling unit configuration than the conventional-style RV refrigerator (freezer above the food storage compartment).
In Illustration #1, which depicts the conventional refrigerator, note that the primary evaporator makes a turn through the freezing compartment first, and then by gravity it descends to make a turn behind the bank of aluminum fins in the food storage compartment. The fins average about 5 inches in vertical height and run the full width of the compartment. Also note that the bank of fins are in full contact with the evaporator as it passes behind the fins.
In Illustration #2, which depicts the side-by-side refrigerator, you will see that the primary evaporator uses about 8 inches of vertical drop before going to the secondary evaporator in the food storage compartment. The aluminum fins in the side-by-side are about 15 inches vertical, and the secondary evaporator is able to contact only the lower half of the aluminum fins. Therefore, the lower half of these long fins will be slightly cooler than the upper half. The normal location for the thermistor would be midway, and some models will have an arrow to indicate such.
Conclusion: On the standard refrigerator, there is very little difference in the temperature of the fins from top to bottom, since the evaporator is in good contact with the bulk of the aluminum fins. So, moving the sensor up or down will have virtually no effect on the resulting cabinet temperature in the standard refrigerator.
However, on the side-by-side, there is contact on only the lower half of the fins, so the lower half will be slightly cooler than the upper half. By moving the sensor upward, the device will sense slightly warmer temperatures, keeping the cooling unit operating a bit longer, and thereby producing a slightly cooler cabinet temperature. How much this will affect the cabinet temperature is dependent on several factors “” such as other heat loads, condition of door gaskets, available air circulation within, etc. I have heard that some cabinet temperatures may cool by as much as 5 degrees Fahrenheit if the sensor is moved from the bottom to the very top, but I believe the average would be 2 or 3 degrees difference “” and, again, this is only for a side-by-side style.
I hope this lengthy explanation will eliminate some of the confusion regarding the thermistor location.