Our Outdoors RV 21RBS came with a Norcold N611CL fridge - a 6 cubic foot combination fridge-freezer that can run on 120V AC or propane. It can do this because it's what's known as a gas absorption fridge, rather than a compressor fridge like you might typically find in a house.
One of the limitations of this type of fridge is that they don't work well when exposed to prolonged temperatures below 32°F. That is, unless your fridge is specially equipped to handle it!
If you have a Norcold fridge, one such solution is the Norcold Cold Weather Kit which claims to help your fridge continue to operate all the way down to 0°F! But how does it work, and more importantly, does it actually work? Let's find out!
How RV fridges work
In a standard residential fridge, a refrigerant is continually circulated around coils where it changes state from gas to liquid (by the compressor) and back to a gas, moving thermal energy from inside the fridge to coils on the outside.
By contrast, many RVs use a gas absorption fridge. These work in much the same way as a compressor fridge - heat is transferred by changing state of a refrigerant. However, in an absorption fridge the system is filled with a pressurized mixture of water, ammonia, sodium chromate and hydrogen.
Perhaps counter-intuitively, it works by boiling the ammonia solution and the cooling effect is provided when the pure ammonia re-condenses once again. This heating is provided by a boiler, and this can be powered either by 120V AC electricity or by propane.
The problem comes when it gets too cold - the ammonia solution can become viscous and gel-like, restricting its flow through the coils. When this happens, the fridge will no longer work effectively.
Norcold Cold Weather Kit
This is where the Norcold Cold Weather Kit comes in. Put simply, it's a heater for your fridge!
It draws 12V from the back of the fridge to power a heating strip on the weak solution tube - in essence warming the ammonia solution just enough to stop it becoming gelatinous. It's thermostatically controlled so it only turns on when the temperature in the outside fridge compartment drops below a threshold, and turns off again once the temperature rises past a cutoff.
Our RV came equipped from the factory with the Norcold Cold Weather Kit, but it's also available as an aftermarket add-on and the instructions seem to suggest installation is straightforward.
Last night, temperatures dropped cold enough for the Norcold Cold Weather Kit thermostat to turn on the heating strip. As part of our home automation system, we have a number of sensors monitoring the fridge. The graph below shows temperatures in a 24-hour period from sensors on the fridge coil (red), outside rear fridge compartment (orange), fridge (green), freezer (blue) and outside the RV.
Each spike of the coil temperature (red) is caused by the fridge heater cycling on and off - about 30 minutes on and 10 minutes off. This is how the fridge regulates the temperature inside. You can see that throughout the entire 24 hour period, the temperature inside the fridge (green) and freezer (blue) stayed very stable - the fridge at 35±2°F and freezer at 0±2°F.
The temperature in the outside rear fridge compartment (orange) tracks at about 12°F above the ambient outside temperature (black), but with little variations up and down corresponding to the fridge cycling on and off - this is just heat radiating off the coils.
At 8:34pm, something interesting happened - the coil temperature suddenly shot up. This is because the temperature in the fridge rear compartment dropped below the threshold for the Norcold Cold Weather Kit thermostat - my compartment sensor isn't placed exactly next to it, but it registered the temperature at 43°F.
This caused the temperature of the coil to rise from ~39°F to ~71°F, or an increase of around +32°F. Zooming in on the chart a little more, we can see what's going on.
In the chart above, I've removed all the temperatures except the outside rear compartment (orange) and coil (red). I've also overlaid a purple line, showing when the fridge heater was running.
As you can see, there's some lag (about 10 minutes) between when the heater cycles on each time and when the coil temperature increases. This is because the coil temperature sensor is at the end of the system, furthest from the boiler.
But the chart nicely shows how once the Norcold Cold Weather Kit is running, a new baseline for the coil temperature is formed - about +30°F above what it would be otherwise. This is inline with the expectation that it should allow the fridge to operate down to temperatures of around 0°F.
Propane vs Electric
This test was done while the fridge was running on 120V AC, but does it still work if the fridge is running on propane?
Yes! I switched the fridge over to propane mode, and here's what I found.
Note that the baseline temperature between cycles still stays high - around ~70°F vs an outside temperature of ~40°F. The chart seems to show higher coil temperatures while running on propane - both the highs and lows are 5-10°F higher.
This may be a factor of radiated heat from the propane burner heating up my sensor on the weak solution tube, however my sensor is directly contacting the metal pipe so it should be representative of the temperature of the solution inside.
However, since both the outside temperature and compartment temperature rise by about the same margin during this time, I suspect it's likely that instead, the increased coil temperature is due to warmer outside temperature - the compartment is in direct sunlight during the morning hours.
Lastly, we can see the coil temperature continuing to drop at the end of the chart - this makes me believe that the Norcold Cold Weather Kit turned off when I switched it from propane back to electric. Perhaps coincidence, or more likely the change in power source triggered a re-evaluation of the thermostat.
Either way, the Norcold Cold Weather Kit did its job - raising the coil temperature to prevent the ammonia solution from becoming a gel.
According to Norcold, our fridge should not be operated for prolonged periods at temperatures below 32°F without the use of a cold weather kit. While they don't elaborate on what constitutes a "prolonged period", I'd suggest that if you're planning on using your RV in conditions where temperatures may drop below freezing, you might want to consider an upgrade such as this.
While our RV came with the Norcold Cold Weather Kit pre-installed from the factory, the kit is relatively inexpensive and the instructions seem to suggest it's a fairly straightforward installation.
Unfortunately I don't have data on how much power the heating strip draws, but my understanding is that it's about 1.5-2 Amps @ 12VDC, or somewhere around 20W.
We're expecting temperatures well below freezing this winter, so for us this is a must-have - it's good to see it working as expected! The question is, will it be enough as temperatures for us continue to drop? I guess we'll find out!