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Freezer fridge defrosts when operated in a cold room
Freesurfer:
I have a fridge/freezer unit in an off-grid solar powered cabin. The problem is that when the outside temperature drops and stays below below 8-9C or so during the colder season, the freezer compartment can't maintain its freezing temperatures, so the food inside defrosts and needs to be disposed off. There is no heating in the cabin during this period.
It is not a fault, the explanation is simple: The fridge only has one compressor cooling both compartments, and the thermostat only checks the temperature in the fridge compartment. When it gets cold, the compressor doesn't have to start as often (or at all) to maintain 4C in the fridge compartment. Which is fine for the fridge, but the same can't be said for the freezer. The the thermostat knob is cranked all the way to the coolest setting.
This is a common problem with all ordinary household fridges that are operated in places like a cold garage. In fact if you check the manual, it does state the temperature range it is designed for, so I can't really blame it.
However I didn't consider this issue when installing the fridge. So now I wonder, can someone think of a simple solution to the issue? One thing I have thought of is to modify it in such a way that the bulb inside always stays on and thus generates some heat that needs to be expelled from the fridge, thus forcing the compressor to run more often. But I have no idea what sort of wattage a common fridge can cope with. Installing too much will overwhelm the compressor and drain the battery bank. Install too little and it may not be enough to maintain sufficient cooling.
Twoflower:
Have you checked, if the 4°C is actually maintained by the compressor? I could imagine that the cooling performance degraded or complete lost as the liquid/gas mixture in the cooling loop is too much condensed for the compressor to actually make the liquid evaporate in the inside of the fridge/freezer.
Nusa:
For your existing fridge, look for "garage refrigerator kit", which basically is a heater for the refrigerator side for a cold weather environment. For off-grid, the increased energy requirements will be a factor, like you said. But any other approach might solve the freezer side at the expense of freezing the refrigerator side.
If these temperatures are only a night-time issue, and it warms up during the day, you may be able to add enough thermal mass in the freezer (e.g. frozen water in milk jugs) so the freezer will stay sufficiently cold without running over the night.
As for a permanent solution, I'd suggest buying a small chest freezer. Dodges the entire problem by not having a refrigerator side. Likely to be the cheapest solution as well, long term. You may be able to unplug the regular refrigerator entirely during cold weather.
Gyro:
Yes, single compressor fridge freezers are fundamentally flawed when it comes to running at an ambient below the fridge thermostat setting. Separate fridge and freezer are of course the ultimate solution (checking the specs to ensure that they are suitable for "outbuilding" use.
One thing you could maybe try is buying a spare part freezer thermostat, fitting it in the freezer compartment and wiring it in place of (even in parallel with?) the fridge thermostat. I suspect that the fridge compartment will turn into an auxiliary freezer compartment, but it would probably solve your thawing problem.
NiHaoMike:
Simple solution is to add a thermostat to turn on the fridge light when the freezer is not cold enough and the compressor is not running. If it's incandescent, also fit a series diode to run it at half power. A SCR can be a clever way to do the part where it only turns on the light when the compressor is not running, by having the cathode connect to hot via the new thermostat, the gate to the existing thermostat via a diode and resistor, and the anode to the light.
I wonder if it might be possible to DIY an "ice motor" (kind of like a wax motor) to close off the vents to the fridge section and have the thermostat only operate based on the freezer temperature.
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