Posted by Mark on 29 Jan, 2017
Our fridge broke during the summer of 2015, more or less on the hottest day. It had only been five years old and we had bought it to be eco-friendly but affordable, so it has an A+ rating (in 2010's standards, more on this below).
It turns out that a typical cause for this is a broken thermostat and in fact it was in our case. The replacement thermostat is a 30 EUR to 40 EUR mechanical part that can be easily found on eBay. It looks like this: https://ebay.de/itm/171288968757. Unfortunately, in our fridge, the capillary (the metal wire) is installed behind the insulation. To replace the part, one would have to disassemble the whole fridge, dismantling it first. Commercially, it makes no sense to have it repaired by a technician as this would be far too expensive.
But we didn't want to throw out the whole fridge just because of a single broken part. So I figured, it would be the ideal reason to start tinkering with an Arduino microcontroller. After some planning, I bought parts that cost about 40 EUR in total, the most expensive part being the Arduino itself, costing 20 EUR. A fully calibrated digital temperature sensor is about 1 EUR to 2 EUR, a mechanical relays fitted for Arduino is 5 EUR to 10 EUR, and you will need at least a pull-up resistor and some wires (< 1 EUR). There is a lot of source code out there already and documentation on temperature control projects. The actual work was about one day, including understanding Arduino, but of course since this is fun, I started tweaking and optimizing it. I've put some documentation online on the Arduino Playground Wiki. It needs some updates, plus I will put the source code online somewhere in the next days.
I had fixed it in 2015 originally, but now it runs for more than a year. After a couple of months, the original relays broke, so I had to solder in a new one, and doing this, I moved the relays into the housing of the original thermostat, so it looks a little more tidy.
Regarding the ecology, I am not sure if the fridge is really energy saving one, now that it is nearly seven years old. It may have never been as eco-friendly as stated by the manufacturer, which you can see from the following calculation:
The model is a Bauknecht KGA 325 Pure/1 Optima WS and it is supposed to consume 0.74 kWh per 24 hours or 270 kWh per year, which gave it its A+ rating. The rating plate lists 0.7 A at 250 V, which lets me assume that the compressor actually would consume 4.2 kWh a day, if run continuously. In other words: the manufacturer assumes that the compressor is on about 16 percent of the time. During summer, it seems to run more like 60 % to 80 % of the time, now in winter, I'd say it's 40 % to 50 %. The room it is standing in is heated to 19.5 C and I've programmed it to keep about 8 C at the thermostat. This seems to be a rather typical setting, so unless I'm missing something here, the energy rating itself is far from reality.