Tech- Inside a Dometic NDA1402

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Last week, I attended a course on the “New Generation” of Dometic refrigerators. The 3 models we looked at are the RM3762/3962, fairly conventional top/bottom 2 door models with new control circuitry, including constant LED readouts of the temperature in the fresh food cabinet, the RM1350, a large 4 door model, with cold water through the door, the same temperature readouts as the RM3762, a “tag line” (ignition switch) hookup which both locks out LP operation for refueling purposes, and activates an automatic door lock, and the most interesting model (at least to me :))- the NDA1402- a side by side model with the first true auto defrost in an RV refrigerator.

I thought it might be interesting to look “under the hood” to see how this model works….

The NDA1402 is a side by side model, with a small twist in layout- as you can see, the freezer portion is somewhat larger than normal, which- added to the vacuum insulation (meaning thin walls with good insulation) gives a fairly large food storage volume.

But… the most interesting part- both from a use and service point of view- is the automatic defrost.

The first thing you have to do when you first start up this model is to set the clock… yes, this refrigerator has a clock. “Why ?” you may ask- pretty simple. The clock lets the refrigerator go in to defrost mode at 1 AM every day, when the doors are likely to be closed, and the refrigerator will have plenty of time to recover from the added heat of defrosting.

“Added heat?”…. Yes, this model has 4- 12 volt heaters in it to do the actual defrosting, and to keep the drain tubes unplugged.

Here’s a picture of the fresh food heater, which is installed behind the cooling fins in the compartment. Notice there is also a fan to circulate air. There are defrost heaters and fans in both the freezer and fresh food compartments.

To control all of this, the unit uses 4 thermisters (temperature sensors)- 2 in the freezer, 2 in the fresh food compartment. 1 thermister in each is attached to the cooling coils, the other reads the air temperature.

In operation the defrost cycle starts at 1 A.M. First, the cooling unit is turned on for 1 hour to pull the temperature down (it will stop before freezing the fresh food, though).

Next, the unit is shut off for 10 minutes, to allow the cooling unit to “relax”, or equalize the pressures in the system.

The freezer heating element is energized next, along with the freezer drain tube heater. The defrost heater stays on until the freezer plate temperature reaches 41° (or 75 minutes, whichever is first). The fan in the freezer is shut off during this procedure (we just want to defrost the plate, not heat up the freezer), but the freezer fan is run for short periods to throw any water off of the blower.

After the freezer defrost cycle has completed, the cooling unit is started again, and the fresh food defrost actually takes place while the unit is cooling. Because of the slow recovery of absorbtion type refrigerators, the timing lets the cooling unit get a head start on recovery. The fresh food defrost cycle is until the fins reach 41°, or 20 minutes. The fan is also shut off for this cycle, and the drain tube heaters stay running for 30 minutes after the cycle is complete.

The defrost cycle can be started manually, and if it is started manually, and if the freezer door is held open, it goes in to the “drying” mode- which is used for storage. In the drying mode, both heaters are run for 3 hours. At the end of this cycle, the unit shuts completely off. This helps eliminate mold and rust from forming.

A couple more features this model has that I thought I would mention- it has temperature readouts for both the freezer and fresh food compartment, and it has ice and water through the door. The ice maker and ice and water dispenser are standard household models, and run on 120 volts, so while it will make ice running on LP gas, it will not eject or dispense ice unless 120 volt power is available- though the units now use 2 separate 120 volt cords, so the ice maker function can easily be run off an inverter.

It was interesting to me to see where the “state of the art” in RV refrigeration is headed, and it will be interesting to see if some of this technology gets in to the standard 2 door “top freezer” models.

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